discussed below, informs consideration of welfare endpoints and at-risk
species and ecosystems on which to focus the current review, and
consideration of the ambient O3 conditions under which
various welfare effects are known or anticipated to occur. As in past
reviews, we recognize that the available evidence has not provided
identification of a threshold in exposure or ambient O3
concentrations below which it can be concluded with confidence that
O3-attributable effects on vegetation do not occur, when
considering the broad range of O3-sensitive plant species
growing within the U.S and the array of effects. This is due in part to
the fact that research shows that there is variability in sensitivity
between and within species and that numerous factors, i.e., chemical,
physical, biological, and genetic, can influence the direction and
magnitude of the studied effect (U.S. EPA, 2013a, section 9.4.8). In
the absence of evidence for a discernible threshold, the general
approach to considering the available O3 welfare effects
evidence involves characterizing the confidence in conclusions
regarding O3-attributable vegetation effects over the ranges
of cumulative seasonal O3 exposure values evaluated in
chamber studies and in field studies in areas where O3-
sensitive vegetation are known to occur, as well as characterizing the
extent to which these effects can be considered adverse at the plant
level and beyond. With this approach, we consider the evidence for
O3 affecting other ecosystem components (such as soils,
water, and wildlife) and their associated goods and services, through
its effects on vegetation, as well as the associated uncertainties.
    Our general approach further recognizes the complexity of judgments
to be made regarding the identification of particular vegetation
effects as welfare effects and regarding the point that known or
anticipated vegetation-related effects become adverse to the public
welfare. For example, in addition to the magnitude of the ambient
concentrations, the species present, their sensitivity to
O3, and their public welfare importance are also essential
considerations in drawing conclusions regarding the significance of
public welfare impact. Taking this into account, we recognize the
existence of a continuum from relatively higher ambient O3
concentrations and conditions, in areas with sensitive species and
public welfare significance, for which there might be general agreement
that effects on public welfare are likely to occur, through lower
concentrations at which the degree to which public welfare might be
expected to be affected becomes increasingly uncertain.
    The evidence base for this review, summarized in section IV.B
below, includes quantitative information across a broad array of
vegetation effects (e.g., growth impairment during seedling, sapling
and mature tree growth stages, visible foliar injury, and yield loss in
annual crops) and across a diverse set of exposure methods from
laboratory and field studies. While considering the full breadth of
information available, we place greater weight on U.S. studies due to
the often species-, site-, and climate-

specific nature of O3-related vegetation responses, and
particularly emphasize those studies that include O3
exposures that fall within the range of those likely to occur in the
ambient air. We additionally recognize differences across different
study types in what information they provide (U.S. EPA, 2013a, section
9.2.6). For example, because conditions can be controlled in laboratory
studies, responses in such studies may be less variable and smaller
differences may be easier to detect. However, the controlled conditions
may limit the range of responses or incompletely reflect pollutant
bioavailability, so they may not reflect responses that would occur in
the natural environment. Alternatively, field data can provide
important information for assessments of multiple stressors or where
site-specific factors significantly influence exposure. They are also
often useful for analyses of larger geographic scales and higher levels
of biological organization. However, depending on the type of field
study, many field study conditions may not be controlled, which can
make variability higher and differences harder to detect. In some field
studies (e.g., gradient studies), the presence of confounding factors
can also make it difficult to attribute observed effects to specific
stressors.
    In developing quantitative exposure and risk assessments for this
review, summarized in section IV.C below, we have placed greatest
emphasis on studies that have evaluated plant response over multiple
exposure levels and developed exposure-response (E-R) relationships
that allow the estimation of plant responses over the range of
O3 exposures pertinent to judgments on the current and
potential alternative standards. In considering the information from
these assessments, we focus particularly on the quantitative risks
related to three types of O3 effects on vegetation and
associated ecosystem services: visible foliar injury, biomass loss in
trees, and crop yield loss. These risks were assessed in a range of
analyses primarily involving national-scale air quality scenarios
developed using model adjustments and interpolation methods. We
consider particularly the national scale assessments for these
scenarios, while recognizing the uncertainties with regard to the
conditions they represent.
    With regard to the appropriate characterization of exposures
associated with ambient O3 concentrations, as in the 2008
and 1997 reviews, we continue to recognize the relevance of cumulative,
seasonal, concentration-weighted exposures for assessing vegetation
effects. More specifically, in the 2008 review, the EPA concluded and
the CASAC agreed that the W126 cumulative exposure metric was the most
appropriate to use to evaluate both the adequacy of the current
secondary standard and the appropriateness of any potential revisions.
As discussed in section IV.B.1 below, the information available in this
review continues to support the use of such a metric and it is used in
considering potential public welfare impacts in the sections below.

B. Welfare Effects Information

1. Nature of Effects and Biologically Relevant Exposure Metric
    This section describes the nature of O3-induced welfare
effects, including the nature of the exposures that drive the
biological and ecological responses (U.S. EPA, 2013a, chapter 9).
    Ozone's phytotoxic effects were first identified on grape leaves in
a study published in 1958 (Richards et al., 1958). In the almost fifty
years that have followed, extensive research has been conducted both in
and outside of the U.S. to examine the impacts of O3 on
plants and their associated ecosystems, since ``of the phytotoxic
compounds commonly found in the ambient air, O3 is the most
prevalent, impairing crop production and injuring native vegetation and
ecosystems more than any other air pollutant'' (U.S. EPA, 1989, 1996a).
As was established in prior reviews, O3 can interfere with
carbon gain (photosynthesis) and allocation of carbon within the plant.
As a result of decreased carbohydrate availability, fewer carbohydrates
are available for plant growth, reproduction, and/or yield. For seed-
bearing plants, these reproductive effects will culminate in reduced
seed production or yield (U.S. EPA, 1996a, pp. 5-28 and 5-29). Recent
studies, assessed in the ISA, together with this longstanding and well-
established literature on O3-related vegetation effects,
further contribute to the coherence and consistency of the vegetation
effects evidence. As described in the ISA, a variety of factors in
natural environments can either mitigate or exacerbate predicted
O3-plant interactions and are recognized sources of
uncertainty and variability. These include: (1) Multiple genetically
influenced determinants of O3 sensitivity; (2) changing
sensitivity to O3 across vegetative growth stages; (3) co-
occurring stressors and/or modifying environmental factors (U.S. EPA,
2013a, section 9.4.8).
    Among the studies of vegetation effects, the ISA recognizes
controlled chamber studies as the best method for isolating or
characterizing the role of O3 in inducing the observed plant
effects, and in assessing plant response to O3 at the finer
scales (U.S. EPA, