uses. Thus, the Criteria Document concludes that scientific
understanding of the detoxification mechanisms is not yet complete and
requires further investigation (EPA, 2006a).
    Once O3 injury has occurred in leaf tissue, some plants
are able to repair or compensate for the impacts. In general, plants
have a variety of compensatory mechanisms for low levels of stress
including reallocation of resources, changes in root/shoot ratio,
production of new tissue, and/or biochemical shifts, such as increased
photosynthetic capacity in new foliage and changes in respiration
rates, indicating possible repair or replacement of damaged membranes
or enzymes. Since these mechanisms are genetically determined, not all
plants have the same complement or degree of tolerance, nor are all
stages of a plant's development equally sensitive to O3. At
higher levels or over longer periods of O3 stress, some of
these compensatory mechanisms, such as a reallocation of resources away
from storage in the roots in favor of leaves or shoots, could occur at
a cost to the overall health of the plant. However, it is not yet clear
to what

degree or how the use of plant resources for repair or compensatory
processes affects the overall carbohydrate budget or subsequent plant
response to O3 or other stresses (EPA, 1996a, EPA, 2006a).
d. Changes to Plant Metabolism
    Ozone inhibits photosynthesis, the process by which plants produce
energy rich compounds (e.g., carbohydrates) in the leaves. This
impairment can result from direct impact to chloroplast function and/or
O3-induced stomatal closure resulting in reduced uptake of
CO2. A large body of literature published since 1996 has
further elucidated the mechanism of the effect of O3 within
the chloroplast. Pell et al. (1997) showed that O3 exposure
results in a loss of the central carboxylating enzyme that plays an
important role in the production of carbohydrates. Due to its central
importance, any decrease in this enzyme may have severe consequences
for the plant's productivity. Several recent studies have found that
O3 has a greater effect as leaves age, with the greatest
impact of O3 occurring on the oldest leaves (Fiscus et al.,
1997; Reid and Fiscus, 1998; Noormets et al., 2001; Morgan et al.,
2004). The loss of this key enzyme as a function of increasing
O3 exposure is also linked to an early senescence or a
speeding up of normal development leading to senescence. If total plant
photosynthesis is sufficiently reduced, the plant will respond by
reallocating the remaining carbohydrate at the level of the whole
organism (EPA, 1996a, 2006a). This reallocation of carbohydrate away
from the roots into above ground vegetative components can have serious
implications for perennial species.
e. Plant Response to Chronic Ozone Exposures
    Though many changes that occur with O3 exposure can be
observed within hours, or perhaps days, of the exposure, including
those connected with wounding, other effects take longer to occur and
tend to become most obvious after chronic exposures to low
O3 concentrations. These chronic exposures have been linked
to senescence or some other physiological response very closely linked
to senescence. In perennial plant species, a reduction in carbohydrate
storage in one year may result in the limitation of growth the
following year (Andersen et al., 1997). Such ``carry-over'' effects
have been documented in the growth of tree seedlings (Hogsett et al.,
1989; Sasek et al., 1991; Temple et al., 1993; EPA, 1996a) and in roots
(Andersen et al., 1991; EPA, 1996a). Though it is not fully understood
how chronic O3 affects long-term growth and resistance to
other biotic and abiotic insults in long-lived trees, accumulation of
these carry-over effects over time could affect survival and
reproduction.
2. Nature of Effects
    Ozone injury at the cellular level, when it has accumulated
sufficiently, will be propagated to the level of the whole leaf or
plant. These larger scale effects can include: Reduced carbohydrate
production and/or reallocation; reduced growth and/or reproduction;
visible foliar injury and/or premature senescence; and reduced plant
vigor. Much of what is now known about these O3-related
effects, as summarized below, is based on research that was available
in the last review. Recent studies continue to support and expand this
knowledge (EPA, 2006a).
a. Carbohydrate Production and Allocation
    When total plant photosynthesis is sufficiently reduced, the plant
will respond by reallocating the remaining carbohydrate at the level of
the whole organism. Many studies have demonstrated that root growth is
more sensitive to O3 exposure than stem or leaf growth (EPA,
2006a). When fewer carbohydrates are present in the roots, less energy
will be available for root-related functions such as acquisition of
water and nutrients. In addition, by inhibiting photosynthesis and the
amount of carbohydrates available for transfer to the roots
O3 can disrupt the association between soil fungi and host
plants. Fungi in the soil form a symbiotic relationship with many
terrestrial plants. For host plants, these fungi improve the uptake of
nutrients, protect the roots against pathogens, produce plant growth
hormones, and may transport carbohydrates from one plant to another
(EPA, 1996a). These below-ground effects have recently been documented
in the field (Grulke et al., 1998; Grulke and Balduman, 1999). Data
from a long-studied pollution gradient in the San Bernardino Mountains
of southern California suggest that O3 substantially reduces
root growth in natural stands of Ponderosa pine (Pinus ponderosa). Root
growth in mature trees was decreased at least 87 percent in a high-
pollution site as compared to a low-pollution site (Grulke et al.,
1998), and a similar pattern was found in a separate study with whole-
tree harvest along this gradient (Grulke and Balduman, 1999). Though
effects on other ecosystem components were not examined, a reduction of
root growth of this magnitude could have significant implications for
the below-ground communities at those sites. Because effects on leaf
and needle carbohydrate content under O3 stress can range
from a reduction (Barnes et al., 1990; Miller et al., 1989), to no
effect (Alscher et al., 1989), to an increase (Luethy-Krause and
Landolt, 1990), studies that examine only above-ground vegetative
components may miss important O3-induced changes below
ground. These below-ground changes could signal a shift in nutrient
cycling with significance at the ecosystem level (Young and Sanzone,
2002).
b. Growth Effects on Trees
    Studies comparing the O3-related growth response of
different vegetation types (coniferous and deciduous) and growth stages
(e.g., seedling and mature) have established that on average,
individual coniferous trees are less sensitive than deciduous trees,
and deciduous trees are generally less sensitive to O3 than
most annual plants, with the exception of a few fast growing deciduous
tree species (e.g., quaking aspen, black cherry, and cottonwood), which
are highly sensitive and, in some cases, as much or more sensitive to
O3 than sensitive annual plants. In addition, studies have
shown that the relationship between O3 sensitivity in
seedling and mature growth stages of trees can vary widely, with
seedling growth being more sensitive to O3 exposures in some
species, while in others, the mature growth stage is the more
O3 sensitive. In general, mature deciduous trees are likely
to be more sensitive to O3 than deciduous seedlings, and
mature evergreen trees are likely to be less sensitive to O3
than their seedling counterparts. Based on these results, stomatal
conductance, O3 uptake, and O3 effects cannot be
assumed to be equivalent in seedlings and mature trees.
    In the last review (EPA, 1996b), analyses of the effects of
O3 on trees were limited to 11 tree species for which
concentration-response (C-R) functions for the seedling growth stage
had been developed from OTC studies conducted by the National Health
and Environmental Effects Research Lab, Western Ecology Division
(NHEERL-WED). A number of replicate studies were conducted on these
species, leading to a total of 49 experimental cases. The Staff Paper
presented a graph of the composite regression equation that combines
the results of the C-R functions developed for each of the 49 cases.
The NHEERL-WED study predicted relative yield loss at various exposure
levels in terms of a 12-hour W126. For example, 50 percent of the

tree seedling cases would be protected from greater than 10 percent
biomass loss at a 3-month, 12-hour W126 of approximately 24 ppm-hrs,
while 75 percent of cases would be protected from 10 percent biomass
loss at a 3-month, 12-hour W126 level of approximately 16 ppm-hrs.
    Since the 1996 review, only a few studies have developed C-R
functions for additional tree seedling species (EPA, 2006a). One such
study is of particular importance in that it documented growth effects
from O3 exposure in the field without the use of chambers or
other fumigation methods that were as great as those seen in OTC
studies (Gregg et al., 2003). This study placed Eastern cottonwood
(Populus deltoides) saplings at sites along a continuum of ambient
O3 exposures that gradually increased from urban to rural
areas in the New York City area (Gregg et al., 2003). Eastern
cottonwood was selected because it is fast growing, O3
sensitive and important ecologically, along stream banks, and
commercially for pulpwood, furniture manufacturing, and as a possible
new source for energy biomass (Burns and Hankola, 1990). Gregg et al.
(2003) found that the cottonwood saplings grown in New York City grew
faster than saplings grown in downwind rural areas. Because these
saplings were grown in pots with carefully controlled soil nutrient and
moisture levels, the authors were able to control for most of the
differences between sites. After carefully considering these and other
factors, the authors concluded the primary explanation for the
difference in growth was the gradient of cumulative O3
exposures that increased as one moved downwind from urban to less urban
and more rural sites. It was determined that the lower O3
exposure within the city center was due to NOX titration
reactions which removed O3 from the ambient air. The authors
were able to reproduce the growth responses observed in the field in a
companion OTC experiment, confirming O3 as the stressor
inducing the growth loss response (Gregg et al., 2003).
    Another recent set of studies employed a modified Free Air
CO2 Enrichment (FACE) methodology to expose vegetation to
elevated O3 without the use of chambers. This exposure
method was originally developed to expose vegetation to elevated levels
of CO2, but has been modified to include O3
exposure in Illinois (SoyFACE) and Wisconsin (AspenFACE) for soybean
and deciduous trees, respectively (Dickson et al., 2000; Morgan et al.,
2004). The FACE method releases gas (e.g., CO2,
O3) from a series of orifices placed along the length of the
vertical pipes surrounding a circular field plot and uses the
prevailing wind to distribute it. This exposure method has many
characteristics that differ from those associated with the OTC.
    Most significantly, this exposure method more closely replicates
conditions in the field than did OTCs. This is because, except for
O3 levels which are varied across co-located plots, plants
are exposed to the same ambient growing conditions that occur naturally
in the field (e.g., location-specific pollutant mixtures; climate
conditions such as light, temperature and precipitation; insect pests,
pathogens). By using one of several co-located plots as a control
(e.g., receives no additional O3), and by exposing the other
rings to differing levels of elevated O3, the growth
response signal that is due solely to the change in O3
exposure can be clearly determined. Furthermore, the FACE system can
expand vertically with the growth of trees, allowing for exposure
experiments to span numerous years, an especially useful capability in
forest research.
    On the other hand, the FACE methodology also has the undesirable
characteristic of potentially creating hotspots near O3 gas
release orifices or gradients of exposure in the outer ring of trees
within the plots, such that averaging results across the entire ring
potentially overestimates the response. In recognition of this
possibility, researchers at the AspenFACE experimental site only
measured trees in the center core of each ring, (e.g., at least 5-6
meters away from the emission sites of O3) (Dickson et al.,
2000; Karnosky et al. 2005). By taking this precaution, it is unlikely
that their measurements were influenced by any potential hotspots or
gradients of exposure within the FACE rings. Taking all of the above
into account, results from the Wisconsin FACE site on quaking aspen
appear to demonstrate that the detrimental effects of O3
exposure seen on tree growth and symptom expression in OTCs can be
observed in the field using this exposure method (Karnosky et al.,
1999; 2005).
    The Staff Paper thus concluded that the combined evidence from the
AspenFACE \56\ and Gregg et al. (2003) field studies provide compelling
and important support for the appropriateness of continued use of the
C-R functions derived using OTC from the NHEERL-WED studies to estimate
risk to these tree seedlings under ambient field exposure conditions.
These studies make a significant contribution to the coherence in the
weight of evidence available in this review and provide additional
evidence that O3-induced effects observed in chambers also
occur in the field.
---------------------------------------------------------------------------

    \56\ Only a few northern forest types in the U.S. have been well
studied with respect to O3 exposures using the FACE
method, though these systems are being used to expose numerous other
ecosystem types to elevated levels of CO2. Additional
FACE studies with O3 on other U.S. forest types would
provide a better understanding of whether these results can be
extrapolated to other forest types and mature forest stands.
---------------------------------------------------------------------------

    Trees and other perennials, in addition to cumulating the effects
of O3 exposures over the annual growing season, can also
cumulate effects across multiple years. It has been reported that
effects can ``carry over'' from one year to another (EPA, 2006a).
Growth affected by a reduction in carbohydrate storage in one year may
result in the limitation of growth in the following year (Andersen, et
al., 1997). Carry-over effects have been documented in the growth of
some tree seedlings (Hogsett et al. 1989; Simini et al., 1992; Temple
et al., 1993) and in roots (Andersen et al., 1991; EPA, 1996a). On the
basis of past and recent OTC and field study data, ambient
O3 exposures that occur during the growing season in the
United States are sufficient to potentially affect the annual growth of
a number of sensitive seedling tree species. However, because most
studies do not take into account the possibility of carry over effects
on growth in subsequent years, the true implication of these annual
biomass losses may be missed. It is likely that under ambient exposure
conditions, some sensitive trees and perennial plants could experience
compounded impacts that result from multiple year exposures.
c. Visible Foliar Injury
    Cellular injury can and often does become visible. Acute injury
usually appears within 24 hours after exposure to O3 and,
depending on species, can occur under a range of exposures and
durations from 0.040 ppm for a period of 4 hours to 0.410 ppm for 0.5
hours for crops and 0.060 ppm for 4 hours to 0.510 ppm for 1 hour for
trees and shrubs (Jacobson, 1977). Chronic injury may be mild to
severe. In some cases, cell death or premature leaf senescence may
occur. The significance of O3 injury at the leaf and whole
plant levels depends on how much of the total leaf area of the plant
has been affected, as well as the plant's age, size, developmental
stage, and degree of functional redundancy among the existing leaf
area. As a result, it is not presently possible to determine, with
consistency across species and environments, what degree of injury at

the leaf level has significance to the vigor of the whole plant.
    The presence of visible symptoms due to O3 exposures
can, however, by itself, represent an adverse impact to the public
welfare. Specifically, it can reduce the market value of certain leafy
crops (such as spinach, lettuce), impact the aesthetic value of
ornamentals (such as petunia, geranium, and poinsettia) in urban
landscapes, and affect the aesthetic value of scenic vistas in
protected natural areas such as national parks and wilderness areas.
Many businesses rely on healthy looking vegetation for their
livelihoods (e.g., horticulturalists, landscapers, Christmas tree
growers, farmers of leafy crops) and a variety of ornamental species
have been listed as sensitive to O3 (Abt, 1993). Though not
quantified, there is likely some level of economic impact to businesses
and homeowners from O3-related injury on sensitive
ornamental species due to the cost associated with more frequent
replacement and/or increased maintenance (fertilizer or pesticide
application). In addition, because O3 not only results in
discoloration of leaves but can lead to more rapid senescence (early
shedding of leaves) there potentially could be some lost tourist
dollars at sites where fall foliage is less available or attractive.
    The use of sensitive plants as biological indicators to detect
phytotoxic levels of O3 is a longstanding and effective
methodology (Chappelka and Samuelson, 1998; Manning and Krupa, 1992).
Each bioindicator exhibits typical O3 injury symptoms when
exposed under appropriate conditions. These symptoms are considered
diagnostic as they have been verified in exposure-response studies
under experimental conditions. In recent years, field surveys of
visible foliar injury symptoms have become more common, with greater
attention to the standardization of methods and the use of reliable
indicator species (Campbell et al., 2000; Smith et al., 2003).
Specifically, the United States Forest Service (USFS) through the
Forest Health Monitoring Program (FHM) (1990-2001) and currently the
Forest Inventory and Analysis (FIA) Program collects data regarding the
incidence and severity of visible foliar injury on a variety of
O3 sensitive plant species throughout the U.S. (Coulston et
al. 2003, 2004; Smith et al. 2003).
    Since the conclusion of the 1996 NAAQS review, the FIA monitoring
program network and database has continued to expand. This network
continues to document foliar injury symptoms in the field under ambient
exposure conditions. Recent survey results show that O3-
induced foliar injury incidence is widespread across the country. The
visible foliar injury indicator has been identified as a means to track
O3 exposure stress trends in the nation's natural plant
communities as highlighted in EPA's most recent Report on the
Environment (EPA, 2003a; http://www.epa.gov/indicators/roe).

    Previous Criteria Documents have noted the difficulty in relating
visible foliar injury symptoms to other vegetation effects such as
individual tree growth, stand growth, or ecosystem characteristics
(EPA, 1996a) and this difficulty remains to the present day (EPA,
2006a). It is important to note that direct links between O3
induced visible foliar injury symptoms and other adverse effects are
not always found. Therefore, visible foliar injury cannot serve as a
reliable surrogate measure for other O3-related vegetation
effects because other effects (e.g., biomass loss) have been reported
with and without visible injury. In a few cases, visible foliar
symptoms have been correlated with decreased vegetative growth
(Karnosky et al., 1996; Peterson et al., 1987; Somers et al., 1998) and
with impaired reproductive function (Black et al., 2000; Chappelka,
2002). Therefore, the lack of visible injury should not be construed to
indicate a lack of phytotoxic concentrations of O3 nor
absence of other non-visible O3 effects.
d. Reduced Plant Vigor
    Though O3 levels over most of the U.S. are not high
enough to kill vegetation directly, current levels have been shown to
reduce the ability of many sensitive species and genotypes within
species to adapt to or withstand other environmental stresses. These
may include increased susceptibility to freezing temperatures, pest
infestations and/or root disease, and compromised ability to compete
for available resources. For example, when species with differing
O3-sensitivities occur together, the resulting decrease in
growth in O3-sensitive species may lead to an increase in
growth of more O3-tolerant species, which are now able to
better compete for available resources. The result of such above
effects can produce a loss in plant vigor in O3-sensitive
species that over time may lead to premature plant death.
e. Ecosystems
    Ecosystems are comprised of complex assemblages of organisms and
the physical environment with which they interact. Each level of
organization within an ecosystem has functional and structural
characteristics. At the ecosystem level, functional characteristics
include, but are not limited to, energy flow; nutrient, hydrologic, and
biogeochemical cycling; and maintenance of food chains. The sum of the
functions carried out by ecosystem components provides many benefits to
humankind, as in the case of forest ecosystems (Smith, 1992). Some of
these benefits, also termed ``ecosystem goods and services'', include
food, fiber production, aesthetics, genetic diversity, maintenance of
water quality, air quality, and climate, and energy exchange. A
conceptual framework for discussing the effects of O3 on
ecosystems was developed by the EPA Science Advisory Board (Young and
Sanzone, 2002). In this report, the authors identify six essential
ecological attributes (EEAs) include landscape condition, biotic
condition, chemical/physical condition, ecological processes,
hydrology/geomorphology, and natural disturbance regime. Each EEA is
depicted as one of six triangles that together build a hexagon. On the
outside of each triangle is a list of stressors that can act on the
EEA. Tropospheric O3 is listed as a stressor of both biotic
condition and the chemical/physical condition of ecosystems. As each
EEA is linked to all the others, it is clearly envisioned in this
framework that O3 could either directly or indirectly impact
all of the EEAs associated with an ecosystem that is being stressed by
O3.
    Vegetation often plays an influential role in defining the
structure and function of an ecosystem, as evidenced by the use of
dominant vegetation forms to classify many types of natural ecosystems,
e.g., tundra, wetland, deciduous forest, and conifer forest. Plants
simultaneously inhabit both above- and below-ground environments,
integrating and influencing key ecosystem cycles of energy, water, and
nutrients. When a sufficient number of individual plants within a
community have been affected, O3-related effects can be
propagated up to ecosystem-level effects. Thus, through its impact on
vegetation, O3 can be an important ecosystem stressor.
i. Potential Ozone Alteration of Ecosystem Structure and Function
    The Criteria Document (EPA, 2006a) outlines seven case studies
where O3 effects on ecosystems have either been documented
or are suspected. The oldest and clearest example involves the San
Bernardino Mountain forest ecosystem in California. This system
experienced chronic high O3 exposures over a period of 50 or
more years. The

O3-sensitive and co-dominant species of ponderosa and
Jeffrey pine demonstrated severe levels of foliar injury, premature
senescence, and needle fall that decreased the photosynthetic capacity
of stressed pines and reduced the production of carbohydrates resulting
in a decrease in radial growth and in the height of stressed trees. It
was also observed that ponderosa and Jeffrey pines with slight to
severe crown injury lost basal area in relation to competing species
that are more tolerant to O3. Due to a loss of vigor, these
trees eventually succumbed to the bark beetle, leading to elevated
levels of tree death. Increased mortality of susceptible trees shifted
the community composition towards white fir and incense cedar,
effectively reversing the development of the normal fire climax mixture
dominated by ponderosa and Jeffrey pines, and leading to increased fire
susceptibility. At the same time, numerous other organisms and
processes were also affected either directly or indirectly, including
successional patterns of fungal microflora and their relationship to
the decomposer community. Nutrient availability was influenced by the
heavy litter and thick needle layer under stands with the most severe
needle injury and defoliation. The composition of lichens was
significantly reduced. In this example, O3 appeared to be a
predisposing factor that led to increased drought stress, windthrow,
root diseases, and insect infestation (Takemoto et al., 2001). Thus,
through its effects on tree water balance, cold hardiness, tolerance to
wind, and susceptibility to insect and disease pests, O3
potentially impacted the ecosystem-related EEA of natural disturbance
regime (e.g., fire, erosion). Although the role of O3 was
extremely difficult to separate from other confounding factors, such as
high nitrogen deposition, there is evidence that this shift in species
composition has altered the structure and dynamics of associated food
webs (Pronos et al., 1999) and carbon (C) and nitrogen (N) cycling
(Arbaugh et al., 2003). Ongoing and new research in this important
ecosystem is needed to reveal the extent to which ecosystem services
have been affected and to what extent strong causal linkages between
historic and/or current ambient O3 exposures and observed
ecosystem-level effects can be made.
    Ozone has also been reported to be a selective pressure among
sensitive tree species (e.g., eastern white pine) in the east. The
nature of community dynamics in eastern forests is different, however,
than in the west, consisting of a wider diversity of species and uneven
aged stands, and the O3 levels are less severe. Therefore,
lower level chronic O3 stress in the east is more likely to
produce subtle long-term forest responses such as shifts in species
composition, rather than wide-spread community degradation.
    One of the best-documented studies of population and community
response to O3 effects are the long-term studies of common
plantain (Plantago major) in native plant communities in the United
Kingdom (Davison and Reiling, 1995; Lyons et al., 1997; Reiling and
Davison, 1992c). Elevated O3 significantly decreased the
growth of sensitive populations of common plantain (Pearson et al.,
1996; Reiling and Davison, 1992a, b; Whitfield et al., 1997) and
reduced its fitness as determined by decreased reproductive success
(Pearson et al., 1996; Reiling and Davison, 1992a). While spatial
comparisons of population responses to O3 are complicated by
other environmental factors, rapid changes in O3 resistance
were imposed by ambient levels and variations in O3 exposure
(Davison and Reiling, 1995). Specifically, in this case study, it
appeared that O3-sensitive individuals are being removed by
O3 stress and the genetic variation represented in the
population could be declining. If genetic diversity and variation is
lost in ecosystems, there may be increased vulnerability of the system
to other biotic and abiotic stressors, and ultimately a change in the
EEAs and associated services provided by those ecosystems.
    Recent free-air exposure experiments have also provided new insight
into how O3 may be altering ecosystem structure and function
(Karnosky et al., 2005). For example, a field O3 exposure
experiment at the AspenFACE site in Wisconsin (described in section
IV.A.2.b. above) was designed to examine the effects of both elevated
CO2 and O3 on mixed stands of aspen (Populus
tremuloides), birch (Betula papyrifera), and sugar maple (Acer
saccharum) that are characteristic of Great Lakes aspen-dominated
forests (Karnosky et al., 2003; Karnosky et al., 1999). They found
evidence that the effects on above- and below-ground growth and
physiological processes have cascaded through the ecosystem, even
affecting microbial communities (Larson et al., 2002; Phillips et al.,
2002). This study also confirmed earlier observations of O3-
induced changes in trophic interactions involving keystone tree
species, as well as important insect pests and their natural enemies
(Awmack et al., 2004; Holton et al., 2003; Percy et al., 2002).
    Collectively these examples suggest that O3 is an
important stressor in natural ecosystems, but it is difficult to
quantify the contribution of O3 due to the combination of
other stresses present in ecosystems. In most cases, because only a few
components in each of these ecosystems have been examined and
characterized for O3 effects, the full extent of ecosystem
changes in these example ecosystems is not fully understood. Clearly,
there is a need for highly integrated ecosystem studies that
specifically investigate the effect of O3 on ecosystem
structure and function in order to fully determine the extent to which
O3 is altering ecosystem services. Continued research,
employing new approaches, will be necessary to fully understand the
extent to which O3 is affecting ecosystem services.
ii. Effects on Ecosystem Services and Carbon Sequestration
    Since it has been established that O3 affects
photosynthesis and growth of plants, O3 is most likely
affecting the productivity of forest ecosystems. Therefore, it is
desirable to link effects on growth and productivity to essential
ecosystem services. However, it is very difficult to quantify
ecosystem-level productivity losses because of the amount of complexity
in scaling from the leaf-level or individual plant to the ecosystem
level, and because not all organisms in an ecosystem are equally
affected by O3.
    Terrestrial ecosystems are important in the Earth's carbon (C)
balance and could help offset emissions of CO2 by humans if
anthropogenic C is sequestered in vegetation and soils. The annual
increase in atmospheric CO2 is less than the total inputs
from fossil fuel burning and land use changes (Prentice et al., 2001)
and much of this discrepancy is thought to be attributable to
CO2 uptake by plant photosynthesis (Tans & White, 1998).
Temperate forests of the northern hemisphere have been estimated to be
a net sink of C per year (Goodale et al. 2002). Ozone interferes with
photosynthesis, causes some plants to senesce leaves prematurely and in
some cases, reduces allocation to stem and root tissue. Thus,
O3 decreases the potential for C sequestration. For the
purposes of this discussion, C sequestration is defined as the net
exchange of carbon by the terrestrial biosphere. However, long-term
storage in the soil organic matter is considered to be the most stable
form of C storage in ecosystems.
    In a study including all ecosystem types, Felzer et al. (2004),
estimated that

U.S. net primary production (net flux of C into an ecosystem) was
decreased by 2.6-6.8 percent due to O3 pollution in the late
1980's to early 1990's. Ozone not only reduces C sequestration in
existing forests, it can also affect reforestation projects (Beedlow et
al. 2004). This effect, in turn, has been found to ultimately inhibit C
sequestration in forest soils which act as long-term C storage (Loya et
al., 2003; Beedlow et al. 2004). The interaction of rising
O3 pollution and rising CO2 concentrations in the
coming decades complicates predictions of future sequestration
potential. Models generally predict that, in the future, C
sequestration will increase with increasing CO2, but often
do not account for the decrease in productivity due to the local
effects of tropospheric O3. In the presence of high
O3 levels, the stimulatory effect of rising CO2
concentrations on forest productivity has been estimated to be reduced
by more that 20 percent (Tingey et al., 2001; Ollinger et al. 2002;
Karnosky et al., 2003).
    In summary, it would be anticipated that meeting lower
O3 standards would increase the amount of CO2
uptake by many ecosystems in the U.S. However, the amount of this
improvement would be heavily dependent on the species composition of
those ecosystems. Many ecosystems in the U.S. do have O3
sensitive plants. For, example forest ecosystems with dominant species
such as aspen or ponderosa pine would be expected to increase
CO2 uptake more with lower O3 than forests with
more O3 tolerant species.
    A recent critique of the secondary NAAQS review process published
in the report by the National Academy of Sciences on Air Quality
Management in the United States (NRC, 2004) stated that ``EPA's current
practice for setting secondary standards for most criteria pollutants
does not appear to be sufficiently protective of sensitive crops and
ecosystems * * *.'' This report made several specific recommendations
for improving the secondary NAAQS process and concluded that ``There is
growing evidence that tighter standards to protect sensitive ecosystems
in the United States are needed * * *.'' An effort has been recently
initiated within the Agency to identify indicators of ecological
condition whose responses can be clearly linked to changes in air
quality that are attributable to Agency environmental programs. Using a
single indicator to represent the complex linkages and dynamic cycles
that define ecosystem condition will always have limitations. With
respect to O3-related impacts on ecosystem condition, only
two candidate indicators, foliar injury (as described above) and radial
growth in trees, have been suggested. Thus, while at the present time,
most O3-related effects on ecosystems must be inferred from
observed or predicted O3-related effects on individual
plants, additional research at the ecosystem level could identify new
indicators and/or establish stronger causal linkages between
O3-induced plant effects and ecosystem condition.
f. Yield Reductions in Crops
    Ozone can interfere with carbon gain (photosynthesis) and
allocation of carbon with or without the presence of visible foliar
injury. As a result of decreased carbohydrate availability, fewer
carbohydrates are available for plant growth, reproduction, and/or
yield. Recent studies have further confirmed and demonstrated
O3 effects on different stages of plant reproduction,
including pollen germination, pollen tube growth, fertilization, and
abortion of reproductive structures, as reviewed by Black et al.
(2000). For seed-bearing plants, these reproductive effects will
culminate in reduced seed production or yield.
    As described in the last review and again in the current Criteria
Document and Staff Paper, the National Crop Loss Assessment Network
(NCLAN) studies undertaken in the early to mid-1980's provide the
largest, most uniform database on the effects of O3 on
agricultural crop yields. The NCLAN protocol was designed to produce
crop exposure-response data representative of the areas in the U.S.
where the crops were typically grown. In total, 15 species (e.g., corn,
soybean, winter wheat, tobacco, sorghum, cotton, barley, peanuts, dry
beans, potato, lettuce, turnip, and hay [alfalfa, clover, and fescue]),
accounting for greater than 85 percent of U.S. agricultural acreage
planted at that time, were studied. Of these 15 species, 13 species
including 38 different cultivars were combined in 54 cases representing
unique combinations of cultivars, sites, water regimes, and exposure
conditions. Crops were grown under typical farm conditions and exposed
in open-top chambers to ambient O3 and increased
O3 above ambient (i.e., modified ambient). Robust C-R
functions were developed for each of these crop species. These results
showed that 50 percent of the studied cases would be protected from
greater than 10 percent yield loss at a W126 level of 21 ppm-hour,
while a W126 of 13 ppm-hour would provide protection for 75 percent of
the cases studied from greater than 10 percent yield loss.
    Recent studies continue to find yield loss levels in crop species
studied previously under NCLAN that reflect the earlier findings\57\.
In other words, there has been no evidence that crops are becoming more
tolerant of O3 (EPA, 2006a). For cotton, some newer
varieties have been found to have higher yield loss due to
O3 compared to older varieties (Olszyk et al., 1993, Grantz
and McCool, 1992). In a meta-analysis of 53 studies, Morgan et al.
(2003) found consistent deleterious effects of O3 exposures
on soybean from studies published between 1973 and 2001. Further, early
results from the field-based exposure experiment SoyFACE in Illinois
indicate a lack of any apparent difference in the O3
tolerance of old and recent cultivars of soybean in a study of 22
soybean varieties (Long et al., 2002). Thus, the Staff Paper concluded
that the recent scientific literature continues to support the
conclusions of the 1996 Criteria Document that ambient O3
concentrations are reducing the yield of major crops in the U.S.
---------------------------------------------------------------------------

    \57\ Given the usefulness of generating robust C-R functions
such as have been developed under NCLAN, it would be beneficial to
employ a similar protocol to update and expand this research to
include more recent and additional crop species and varieties, such
as fruit and vegetable species, as well as recent O3 air
quality.
---------------------------------------------------------------------------

    In addition to the effects described on annual crop species,
several studies published since the last review have focused on
perennial forage crops (EPA, 2006a). These recent results confirm that
O3 is also impacting yields and quality of multiple-year
forage crops at sufficient magnitude to have nutritional and possibly
economic implications to their use as ruminant animal feed at
O3 exposures that occur in some years over large areas of
the U.S.
3. Adversity of Effects
    The Staff Paper recognized that the statute requires that a
secondary standard be protective against ``adverse'' O3
effects, not all identifiable effects. In considering what constitutes
a vegetation effect that is adverse to the public welfare, the Staff
Paper recognizes that O3 can cause a variety of vegetation
effects, beginning at the level of the individual cell and accumulating
up to the level of whole leaves, plants, plant populations, communities
and whole ecosystems, not all of which have been classified in past
reviews as ``adverse'' to public welfare.
    Previous reviews have classified O3 vegetation effects
as either ``injury'' or ``damage'' to help in determining adversity.
Specifically, ``injury'' is

defined as encompassing all plant reactions, including reversible
changes or changes in plant metabolism (e.g., altered photosynthetic
rate), altered plant quality, or reduced growth, that does not impair
the intended use or value of the plant (Guderian, 1977). In contrast,
``damage'' has been defined to include those injury effects that reach
sufficient magnitude as to also reduce or impair the intended use or
value of the plant. Examples of effects that are classified as damage
include reductions in aesthetic values (e.g., foliar injury in
ornamental species) as well as losses in terms of weight, number, or
size of the plant part that is harvested (reduced yield or biomass
production). Yield loss also may include changes in crop quality, i.e.,
physical appearance, chemical composition, or the ability to withstand
storage, while biomass loss includes slower growth in species harvested
for timber or other fiber uses. While this construct has proved useful
in the past, it appears to be most useful in the context of evaluating
effects on single plants or species grown in monocultures such as
agricultural crops or managed forests. It is less clear how it might
apply to potential effects on natural forests or entire ecosystems when
O3-induced species level impacts lead to shifts in species
composition and/or associated ecosystem services such as nutrient
cycling or hydrologic cycles, where the intended use or value of the
system has not been specifically identified.
    A more recent construct for assessing risks to forests described in
Hogsett et al. (1997) suggests that ``adverse effects could be
classified into one or more of the following categories: (1) Economic
production, (2) ecological structure, (3) genetic resources, and (4)
cultural values.'' This approach expands the context for evaluating the
adversity of O3-related effects beyond the species level.
Another recent publication, A Framework for Assessing and Reporting on
Ecological Condition: an SAB report (Young and Sanzone, 2002), provides
additional support for expanding the consideration of adversity beyond
the species level by making explicit the linkages between stress-
related effects (e.g., O3 exposure) at the species level and
at higher levels within an ecosystem hierarchy. Taking this recent
literature into account, the Staff Paper concludes that a determination
of what constitutes an ``adverse'' welfare effect in the context of the
secondary NAAQS review can appropriately occur within this broader
paradigm.

B. Biologically Relevant Exposure Indices

    The Criteria Document concluded that O3 exposure indices
that cumulate differentially weighted hourly concentrations are the
best candidates for relating exposure to plant growth responses (EPA,
2006a). This conclusion follows from the extensive evaluation of the
relevant studies in the 1996 Criteria Document (EPA, 1996a) and the
recent evaluation of studies that have been published since that time
(EPA, 2006a). The following selections, taken from section 5.5 the 1996
Criteria Document (EPA, 1996a), further elucidate the depth and
strength of these conclusions. Specifically, with respect to the
importance of taking into account exposure duration, the 1996 Criteria
Document stated, ``when O3 effects are the primary cause of
variation in plant response, plants from replicate studies of varying
duration showed greater reductions in yield or growth when exposed for
the longer duration'' and ``the mean exposure index of unspecified
duration could not account for the year-to-year variation in response''
(EPA, 1996a, pg. 5-96). Further, ``because the mean exposure index
treats all concentrations equally and does not specifically include an
exposure duration component, the use of a mean exposure index for
characterizing plant exposures appears inappropriate for relating
exposure with vegetation effects'' (EPA, 1996a, pg. 5-88). Regarding
the relative importance of higher concentrations than lower in
determining plant response, the 1996 Criteria Document concluded that
``the ultimate impact of long-term exposures to O3 on crops
and seedling biomass response depends on the integration of repeated
peak concentrations during the growth of the plant'' (EPA, 1996a, pg.
5-104). Further, ``at this time, exposure indices that weight the
hourly O3 concentrations differentially appear to be the
best candidates for relating exposure with predicted plant response''
(EPA, 1996a, pgs. 5-136).
    At the conclusion of the last review, the biological basis for a
cumulative, seasonal form was not in dispute. There was general
agreement between the EPA staff, CASAC, and the Administrator, based on
their review of the air quality criteria, that a cumulative, seasonal
form was more biologically based than the then current 1-hour and newly
proposed 8-hour average form. However, in selecting a specific form
appropriate for a secondary standard, there was less agreement. An
evaluation of the performance of several seasonal cumulative forms in
predicting plant response data taken from OTC experiments had found
that all performed about equally well and was unable to distinguish
between them (EPA, 1996a). In selecting between two of these cumulative
forms, the SUM06 \58\ and W126, in the absence of biological evidence
to distinguish between them, the Administrator based her decision on
both science and policy considerations. Specifically, these were: (1)
All cumulative, peak-weighted exposure indices considered, including
W126 and SUM06, were about equally good as exposure measures to predict
exposure-response relationships reported in the NCLAN crop studies; and
(2) the SUM06 form would not be influenced by PRB O3
concentrations (defined at the time as 0.03 to 0.05 ppm) under many
typical air quality distributions. On the basis of these
considerations, the Administrator chose the SUM06 as the most
appropriate cumulative, seasonal form to consider when proposing an
alternative secondary standard form (61 FR 65716).
---------------------------------------------------------------------------

    \58\ SUM06: Sum of all hourly O3 concentrations
greater or equal to 0.06 ppm over a specified time.
---------------------------------------------------------------------------

    Though the scientific justification for a cumulative, seasonal form
was generally accepted in the last review, an analysis undertaken by
EPA at that time had shown that there was considerable overlap between
areas that would be expected not to meet the range of alternative 8-
hour standards being considered for the primary NAAQS and those
expected not to meet the range of values (expressed in terms of the
seasonal SUM06 index) of concern for vegetation. This result suggested
that improvements in national air quality expected to result from
attaining an 8-hour primary standard within the recommended range of
levels would also be expected to significantly reduce levels of concern
for vegetation in those same areas. Thus, in the notice of proposed
rulemaking, the Administrator proposed two alternatives for
consideration: one alternative was to make the secondary standard equal
in every way to the proposed 8-hour, 0.08 ppm primary standard; and the
second was to establish a cumulative, seasonal secondary standard in
terms of a SUM06 form as also appropriate to protect public welfare
from known or anticipated adverse effects given the available
scientific knowledge and that such a seasonal standard ``* * * is more
biologically relevant * * *'' (61 FR 65716).
    In the 1997 final rule, the Administrator decided to make the
secondary standard identical to the primary standard. She acknowledged,

however, that ``it remained uncertain as to the extent to which air
quality improvements designed to reduce 8-hr average O3
concentrations averaged over a 3-year period would reduce O3
exposures measured by a seasonal SUM06 index.'' (62 FR 38876) In other
words, it was uncertain as to whether the 8-hour average form would, in
practice, provide sufficient protection for vegetation from the
cumulative, seasonal and concentration-weighted exposures described in
the scientific literature as of concern.
    On the basis of that history, chapter 7 of the current Staff Paper
revisited the issue of whether the SUM06 was still the most appropriate
choice of cumulative, seasonal form for a secondary standard to protect
the public welfare from known and anticipated adverse vegetation
effects in light of the new information available in this review.
Specifically, the Staff Paper considered: (1) The continued lack of
evidence within the vegetation effects literature of a biological
threshold for vegetation exposures of concern; and (2) new estimates of
PRB that are lower than in the last review. The W126 form, also
evaluated in the last review, was again selected for comparison with
the SUM06 form. Regarding the first consideration, the Staff Paper
noted that W126 form, by its incorporation of a continuous sigmoidal
weighting scheme, does not create an artificially imposed concentration
threshold, yet also gives proportionally more weight to the higher and
typically more biologically potent concentrations, as supported by the
scientific evidence. Second, the index value is not significantly
influenced by O3 concentrations within the range of
estimated PRB, as the weights assigned to concentrations in this range
are very small. Thus, it would also provide a more appropriate target
for air quality management programs designed to reduce emissions from
anthropogenic sources contributing to O3 formation. On the
basis of these considerations, the Staff Paper concludes that the W126
form is the most biologically-relevant cumulative, seasonal form
appropriate to consider in the context of the secondary standard
review.

C. Vegetation Exposure and Impact Assessment

    The vegetation exposure and impact assessment conducted for the
current review and described in the Staff paper, consisted of exposure,
risk and benefits analyses and improves and builds upon similar
analyses performed in the last review (EPA 1996b). The vegetation
exposure assessment was performed using interpolation and included
information from ambient monitoring networks and results from air
quality modeling. The vegetation risk assessment included both tree and
crop analyses. The tree risk analysis includes three distinct lines of
evidence: (1) Observations of visible foliar injury in the field linked
to recent monitored O3 air quality for the years 2001-2004;
(2) estimates of seedling growth loss under current and alternative
O3 exposure conditions; and (3) simulated mature tree growth
reductions using the TREGRO model to simulate the effect of meeting
alternative air quality standards on the predicted annual growth of a
single western species (ponderosa pine) and two eastern species (red
maple and tulip poplar). The crop analysis includes estimates of the
risks to crop yields from current and alternative O3
exposure conditions and the associated change in economic benefits
expected to accrue in the agriculture sector upon meeting the levels of
various alternative standards. Each element of the assessment is
described below, including discussions of known sources and ranges of
uncertainties associated with the elements of this assessment.
1. Exposure Characterization
    Though numerous effects of O3 on vegetation have been
documented as discussed above, it is important in considering risk to
examine O3 air quality patterns in the U.S. relative to the
location of O3 sensitive species that have a known
concentration-response in order to predict whether adverse effects are
occurring at current levels of air quality, and whether they are likely
to occur under alternative standard forms and levels.
    The most important information about exposure to vegetation comes
from the O3 monitoring data that are available from two
national networks: (1) Air Quality System (AQS; http://www.epa.gov/ttn/airs/airsaqs
) and (2) Clean Air Status and Trends Network (CASTNET;

http://www.epa.gov/castnet/). The AQS monitoring network currently has

over 1100 active O3 monitors which are generally sited near
population centers. However, this network also includes approximately
36 monitors located in national parks. CASTNET is the nation's primary
source for data on dry acidic deposition and rural, ground-level
O3. It consists of over 80 sites across the eastern and
western U.S. and is cooperatively operated and funded with the National
Park Service. In the 1997 final O3 rule, it was acknowledged
that because the national air quality surveillance network for
O3 was designed principally to monitor O3
exposure in populated areas, there was limited measured data available
to characterize O3 air quality in rural and remote sites.
Since the last review, there has been a small increase in the number of
CASTNET sites (from approximately 52 sites in 1992 to 84 sites in
2004), however these monitors are not used for attainment designations.
    National parks represent areas of nationally recognized ecological
and public welfare significance, which are afforded a higher level of
protection. Two recent reports presented some discussion of
O3 trends in a subset of national parks: The Ozone Report:
Measuring Progress Through 2003 (EPA, 2004), and 2005 Annual
Performance and Progress Report: Air Quality in National Parks (NPS,
2005). Unfortunately, much of this information is presented only in
terms of the current 8-hr average form. The Staff Paper analyzed
available air quality data in terms of the cumulative 12-hour W126 form
from 2001 to 2005 for a subset of national parks and other significant
natural areas representing 4 general regions of the U.S. Many of these
national parks and natural areas have monitored O3 levels
above concentrations that have been shown to decrease plant growth and
above the 12-hour W126 levels analyzed in this review. For example, the
Great Smokey Mountain, Rocky Mountain, Grand Canyon, Yosemite and
Sequoia National Parks all had more than one year within the 2001-2005
period with a 12-hour W126 above 21 ppm-hour. This level of exposure
has been associated with approximately no more than 10 percent biomass
loss in 50 percent of the 49 tree seedling cases studied in the NHEERL-
WED experiments (Lee and Hogsett, 1996). Black cherry (Prunus
serotina), an important O3-sensitive tree species in the
eastern U.S., occurs in the Great Smoky Mountain National Park and is
estimated to have O3-related seedling biomass loss of
approximately 40 percent when exposed to a 3-month, 12-hour W126
O3 level greater than 21 ppm-hour. Ponderosa pine (Pinus
ponderosa) which occurs in the Grand Canyon, Yosemite and Sequoia
National Parks has been reported to have approximately 10 percent
biomass losses at 3-month, 12-hour W126 O3 levels as low as
17 ppm-hour (Lee and Hogsett, 1996). Impacts on seedlings may
potentially affect long-term tree growth and survival, ultimately
affecting the competitiveness of O3-sensitive tree species
and genotypes within forest stands.
    In order to characterize exposures to vegetation at the national
scale,

however, the Staff Paper concluded that it could not rely solely on
limited site-specific monitoring data, and that it was necessary to
select an interpolation method that could be used to characterize
O3 air quality over broad geographic areas. The Staff Paper
therefore investigated the appropriateness of using the O3
outputs from the EPA/NOAA Community Multi-scale Air Quality (CMAQ) \59\
model system (http://www.epa.gov/asmdnerl/CMAQ, Byun and Ching, 1999;

Arnold et al. 2003, Eder and Yu, 2005) to improve spatial
interpolations based solely on existing monitoring networks. Due to the
significant resources required to run CMAQ, model outputs were only
available for a limited number of years. For this review, 2001 outputs
from CMAQ version 4.5 were the most recent available.
---------------------------------------------------------------------------

    \59\ The CMAQ model is a multi-pollutant, multiscale air quality
model that contains state-of-the-science techniques for simulating
all atmospheric and land processes that affect the transport,
transformation, and deposition of atmospheric pollutants and/or
their precursors on both regional and urban scales. It is designed
as a science-based modeling tool for handling many major pollutants
(including photochemical oxidants/O3, particulate matter,
and nutrient deposition) holistically. The CMAQ model can generate
estimates of hourly O3 concentrations for the contiguous
U.S., making it possible to express model outputs in terms of a
variety of exposure indices (e.g., W126, 8-hour average).
---------------------------------------------------------------------------

    Based on the significant difference in monitor network density
between the eastern and western U.S., the Staff Paper concluded that it
was appropriate to use separate interpolation techniques in these two
regions. AQS and CASTNET monitoring data were solely used for the
eastern interpolation since it was determined that enhancing the
interpolation with CMAQ data did not add much information to the
eastern U.S. interpolation. In the western U.S., where rural monitoring
is more sparse, O3 values generated by the CMAQ model were
used to develop scaling factors to augment the interpolation.
    In order to characterize uncertainty in the interpolation method,
monitored O3 concentrations were systematically compared to
interpolated O3 concentrations in areas where monitors were
located. In general, the interpolation method used in the current
review performed well in many areas in the U.S., although it under-
predicted higher 12-hour W126 exposures in rural areas. Due to the
important influence of higher exposures in determining risks to plants,
this feature of the interpolated surface could result in an under-
estimation of risks to vegetation in some areas. Taking these
uncertainties into account, and given the absence of more complete
rural monitoring data, this approach was used in developing national
vegetation exposure and risk assessments that estimate relative changes
in risk for the various alternative standards analyzed.
    To evaluate changing vegetation exposures and risks under selected
air quality scenarios, the Staff Paper utilized adjusted 2001 base year
O3 air quality distributions with a rollback method (Horst
and Duff, 1995; Rizzo, 2005 & 2006) to reflect meeting the current and
alternative secondary standard options. This technique combines both
linear and quadratic elements to reduce higher O3
concentrations more than lower ones. In this regard, the rollback
method attempts to account for reductions in emissions without greatly
affecting lower concentrations. The following O3 air quality
scenarios were analyzed: (1) 4th-highest daily maximum 8-hour average:
0.084 ppm (the effective level of the current standard) and 0.070 ppm
levels; (2) 3-month, 12-hour. SUM06: 25 ppm-hour (proposed in the 1996
review) and 15 ppm-hour levels; and (3) 3-month, 12-hour. W126: 21 ppm-
hour and 13 ppm-hour levels.
    The two 8-hour average levels were chosen as possible alternatives
of the current form for comparison with the cumulative, seasonal
alternative forms. The SUM06 scenarios were very similar to the W126
scenarios. Since the W126 was judged to be the more biologically-
relevant cumulative, seasonal form, only the results for the W126
scenarios are summarized below. For the W126 form, the two levels were
selected on the basis of the associated levels of tree seedling biomass
loss and crop yield loss protection identified in the NHEERL-WED and
NCLAN studies, respectively. Specifically, the upper level of W126 (21
ppm-hour) was associated with a level of tree and crop protection of
approximately no more than 10 percent growth or yield loss in 50
percent of cases studied. Alternatively, the lower level of W126 (13
ppm-hour) was associated with a level of tree seedling and crop
protection of approximately no more than 10 percent growth or yield
loss in 75 percent of studied cases.
    The following discussion highlights key observations drawn from
comparing predicted changes in interpolated air quality under each
alternative standard form and level scenario for the base year, 2001:
    (1) Under the base year (2001) ``as is'' air quality, a large
portion of California had 12-hr W126 O3 levels above 31 ppm-
hour, which has been associated with approximately no more than 14
percent biomass loss in 50 percent of tree seedling cases studies.
Broader multi-state regions in the east (NC, TN, KY, IN, OH, PA, NJ,
NY, DE, MD, VA) and west (CA, NV, AZ, OK, TX) are predicted to have
levels of air quality above the W126 level of 21 ppm-hour, which is
approximately equal to the secondary standard proposed in 1996 and is
associated with approximately no more than 10 percent biomass loss in
50 percent of tree seedling cases studied. Much of the east and Arizona
and California have 12-hour W126 O3 levels above 13 ppm-hour
which has been associated with approximately no more than 10 percent
biomass loss in 75 percent of tree seedling cases studied. The results
of the exposure assessment indicate that current air quality levels
could result in significant impacts to vegetation in some areas.
    (2) When 2001 air quality is rolled back to meet the current 8-hour
secondary standard, the overall 3-month 12-hour W126 O3
levels were somewhat improved, but not substantially. Under this
scenario, there were still many areas in California with 12-hour W126
O3 levels above 31 ppm-hour. A broad multi-state region in
the east (NC, TN, KY, IN, OH, PA, MD) and west (CA, NV, AZ, OK, TX)
were still predicted to have O3 levels above the W126 level
of 21 ppm-hour.
    (3) Exposures generated for just meeting a 0.070 ppm, 4th-highest
maximum 8-hour average alternative standard showed substantially
improved O3 air quality when compared to just meeting the
current 0.08 ppm, 8-hour standard. Most areas were predicted to have
O3 levels below the W126 level of 21 ppm-hr, although some
areas in the east (KY, TN, MI, AR, MO, IL) and west (CA, NV, AZ, UT,
NM, CO, OK, TX) were still predicted to have O3 levels above
the W126 level of 13 ppm-hour.
    These results suggest that meeting a proposed 0.070 ppm, 8-hour
secondary standard would provide substantially improved protection in
some areas for vegetation from seasonal O3 exposures of
concern. The Staff Paper recognizes, however, that some areas meeting a
0.070 ppm 8-hour standard could continue to have elevated seasonal
exposures, including forested park lands and other natural areas, and
Class I areas which are federally mandated to preserve certain air
quality related values. This is especially important in the high
elevation forests in the Western U.S. where there are few O3
monitors. This is because the air quality patterns in remote areas can
result in relatively low 8-hour averages while still experiencing
relatively high cumulative exposures.

    To further characterize O3 air quality in terms of
current and alternative secondary standard forms, an analysis was
performed in the Staff Paper to evaluate the extent to which county-
level O3 air quality measured in terms of various levels of
the current 8-hour average form overlapped with that measured in terms
of various levels of the 12-hour W126 cumulative, seasonal form. The
Staff Paper presented this analysis using recent (2002-2004) \60\
county-level O3 air quality data from AQS sites and the
subset of CASTNET sites having the highest O3 levels for the
counties in which they are located. Since the current 8-hour average
secondary form is a 3-year average, the analysis initially compared the
3-year averages of both the 8-hour and W126 forms. In addition,
recognizing that some vegetation effects (e.g. crop yield loss and
foliar injury) are driven solely by annual O3 exposures and
are typically evaluated with respect to exposures within the annual
growing season, the Staff Paper also presented a comparison of the
current 3-year average 8-hour form to the annual W126 form for the
individual years, 2002 and 2004.
---------------------------------------------------------------------------

    \60\ This analysis was updated using 2003-2005 air quality as it
became available, finding similar results.
---------------------------------------------------------------------------

    Results of the 3-year average comparisons showed that of the
counties with air quality meeting the 3-year average form of the 0.08
ppm, 8-hour average standard, 7 counties showed 3-year average W126
values above the 21 ppm-hour level. At the lower W126 level of 13 ppm-
hours, 135 counties with air quality meeting the 3-year average form of
the 0.08 ppm, 8-hour average standard, would be above this W126 level.
In addition, when the 3-year average of the 8-hour form was compared to
annual W126 values, further variability in the degree of overlap
between the 8-hour form and W126 form became apparent. For example, the
relatively high 2002 O3 air quality year showed a greater
degree of overlap between those areas that would meet the levels
analyzed for the current 8-hour and alternative levels of the W126 form
than did the relatively low O3 2004 air quality year. This
lack of a consistent degree of overlap between the two forms in
different air quality years demonstrates that annual vegetation would
be expected to receive widely differing degrees of protection from
cumulative seasonal exposures in some areas from year to year, even
when the 3-year average of the 8-hour form was consistently met.
    It is clear that this analysis is limited by the lack of monitoring
in rural areas where important vegetation and ecosystems are located,
especially at higher elevation sites. This is because O3 air
quality distributions at high elevation sites often do not reflect the
typical urban and near-urban pattern of low morning and evening
O3 concentrations with a high mid-day peak, but instead
maintain relatively flat patterns with many concentrations in the mid-
range (e.g., 0.05-0.09 ppm) for extended periods. These conditions can
lead to relatively low daily maximum 8-hour averages concurrently with
high cumulative values so that there is potentially less overlap
between an 8-hour average and a cumulative, seasonal form at these
sites. The Staff Paper concludes that it is reasonable to anticipate
that additional unmonitored rural high elevation areas important for
vegetation may not be adequately protected even with a lower level of
the 8-hour form.
    The Criteria Document (EPA, 2006a), discusses policy relevant
background (PRB) levels for high elevation sites and makes the
following observations: (1) PRB concentrations of 0.04 to 0.05 ppm
occur occasionally at high-elevation sites (e.g., >1.5 km) in the
spring due to the free-tropospheric influence, including some limited
contribution from hemispheric pollution (O3 produced from
anthropogenic emissions outside North America); and (2) stratospheric
intrusions might occasionally elevate O3 at high-altitude
sites, however, these events are rare. Therefore, the Staff Paper
concludes that springtime PRB levels in the range identified above and
rare stratospheric intrusions of O3 are unlikely to
influence 3 month cumulative seasonal W126 values significantly.
    It further remains uncertain as to the extent to which air quality
improvements designed to reduce 8-hour O3 average
concentrations would reduce O3 exposures measured by a
seasonal, cumulative W126 index. The Staff Paper indicated this to be
an important consideration because: (1) The biological database
stresses the importance of cumulative, seasonal exposures in
determining plant response; (2) plants have not been specifically
tested for the importance of daily maximum 8-hour O3
concentrations in relation to plant response; and (3) the effects of
attainment of an 8-hour standard in upwind urban areas on rural air
quality distributions cannot be characterized with confidence due to
the lack of monitoring data in rural and remote areas. These factors
are important considerations in determining whether the current 8-hour
form can appropriately provide requisite protection for vegetation.
2. Assessment of Risk to Vegetation
    The Staff Paper presents results from quantitative and qualitative
risk assessments of O3 risks to vegetation (EPA, 2007). In
the last review, crop yield and seedling biomass loss OTC data provided
the basis for staff analyses, conclusions, and recommendations (EPA,
1996b). Since then, several additional lines of evidence have
progressed sufficiently to provide staff with a more complete and
coherent picture of the scope of O3-related vegetation
risks, especially those currently faced by seedling, sapling and mature
tree species growing in field settings, and indirectly, forested
ecosystems. Specifically, new research reflects an increased emphasis
on field-based exposure methods (e.g., free air exposure and ambient
gradient), improved field survey biomonitoring techniques, and
mechanistic tree process models. Findings from each of these research
areas are discussed separately below. In conducting these assessments,
the Staff Paper analyses relied on both measured and modeled air
quality information. For some effects, like visible foliar injury and
modeled mature tree growth response, only monitored air quality
information was used. For other effects categories (e.g., crop yield
and tree seedling growth), staff relied on interpolated O3
exposures.
a. Visible Foliar Injury
    As discussed earlier (Section A), recent systematic injury surveys
continue to document visible foliar injury symptoms diagnostic of
phytotoxic O3 exposures on sensitive bioindicator plants.
These surveys produced more expansive evidence than that available at
the time of the last review that visible foliar injury is occurring in
many areas of the U.S. under current ambient conditions. The Staff
Paper presents an assessment combining recent U.S. Forest Service
Forest Inventory and Analysis (FIA) biomonitoring site data with the
county level air quality data for those counties containing the FIA
biomonitoring sites. This assessment showed that incidence of visible
foliar injury ranged from 21 to 39 percent during the four-year period
(2001-2004) across all counties with air quality levels at or below
that of the current 0.08 8-hour standard. Of the counties that met an
8-hour level of 0.07 ppm in those years, 11 to 30 percent still had
incidence of visible foliar

injury. The magnitude of these percentages suggests that phytotoxic
exposures sufficient to induce visible foliar injury would still occur
in many areas after meeting the level of the current secondary standard
or alternative 0.07 ppm 8-hour standard. Additionally, the data show
that visible foliar injury occurrence is geographically widespread and
is occurring on a variety of plant species in forested and other
natural systems. Linking visible foliar injury to other plant effects
is still problematic. However, its presence indicates that other
O3-related vegetation effects could also be present.
b. Seedling and Mature Tree Biomass Loss
    In the last review (EPA, 1996b), analyses of the effects of
O3 on trees were limited to 11 tree species for which C-R
functions for the seedling growth stage had been developed from OTC
studies conducted by the NHEERL-WED. Important tree species such as
quaking aspen, ponderosa pine, black cherry, tulip poplar were found to
be sensitive to cumulative seasonal O3 exposures. Work done
since the 1996 review at the AspenFACE site in Wisconsin on quaking
aspen (Karnosky et al., 2005) and a gradient study performed in the New
York City area (Gregg et al. 2003) has confirmed the detrimental
effects of O3 exposure on tree growth in field studies
without chambers and beyond the seedling stage (King et al., 2005).
These field studies are discussed above in section IV.A.
    To update the seedling biomass loss analysis, C-R functions for
biomass loss for available seedling tree species taken from the CD and
information on tree growing regions derived from the U.S. Department of
Agriculture's Atlas of United States Trees were combined with
projections of air quality based on 2001 interpolated exposures, to
produce estimated biomass loss for each of the seedling tree species
individually. Maps of these biomass loss projections are presented in
the Staff Paper. For example, quaking aspen had a wide range of
O3 exposure across its growing range and therefore, showed
significant variability in projected seedling biomass loss across its
range. Quaking aspen seedling biomass loss was projected to be greater
than 4 percent over much of its geographic range, though it can reach
above 10 percent in areas of Ohio, Pennsylvania, New York, New Jersey
and California. Biomass loss for black cherry was projected to be
greater than 20 percent in approximately half its range. Greater than
30 percent biomass loss for black cherry was projected in North
Carolina, Tennessee, Indiana, Ohio, Pennsylvania, Arizona, Michigan,
New York, New Jersey, Maryland and Delaware. For ponderosa pine, an
important tree species in the western U.S., biomass loss was projected
to be above 10 percent in much of its range in California. Biomass loss
still occurred in many tree species when O3 air quality was
adjusted to meet the current 8-hour standard. For instance, black
cherry, ponderosa pine, eastern white pine, and aspen had estimated
median seedling biomass losses over portions of their growing range as
high as 24, 11, 6, and 6 percent, respectively, when O3 air
quality was rolled back to just meet the current 8-hour standard. The
Staff Paper noted that these results are for tree seedlings and that
mature trees of the same species may have more or less of a response to
O3 exposure. Due to the potential for compounding effects
over multiple years, a consensus workshop on O3 effects
reported that a biomass loss greater than 2 percent annually can be
significant (Heck and Cowling, 1997). Decreased seedling root growth
and survivability could affect overall stand health and composition in
the long term.
    In addition to the estimation of O3 effects on seedling
growth, recent work has enhanced our understanding of risks beyond the
seedling stage. In order to better characterize the potential
O3 effects on mature tree growth, a tree growth model
(TREGRO) was used as a tool to evaluate the effect of changing
O3 air quality scenarios from just meeting alternative
O3 standards on the growth of mature trees. TREGRO is a
process-based, individual tree growth simulation model (Weinstein et
al., 1991) and has been used to evaluate the effects of a variety of
O3 scenarios and linked with concurrent climate data to
account for O3 and climate/meteorology interactions on
several species of trees in different regions of the U.S. (Tingey et
al., 2001; Weinstein et al., 1991; Retzlaff et al., 2000; Laurence et
al., 1993; Laurence et al., 2001; Weinstein et al., 2005). The model
provides an analytical framework that accounts for the nonlinear
relationship between O3 exposure and response. The
interactions between O3 exposure, precipitation and
temperature are integrated as they affect vegetation, thus providing an
internal consistency for comparing effects in trees under different
exposure scenarios and climatic conditions. An earlier assessment of
the effectiveness of national ambient air quality standards in place
since the early 1970s took advantage of 40 years of air quality and
climate data for the Crestline site in the San Bernardino Mountains of
California to simulate ponderosa pine growth over time with the
improving air quality using TREGRO (Tingey et al., 2004).
    The TREGRO model was used to assess growth of Ponderosa pine in the
San Bernardino Mountains of California (Crestline) and the growth of
yellow poplar and red maple in the Appalachian mountains of Virginia
and North Carolina, Shenandoah National Park (Big Meadows) and Linville
Gorge Wilderness Area (Cranberry), respectively. Total tree growth
associated with 'as is' air quality, and air quality adjusted to just
meet alternative O3 standards was assessed. Ponderosa pine
is one of the most widely distributed pines in western North America, a
major source of timber, important as wildlife habitat, and valued for
aesthetics (Burns and Honkala, 1990). Red maple is one of the most
abundant species in the eastern U.S. and is important for its brilliant
fall foliage and highly desirable wildlife browse food (Burns and
Honkala, 1990). Yellow poplar is an abundant species in the southern
Appalachian forest. It is 10 percent of the cove hardwood stands in
southern Appalachians which are widely viewed as some of the country's
most treasured forests because the protected, rich, moist set of
conditions permit trees to grow the largest in the eastern U.S. The
wood has high commercial value because of its versatility and as a
substitute for increasingly scarce softwoods in furniture and framing
construction. Yellow poplar is also valued as a honey tree, a source of
wildlife food, and a shade tree for large areas (Burns and Honkala,
1990).
    The Staff Paper analyses found that just meeting the current
standard would likely continue to allow O3-related
reductions in annual net biomass gain in these species. This is based
on model outputs that estimate that as O3 levels are reduced
below those of the current standard, significant improvements in growth
would occur. For instance, estimated growth in red maple increased by 4
and 3 percent at Big Meadows and Cranberry sites, respectively, when
air quality was rolled back to just met a W126 value of 13 ppm-hour.
Yellow poplar was projected to have a growth increase between 0.6 and 8
percent under the same scenario at the two eastern sites.
    Though there is uncertainty associated with the above analyses,
this information should be given careful consideration in light of
several other pieces of evidence. Specifically, new evidence from
experimental studies that go beyond the seedling growth stage

continues to show decreased growth under elevated O3 (King
et al., 2005). Some mature trees such as red oak have shown an even
greater sensitivity of photosynthesis to O3 than seedlings
of the same species (Hanson et al., 1994). As indicated above, smaller
growth loss increments may be significant for perennial species. The
potential for cumulative ``carry over'' effects as well as compounding
must be considered. The accumulation of such ``carry-over'' effects
over time may affect long-term survival and reproduction of individuals
and ultimately the abundance of sensitive tree species in forest
stands.
c. Crops
    As discussed in the Staff Paper, risk of O3 exposure and
associated monetized benefits were estimated for commodity crops,
fruits and vegetables. Similar to the tree seedling analysis, this
analysis combined C-R information on crops, crop growing regions and
interpolated exposures during each crop growing season. NCLAN crop
functions were used for commodity crops. According to USDA National
Agricultural Statistical Survey (NASS) data, the 9 commodity crop
species (e.g., cotton, field corn, grain sorghum, peanut, soybean,
winter wheat, lettuce, kidney bean, potato) included in the Staff Paper
analysis accounted for 69 percent of 2004 principal crop acreage
planted in the U.S. in 2004.\61\ The C-R functions for six fruit and
vegetable species (tomatoes-processing, grapes, onions, rice,
cantaloupes, Valencia oranges) were identified from the California
fruit and vegetable analysis from the last review (Abt 1995). The Staff
Paper noted that fruit and vegetable studies were not part of the NCLAN
program and C-R functions were available only in terms of seasonal 7-
hour or 12-hour mean index. This index form is considered less
effective in predicting plant response for a given change in air
quality than the cumulative form used with other crops. Therefore, the
fruit and vegetable C-R functions were considered more uncertain than
those for commodity crops.
---------------------------------------------------------------------------

    \61\ Principal crops as defined by the USDA include corn,
sorghum, oats, barley, winter wheat, rye, Durum wheat, other spring
wheat, rice, soybeans, peanuts, sunflower, cotton, dry edible beans,
potatoes, sugar beets, canola, proso millet, hay, tobacco, and
sugarcane. Acreage data for the principal crops were taken from the
USDA NASS 2005 Acreage Report (http://usda.mannlib.cornell.edu/reports/nassr/field/pcp-bba/acrg0605.pdf
).

---------------------------------------------------------------------------

    Analyses in the Staff Paper showed that some of the most important
commodity crops such as soybean, winter wheat and cotton had some
projected losses under the 2001 base year air quality. Soybean yield
losses were projected to be 2-4 percent in parts of Pennsylvania, New
Jersey, Maryland and Texas. Winter wheat was projected to have yield
losses of 2-6 percent in parts of California. Additionally, cotton was
projected to have yield losses of above 6 percent in parts of
California, Texas and North Carolina in 2001. The risk assessment
estimated that just meeting the current 8-hour standard would still
allow O3-related yield loss to occur in some commodity crop
species and fruit and vegetable species currently grown in the U.S. For
example, based on median C-R function response, in counties with the
highest O3 levels, potatoes and cotton had estimated yield
losses of 9-15 percent and 5-10 percent, respectively, when
O3 air quality just met the level of the current standard.
Estimated yield improved in these counties when the alternative W126
standard levels were met. The very important soybean crop had generally
small yield losses throughout the country under just meeting the
current standard (0-4 percent).
    The Staff Paper also presented estimates of monetized benefits for
crops associated with the current and alternative standards. The
Agriculture Simulation Model (AGSIM) (Taylor, 1994; Taylor, 1993) was
used to calculate annual average changes in total undiscounted economic
surplus for commodity crops and fruits and vegetables when current and
alternative standard levels were met. Meeting the various alternative
standards did show some significant benefits beyond the current 8-hour
standard. However, the Staff Paper recognized the AGSIM modeled
economic benefits had many uncertainties: For example, much of the
economic benefits were from the fruits and vegetables which had
uncertain C-R relationships, there was uncertainty in assumptions about
the treatment and effect of government farm payment programs, and there
was also uncertainty about near-term changes in agriculture sector due
to the increased use of crops as biofuels. Although the AGSIM model
results provided a relative comparison of agricultural benefits between
alternative standards, the uncertainties limited the utility of the
absolute numbers.

D. Conclusions on the Adequacy of the Current Standard

1. Background
    The initial issue to be addressed in the current review of the
secondary O3 standard is whether, in view of the advances in
scientific knowledge reflected in the Criteria Document and additional
information on exposure and risk discussed in the Staff Paper, the
existing standard should be revised. The current secondary standard is
a 3-year average of the annual 4th-highest maximum 8-hour average
O3 concentration set at a level of 0.08 ppm. In evaluating
whether it is appropriate to retain or revise the current secondary
O3 standard, the Administrator adopts an approach in this
review that builds upon the general approach used in the last review
and reflects the broader body of evidence now available.
    In developing proposed conclusions on the adequacy of the current
secondary O3 standard, the Administrator has considered a
weight-of-evidence approach that evaluated information across the
variety of vegetation-related research areas described in the Criteria
Document (e.g., seedling, sapling and mature forest tree species growth
stages and commodity, fruit, vegetable and forage crop species), and
included the assessments of air quality, exposures, and qualitative and
quantitative risks associated with alternative air quality scenarios.
Evidence-based considerations included assessment of vegetation effects
evidence obtained from chamber, free air, gradient, model and field-
based observation studies across an array of vegetation effects
endpoints. Exposure- and risk-based considerations were drawn from
exposure and risk assessments that relied upon both monitored and
interpolated O3 exposures as described in the Staff Paper.
These assessments reflect the availability of new tools and assessment
methods, as well as the larger and more diverse body of evidence
available since the last review. Specifically, estimates of exposures
and risks associated with recent O3 air quality levels, as
well as estimates of the relative magnitude of exposure and risk
reductions potentially associated with meeting the current 8-hour
secondary O3 NAAQS and alternative standards, have also been
considered, along with all known associated uncertainties.
    In this review, a series of general questions frames the approach
to reaching a proposed decision on the adequacy of the current
standard, beginning with: (1) To what extent does newly available
information reinforce or call into question evidence of associations of
O3 exposures with effects identified in the last review?;
(2) to what extent does newly available information reinforce or call
into question any of the basic elements of the current standard?;

and (3) to what extent have important uncertainties identified in the
last review been reduced and have new uncertainties emerged? To the
extent the available information suggests that revision of the current
standard may be appropriate, the question of whether the available
information supports consideration of a standard that is either more or
less protective than the current standard is addressed, including: (1)
Whether there is evidence that vegetation effects extend to ambient
O3 concentration levels that are as low as or lower than had
previously been observed, and what are the important uncertainties
associated with that evidence?; (2) whether vegetation exposures and
risks of concern estimated to occur in areas upon meeting the current
standard are considered important from a public welfare perspective;
and (3) what are the important uncertainties associated with the
estimated risks?
    The current secondary standard was selected to provide protection
to the public welfare against a range of O3-induced
vegetation effects, particularly yield loss in agricultural crops and
biomass loss in tree seedlings. As an introduction to the discussion in
this section of the adequacy of the current O3 standard, it
is useful to summarize the key factors that formed the basis of the
decision in the last review to revise the averaging time, level and
form of the then current 1-hour secondary standard.
    In the 1996 proposal notice (61 FR 65716), the Administrator
proposed to replace the then existing 1-hour O3 secondary
NAAQS with one of two alternative new standards: a standard identical
to the proposed and now current 0.08 ppm, 8-hour primary standard
(described above), or alternatively, a new seasonal standard, SUM06,
expressed as a sum of hourly concentrations greater than or equal to
0.06 ppm, cumulated daily over a 12 hour daylight window (8 am to 8 pm)
during the maximum consecutive 3-month period (e.g., the consecutive 3
month period with the highest SUM06 index value) during the
O3 monitoring season, set at a level of 25 ppm-hours. The
latter form and level were selected to provide protection to vegetation
on the basis of annual, rather than 3-year average, exposures.
    In the final rule for the O3 NAAQS published in July
1997 (62 FR 38877), the Administrator decided to replace the then
current 1-hour, 0.12-ppm secondary NAAQS with a standard that was
identical in every way to the new revised primary standard of an 0.08
ppm annual 4th-highest maximum 8-hour average standard averaged over 3
years. Her decision was based on: (1) Her judgment that the then
existing secondary standard did not provide adequate protection for
vegetation against the adverse welfare effects of O3; (2)
CASAC advice ``that a secondary NAAQS, more stringent than the present
primary standard, was necessary to protect vegetation from
O3'' (Wolff, 1996); (3) her judgment that the new 8-hour
average standard would provide substantially improved protection for
vegetation from O3-related adverse effects as compared to
the level of protection provided by the then current 1-hour, 0.12-ppm
secondary standard; (4) recognition that significant uncertainties
remained with respect to exposure dynamics, air quality relationships,
and the exposure, risk, and monetized valuation analyses presented in
the proposal, resulting in only rough estimates of the increased public
welfare likely to be afforded by each of the proposed alternative
standards; (5) her judgment that there was value in allowing more time
to obtain additional information to better characterize O3-
related vegetation effects under field conditions from additional
research and to develop a more complete rural monitoring network and
air quality database from which to evaluate the elements of an
appropriate seasonal secondary standard; and (6) her judgment that
there was value in allowing more time to evaluate more specifically the
improvement in rural air quality and in O3-related
vegetation effects resulting from measures designed to attain the new
primary standard (62 FR 38877-78).
    The Administrator further concluded (62 FR 38877-78) that continued
research on the effects of O3 on vegetation under field
conditions and on better characterizing the relationship between
O3 exposure dynamics and plant response would be important
in the next review because: (1) The available biological database
highlighted the importance of cumulative, seasonal exposures as a
primary determinant of plant responses; (2) the association between
daily maximum 8-hour O3 concentrations and plant responses
had not been specifically examined in field tests; (3) the impacts of
attaining an 8-hour, 0.08 ppm primary standard in upwind urban areas on
rural air quality distributions could not be characterized with
confidence due to limited monitoring data and air quality modeling in
rural and remote areas.
2. Evidence- and Exposure/Risk-Based Considerations
    The new evidence available in this review as described in the
Criteria Document continues to support and strengthen key policy-
relevant conclusions drawn in the previous review (EPA, 2006a). Based
on this new evidence, the current Criteria Document once more concludes
that: (1) A plant's response to O3 depends upon the
cumulative nature of ambient exposure as well as the temporal dynamics
of those concentrations; (2) current ambient concentrations in many
areas of the country are sufficient to impair growth of numerous common
and economically valuable plant and tree species; (3) the entrance of
O3 into the leaf through the stomata is the critical step in
O3 effects; (4) effects can occur with only a few hourly
concentrations above 0.08 ppm; (5) other environmental biotic and
abiotic factors are also influential to the overall impact of
O3 on plants and trees; and (6) a high degree of uncertainty
remains in our ability to assess the impact of O3 on
ecosystem services.
    In light of the new evidence, as described in the Criteria
Document, the Staff Paper evaluates the adequacy of the current
standard based on assessments of both the most policy-relevant
vegetation effects evidence and exposure and risk-based information, as
summarized above in sections IV.A and IV.C, respectively. In evaluating
the strength of this information, the Staff Paper takes into account
the uncertainties and limitations in the scientific evidence and
analyses as well as the views of CASAC. The Staff Paper concludes that
progress has been made since the last review and generally finds
support in the available effects- and exposure/risk-based information
for consideration of an O3 standard that is more protective
than the current standard. The Staff Paper further concludes that there
is no support for consideration of an O3 standard that is
less protective than the current standard. This general conclusion is
consistent with the advice and recommendations of CASAC.
a. Evidence-Based Considerations
    In the last review, crop yield and tree seedling biomass loss data
obtained in OTC studies provided the basis for the Administrator's
judgment that the then current 1-hour, 0.12 ppm secondary standard was
inadequate (EPA, 1996b). Since then, several additional lines of
evidence have progressed sufficiently to provide a more complete and
coherent picture of the scope of O3-related

vegetation risks, especially those currently faced by sensitive
seedling, sapling and mature growth stage tree species growing in field
settings, and their associated forested ecosystems. Specifically, new
research reflects an increased emphasis on field-based exposure methods
(e.g., free air, ambient gradient, and biomonitoring surveys). In
reaching conclusions regarding the adequacy of the current standard,
the Staff Paper has considered the combined information from all these
areas together, along with associated uncertainties, in an integrated,
weight-of-evidence approach.
    Regarding the O3-induced effect of visible foliar
injury, observations for the years 2001 to 2004 at USDA FIA
biomonitoring sites showed widespread O3-induced leaf injury
occurring in the field, including in forested ecosystems, under current
ambient O3 conditions. For a few studied species, it has
been shown that the presence of visible foliar injury is further linked
to the presence of other vegetation effects (e.g., reduced plant growth
and impaired below ground root development) (EPA, 2006), though for
most species, this linkage has not been specifically studied or where
studied, has not been found. Nevertheless, when visible foliar injury
is present, the possibility that other O3-induced vegetation
effects could also be present for some species should be considered.
Likewise, the absence of visible foliar injury should not be construed
to demonstrate the absence of other O3-induced vegetation
effects. The Staff Paper concludes that it is not possible at this time
to quantitatively assess the degree of visible foliar injury that
should be judged adverse in all settings and across all species, and
that other environmental factors can mitigate or exacerbate the degree
of O3-induced visible foliar injury expressed at any given
concentration of O3. However, the Staff Paper also concludes
that the presence of visible foliar injury alone can be adverse to the
public welfare, especially when it occurs in protected areas such as
national parks and wilderness areas. Thus, on the basis of the
available information on the widespread distribution of O3-
sensitive species within the U.S. including in areas, such as national
parks, which are afforded a higher degree of protection, the Staff
Paper concludes that the current standard continues to allow levels of
visible foliar injury in some locations that could reasonably be
considered to be adverse from a public welfare perspective. Additional
monitoring of both O3 air quality and foliar injury levels
are needed in these areas of national significance to more fully
characterize the spatial extent of this public welfare impact.
    With respect to O3-induced biomass loss in trees, the
Staff Paper concludes that the significant new body of field-based
research on trees strengthens the conclusions drawn on tree seedling
biomass loss from earlier OTC work by documenting similar seedling
responses in the field. For example, recent empirical studies conducted
on quaking aspen at the AspenFACE site in Wisconsin have confirmed the
detrimental effects of O3 exposure on tree growth in a field
setting without chambers (Isebrands et al., 2000, 2001). In addition,
results from an ambient gradient study (Gregg et al., 2003), which
evaluated biomass loss in cottonwood along an urban-to-rural gradient
at several locations, found that conditions in the field were
sufficient to produce substantial biomass loss in cottonwood, with
larger impacts observed in downwind rural areas due to the presence of
higher O3 concentrations. These gradients from low urban to
higher rural O3 concentrations occur when O3
precursors generated in urban areas are transported to downwind sites
and are transformed into O3. In addition,O3
concentrations typically fall to near 0 ppm at night in urban areas due
to scavenging of O3 by NOX and other compounds.
In contrast, rural areas, due to a lack of nighttime scavenging, tend
to maintain elevated O3 concentrations for longer periods.
On the basis of such key studies, the Staff Paper concludes that the
expanded body of field-based evidence, in combination with the
substantial corroborating evidence from OTC data, provides stronger
evidence than that available in the last review that ambient levels of
O3 are sufficient to produce visible foliar injury symptoms
and biomass loss in sensitive vegetative species growing in natural
environments. Further, the Staff Paper judges that the consistency in
response in studied species/genotypes to O3 under a variety
of exposure conditions and methodologies demonstrates that these
sensitive genotypes and populations of plants are susceptible to
adverse impacts from O3 exposures at levels known to occur
in the ambient air. Due to the potential for compounded risks from
repeated insults over multiple years in perennial species, the Staff
Paper concludes that these sensitive subpopulations are not afforded
adequate protection under the current secondary O3 standard.
Despite the fact that only a relatively small portion of U.S. plant
species have been studied with respect to O3 sensitivity,
those species/genotypes shown to have O3 sensitivity span a
broad range of vegetation types and public use categories, including
direct-use categories like food production for human and domestic
animal consumption; fiber, materials, and medicinal production; urban/
private landscaping. Many of these species also contribute to the
structure and functioning of natural ecosystems (e.g., the EEAs) and
thus, to the goods and services those ecosystems provide (Young and
Sanzone, 2002), including non-use categories such as relevance to
public welfare based on their aesthetic, existence or wildlife habitat
value.
    The Staff Paper therefore concludes that the current secondary
standard is inadequate to protect the public welfare against the
occurrence of known adverse levels of visible foliar injury and tree
seedling biomass loss occurring in tree species (e.g., ponderosa pine,
aspen, black cherry, cottonwood) that are sensitive and clearly
important to the public welfare.
b. Exposure- and Risk-Based Considerations
    The Staff Paper also presents the results of exposure and risk
assessments. Due to multiple sources of uncertainty, both known and
unknown, that continue to be associated with these analyses, the Staff
Paper put less weight on this information in drawing conclusions on the
adequacy of the current standard. However, the Staff Paper also
recognizes that some progress has been made since the last review in
better characterizing some of these associated uncertainties and,
therefore concluded that the results of the exposure and risk
assessments continue to provide information useful to informing
judgments as to the relative changes in risks predicted to occur under
exposure scenarios associated with the different standard alternatives
considered. Importantly, with respect to two key uncertainties, the
uncertainty associated with continued reliance on C-R functions
developed from OTC exposure systems to predict plant response in the
field and the potential for changes in tree seedling and crop
sensitivities in the intervening period since the C-R functions were
developed, the Staff Paper concluded that recent research has provided
information useful in judging how much weight to put on these concerns.
Specifically, new field-based studies, conducted on a limited number of
tree seedling and crop species to date, demonstrate plant growth and
visible foliar injury responses in the field that

are similar in nature and magnitude to those observed previously under
OTC exposure conditions, lending qualitative support to the conclusion
that OTC conditions do not fundamentally alter the nature of the
O3-plant response. Second, nothing in the recent literature
suggests that the O3 sensitivity of crop or tree species
studied in the last review and for which C-R functions were developed
has changed significantly in the intervening period. Indeed, in the few
recent studies where this is examined, O3 sensitivities were
found to be as great as or greater than those observed in the last
review.
i. Seedling and Mature Tree Biomass Loss
    Biomass loss in sensitive tree seedlings is predicted to occur
under O3 exposures that meet the level of the current
secondary standard. For instance, black cherry, ponderosa pine, eastern
white pine, and aspen had estimated median seedling biomass losses as
high as 24, 11, 6, and 6 percent, respectively, over some portions of
their growing ranges when air quality was rolled back to meet the
current 8-hr standard with the 10 percent downward adjustment for the
potential O3 gradient between monitor height and short plant
canopies applied. The Staff Paper notes that these results are for tree
seedlings and that mature trees of the same species may have more or
less of a response to O3 exposure. Decreased root growth
associated with biomass loss has the potential to indirectly affect the
vigor and survivability of tree seedlings. If such effects occur on a
sufficient number of seedlings within a stand, overall stand health and
composition can be affected in the long term. Thus, the Staff Paper
concludes that these levels of estimated tree seedling growth reduction
should be considered significant and potentially adverse, given that
they are well above the 2 percent level of concern identified by the
1997 consensus workshop (Heck and Cowling, 1997).
    Though there is significant uncertainty associated with this
analysis, the Staff Paper recommends that this information should be
given careful consideration in light of several other pieces of
evidence. Specifically, limited evidence from experimental studies that
go beyond the seedling growth stage continues to show decreased growth
under elevated O3 levels (King et al., 2005). Some mature
trees such as red oak have shown an even greater sensitivity of
photosynthesis to O3 than seedlings of the same species
(Hanson et al., 1994). The potential for effects to ``carry over'' to
the following year or cumulate over multiple years, including the
potential for compounding, must be considered. The accumulation of such
``carry-over'' effects over time may affect long-term survival and
reproduction of individual trees and ultimately the abundance of
sensitive tree species in forest stands.
ii. Qualitative Ecosystem Risks
    In addition to the quantifiable risk categories discussed above,
the Staff Paper presents qualitative discussions on a number of other
public welfare effects categories. In so doing, the Staff Paper
concludes that the quantified risks to vegetation estimated to be
occurring under current air quality or upon meeting the current
secondary standard likely represent only a portion of actual risks that
may be occurring for a number of reasons.
    First, as mentioned above, out of the over 43,000 plant species
catalogued as growing within the U.S. (USDA PLANTS database, USDA,
NRCS, 2006), only a small percentage have been studied with respect to
O3 sensitivity. Most of the studied species were selected
because of their commercial importance or observed O3-
induced visible foliar injury in the field. Given that O3
impacts to vegetation also include less obvious but often more
significant impacts, such as reduced annual growth rates and below
ground root loss, the paucity of information on other species means the
number of O3-sensitive species that exists within U.S.,
could be greater than what is now known. Since no state in the lower 48
states has less than seven known O3-sensitive plant species,
with the majority of states having between 11 and 30 (see Appendix 7J-2
in Staff Paper), protecting O3 sensitive vegetation is
clearly important to the public welfare at the national scale.
    Second, the Staff Paper also takes into consideration the
possibility that more subtle and hidden risks to ecosystems are
potentially occurring in areas where vegetation is being significantly
impacted. Given the importance of these qualitative and anticipated
risks to important public welfare effects categories such as ecosystem
impacts leading to potential losses or shifts in ecosystem goods and
services (e.g., carbon sequestration, hydrology, and fire disturbance
regimes), the Staff Paper concludes that any secondary standard set to
protect against the known and quantifiable adverse effects to
vegetation should also consider the anticipated, but currently
unquantifiable, potential effects on natural ecosystems.
iii. Crop Yield Loss
    Exposure and risk assessments in the Staff Paper estimated that
meeting the current 8-hour standard would still allow O3-
related yield loss to occur in several fruit and vegetable and
commodity crop species currently grown in the U.S. These estimates of
crop yield loss are substantially lower than those estimated in the
last review as a result of several factors, including adjusted exposure
levels to reflect the presence of a variable O3 gradient
between monitor height and crop canopies, and use of a different
econometric agricultural benefits model updated to reflect more recent
agricultural policies (EPA, 2006b). Though these sources of uncertainty
associated with the crop risk and benefits assessments were better
documented in this review, the Staff Paper concludes that the presence
of these uncertainties make the risk estimates suitable only as a basis
for understanding potential trends in relative yield loss and economic
benefits. The Staff Paper further recognizes that actual conditions in
the field and management practices vary from farm to farm, that
agricultural systems are heavily managed, and that adverse impacts from
a variety of other factors (e.g., weather, insects, disease) can be
orders of magnitude greater than that of yield impacts predicted for a
given O3 exposure. Thus, the relevance of such estimated
impacts on crop yields to the public welfare are considered highly
uncertain and less useful as a basis for assessing the adequacy of the
current standard. The Staff Paper notes, however, that in some
experimental cases, exposure to O3 has made plants more
sensitive or vulnerable to some of these other important stressors,
including disease, insect pests, and harsh weather (EPA, 2006a). The
Staff Paper therefore concluded that this remains an important area of
uncertainty and that additional research to better characterize the
nature and significance of these interactions between O3 and
other plant stressors would be useful.
c. Summary
    In summary, the Staff Paper concludes that the current secondary
O3 standard is inadequate. This conclusion is based on the
extensive vegetation effects evidence, in particular the recent
empirical field-based evidence on biomass loss in seedlings, saplings
and mature trees, and foliar injury incidence that has become available
in this review, which demonstrates the occurrence of adverse vegetation
effects at ambient

levels of recent O3 air quality, as well as evidence and
exposure- and risk-based analyses indicating that adverse effects would
be predicted to occur under air quality scenarios that meet the current
standard.
3. CASAC Views
    In a letter to the Administrator (Henderson, 2006c), the CASAC
O3 Panel, with full endorsement of the chartered CASAC,
unanimously concluded that ``despite limited recent research, it has
become clear since the last review that adverse effects on a wide range
of vegetation including visible foliar injury are to be expected and
have been observed in areas that are below the level of the current 8-
hour primary and secondary ozone standards.'' Therefore, ``based on the
Ozone Panel's review of Chapters 7 and 8 [of the Staff Paper], the
CASAC unanimously agrees that it is not appropriate to try to protect
vegetation from the substantial, known or anticipated, direct and/or
indirect, adverse effects of ambient O3 by continuing to
promulgate identical primary and secondary standards for O3.
Moreover, the members of the Committee and a substantial majority of
the Ozone Panel agree with EPA staff conclusions and encourage the
Administrator to establish an alternative cumulative secondary standard
for O3 and related photochemical oxidants that is distinctly
different in averaging time, form and level from the currently existing
or potentially revised 8-hour primary standard'' (Henderson,
2006c).\62\
---------------------------------------------------------------------------

    \62\ One CASAC Panel member reached different conclusions from
those of the broader Panel regarding certain aspects of the
vegetation effects information and the appropriate degree of
emphasis that should be placed on the associated uncertainties.
These concerns related to how the results of O3/
vegetation exposure experiments carried out in OTC can be
extrapolated to the ambient environment and how C-R functions
developed in the 1980's can be used today given that he did not
expect that current crop species/cultivars in use in 2002 would have
the same O3 sensitivity as those studied in NCLAN
(Henderson, 2007, pg. C-18).
---------------------------------------------------------------------------

4. Administrator's Proposed Conclusions Concerning Adequacy of Current
Standard
    The Administrator recognizes that the secondary standard is to
protect against ``adverse'' O3 effects, discussed above in
section IV.A.3. In considering what constitutes a vegetation effect
that is also adverse to the public welfare, the Administrator took into
account the Staff Paper conclusions regarding the nature and strength
of the vegetation effects evidence, the exposure and risk assessment
results, the degree to which the associated uncertainties should be
considered in interpreting the results, and the views of CASAC and
members of the public. On these bases, the Administrator proposes that
the current secondary standard is inadequate to protect the public
welfare from known and anticipated adverse O3-related
effects on vegetation and ecosystems. Ozone levels that would be
expected to remain after meeting the current secondary standard are
sufficient to cause visible foliar injury, seedling and mature tree
biomass loss, and crop yield reductions to degrees that could be
considered adverse depending on the intended use of the plant and its
significance to the public welfare, and the current secondary standard
does not provide adequate protection from such effects. Other
O3-induced effects described in the literature, including an
impaired ability of many sensitive species and genotypes within species
to adapt to or withstand other environmental stresses, such as freezing
temperatures, pest infestations and/or disease, and to compete for
available resources, would also be anticipated to occur. In the long
run, the result of these impairments (e.g., loss in vigor) could lead
to premature plant death in O3 sensitive species. Though
effects on other ecosystem components have only been examined in
isolated cases, effects such as those described above could have
significant implications for plant community and associated species
biodiversity and the structure and function of whole ecosystems. These
considerations also support the proposed conclusion that the current
secondary standard is not adequate and that revision is needed to
provide additional public welfare protection.

E. Conclusions on the Elements of the Secondary Standard

    Given his proposed conclusion that the current secondary standard
is inadequate, the Administrator then considered what revisions to the
standard are appropriate. In so doing, the Administrator has focused on
revisions to the key standard elements of indicator, form, averaging
time, and level. On the basis of the strength and coherence of the
vegetation effects evidence suggesting that a biologically-based
standard for vegetation, at a minimum, should cumulate exposures and
differentially-weight higher O3 concentrations, the
Administrator judges that it is appropriate to consider revisions to
the secondary standard that reflect this understanding. In addition,
the Administrator also judges that the current 8-hour average form,
though not based on the most biologically relevant and coherent
vegetation effects literature, can also provide substantially improved
protection to vegetation when set at an appropriate level. Therefore,
the Administrator also considered whether revision to the level of the
current 8-hour secondary standard might provide the requisite level of
public welfare protection. In light of these considerations, as
discussed below, the Administrator is proposing two options for
revising the current secondary standard: one option is a cumulative
seasonal standard (section IV.E.2) and the other option is an 8-hour
average standard consistent with the revised 8-hour average standard
proposed above for the primary standard (section IV.E.3). The
Administrator has also considered an alternative approach to setting a
cumulative, seasonal standard(s) as described below in section IV.E.2.
1. Indicator
    In the last review, EPA focused on a standard for O3 as
the most appropriate surrogate for ambient photochemical oxidants. In
this review, while the complex atmospheric chemistry in which
O3 plays a key role has been highlighted, no alternatives to
O3 have been advanced as being a more appropriate surrogate
for ambient photochemical oxidants. Thus, as is the case for the
primary standard, (discussed above in section II.D.1.), the
Administrator proposes to continue to use O3 as the
indicator for a standard that is intended to address effects associated
with exposure to O3, alone and in combination with related
photochemical oxidants. In so doing, the Administrator recognizes that
measures leading to reductions in vegetation exposures to O3
will also reduce exposures to other photochemical oxidants.
2. Cumulative, Seasonal Standard
    The Administrator proposes to replace the current secondary
standard with a new cumulative, seasonal standard expressed as an index
of the annual sum of weighted hourly concentrations (using the W126
form), set at a level in the range of 7 to 21 ppm-hours. The index
would be cumulated over the 12-hour daylight period (8 a.m. to 8 p.m.)
during the consecutive 3-month period within the O3 season
with the maximum index value. In addition, as discussed below, the
Administrator is considering an alternative approach to setting a
cumulative, seasonal standard(s) that would afford differing degrees of
protection for O3-related impacts on different types of
vegetation with different intended uses.

a. Form
    The current Criteria Document and Staff Paper concluded that the
recent vegetation effects literature evaluated in this review
strengthens and reaffirms conclusions made in the last review that the
use of a cumulative exposure index that differentially-weights ambient
concentrations is best able to relate ambient exposures to vegetation
response at this time (EPA, 2006a, b; see also discussion in IV.B.
above). The 1996 review focused in particular on two of these
cumulative forms, the SUM06 and W126. As described in the last review
(EPA, 1996a, b) it was concluded that, based on statistical reanalysis
of the NCLAN data, these different cumulative forms performed equally
well in predicting crop yield loss response to O3 exposure.
Given that the data available at that time were unable to distinguish
between these forms, the Administrator, based on the policy
consideration of not including O3 concentrations considered
to be within the PRB, concluded that the SUM06 form was the more
appropriate choice for a secondary standard.
    In this review, the Staff Paper evaluated the continued
appropriateness of the SUM06 form in light of two key pieces of
information: new estimates of PRB that are lower than in the last
review, and continued lack of evidence within the vegetation effects
literature of a biological threshold for vegetation exposures of
concern. On the basis of those policy and science-related
considerations, the Staff Paper concluded that the W126 form was more
appropriate in the context of this review. Specifically, the W126, by
its incorporation of a sigmoidal weighting scheme, does not create an
artificially imposed concentration threshold, gives proportionally more
weight to the higher and typically more biologically potent
concentrations, and is not significantly influenced by O3
concentrations within the range of estimated PRB.
    The CASAC, based on its assessment of the same vegetation effects
science, agreed with the Criteria Document and Staff Paper and
unanimously concluded that it is not appropriate to try to protect
vegetation from the known or anticipated adverse effects of ambient
O3 by continuing to promulgate identical primary and
secondary standards for O3. Moreover, the members of CASAC
and a substantial majority of the CASAC O3 Panel agreed with
Staff Paper conclusions and encouraged the Administrator to establish
an alternative cumulative secondary standard for O3 and
related photochemical oxidants that is distinctly different in
averaging time, form, and level from the current or potentially revised
8-hour primary standard. The CASAC also stated that ``the recommended
metric for the secondary ozone standard is the (sigmoidally-weighted)
W126 index'' (Henderson, 2007).\63\
---------------------------------------------------------------------------

    \63\ One CASAC Panel member expressed the view that the
O3 exposure indices, SUM06 and W126, are simply
mathematical expressions of exposure and, thus, cannot be said to
have a biological basis (Henderson, 2007, pg. C-18).
---------------------------------------------------------------------------

    The Administrator agrees with the conclusions drawn in the Criteria
Document, Staff Paper and by CASAC that the scientific evidence
available in the current review continues to demonstrate the cumulative
nature of O3-induced plant effects and the need to give
greater weight to higher concentrations. Thus, the Administrator
concludes that a cumulative exposure index that differentially-weights
O3 concentrations represents a reasonable policy choice for
a seasonal secondary standard to protect against the effects of
O3 on vegetation. The Administrator further agrees with both
the Staff Paper and CASAC that the most appropriate cumulative,
concentration-weighted form to consider in this review is the
sigmoidally weighted W126 form, due to his recognition that there is no
evidence in the literature for an exposure threshold that would be
appropriate across all O3-sensitive vegetation and that this
form is unlikely to be significantly influenced by O3 air
quality within the range of PRB levels identified in this review. Thus,
the Administrator proposes as one option to replace the current 8-hour
average secondary standard form with the cumulative, seasonal W126
form.
b. Averaging Times \64\
---------------------------------------------------------------------------

    \64\ While the term ``averaging time'' is used, for the
cumulative, seasonal standard the time period at issue is one over
which exposures during a specified period of time are cumulated, not
averaged.
---------------------------------------------------------------------------

    The Staff Paper, in addition to form, also considers what
``averaging'' periods or exposure durations are most relevant for
vegetation, which, unlike people, is exposed to ambient air
continuously throughout its lifespan. For annual species, this lifespan
encompasses a period of only one year or less; while for perennials,
lifespans can range from a few years to decades or centuries. However,
because O3 levels are not continuously elevated and plants
are not equally sensitive to O3 over the course of a day,
season or lifetime, it becomes necessary to identify periods of
exposure that have the most relevance for plant response. Exposure
periods are discussed below in terms of a seasonal window, a diurnal
window, and an annual versus 3-year average standard.
    (1) In considering an appropriate seasonal window, the Staff Paper
recognizes that, in general, many annual crops are grown for periods of
a few months before being harvested. In contrast, other annual and
perennial species may be photosynthetically active longer, and for some
species and locations, throughout the entire year. In general, the
period of maximum physiological activity and thus, maximum potential
O3 uptake for annual crops, herbaceous species, and
deciduous trees and shrubs coincides with some or all of the intra-
annual period defined as the O3 season, which varies on a
state-by-state basis. This is because the high temperature and high
light conditions that promote the formation of tropospheric
O3 also promote physiological activity in vegetation.
    The Staff Paper notes that the selection of any single seasonal
exposure period for a national standard would represent a compromise,
given the significant variability in growth patterns and lengths of
growing seasons among the wide range of vegetation species occurring
within the U.S. that may experience adverse effects associated with
O3 exposures. However, the Staff Paper further concludes
that the consecutive 3-month period within the O3 season
with the highest W126 index value (e.g., maximum 3 month period) would,
in most cases, likely coincide with the period of greatest plant
sensitivity on an annual basis. Therefore, the Staff Paper again
concludes, as it did in 1996, that the annual maximum consecutive 3-
month period is a reasonable seasonal time period, when combined with a
cumulative, concentration weighted form, for protection of sensitive
vegetation.
    (2) In considering an appropriate diurnal window, the Staff Paper
recognizes that over the course of the 24-hour diurnal period, plant
stomatal conductance varies in response to changes in light level, soil
moisture and other environmentally and genetically controlled factors.
In general, stomata are most open during daylight hours in order to
allow sufficient CO2 uptake for use in carbohydrate
production through the light-driven process of photosynthesis. At most
locations, O3 concentrations are also highest during the
daytime, and thus, most likely to coincide with maximum stomatal
uptake. It is also known however, that in some species, stomata may
remain

open sufficiently at night to allow for some nocturnal uptake to occur.
In addition, at some rural, high elevation sites, the O3
concentrations remain relatively flat over the course of the day, often
at levels above estimated PRB. At these sites, nighttime W126 values
can be of similar magnitude as daytime values, though the significance
of these exposures is much less certain. This is because O3
uptake during daylight hours is known to impair the light-driven
process of photosynthesis, which can then lead to impacts on
carbohydrate production, plant growth, reproduction (yield) and root
function. It is less clear at this time to what extent and by what
mechanisms O3 uptake at night adversely impacts plant
function. In addition, many species do not take up O3 at
night or occur in areas with elevated nighttime O3
concentrations.
    In light of a recent work on this topic conducted by Musselman and
Minnick (2000), the Staff Paper again revisited the issue of what
diurnal period is of most relevance in influencing O3-
induced effects on vegetation. This work reports that some species take
up O3 at night, but that the degree of nocturnal stomatal
conductance varies widely between species and its relevance to overall
O3-induced vegetation effects remain unclear. In considering
this information, the Staff Paper concludes that for the vast majority
of studied species, daytime exposures represent the majority of diurnal
plant O3 uptake and are responsible for inducing the plant
response of most significance to the health and productivity of the
plant (e.g., reduced carbohydrate production). Until additional
information is available about the extent to which co-occurrence of
sensitive species and elevated nocturnal O3 exposures
exists, and what levels of nighttime uptake are adverse to affected
species, the Staff Paper concludes that this information continues to
be preliminary, and does not provide a basis for reaching a different
conclusion at this time. The Staff Paper further notes that additional
research is needed to address the degree to which a 12-hour diurnal
window may be under protective in areas where elevated nighttime levels
of O3 co-occur with sensitive species with a high degree of
nocturnal stomatal conductance. Thus, as in the last review, the Staff
Paper again concludes that based on the available science, the daytime
12-hour window (8 a.m. to 8 p.m.) is the most appropriate period over
which to cumulate diurnal O3 exposures, specifically those
most relevant to plant growth and yield responses.
    (3) In considering whether an annual or 3-year averaging period is
more appropriate, the Staff Paper recognized that though most
cumulative seasonal exposure levels of concern for vegetation have been
expressed in terms of the annual timeframe, it may be appropriate to
consider a 3-year averaging period for purposes of standard stability.
However, the Staff Paper notes that for certain welfare effects of
concern (e.g., foliar injury, yield loss for annual crops, growth
effects on other annual vegetation and potentially tree seedlings), an
annual time frame may be a more appropriate period in which to assess
what level would provide the requisite degree of protection, while for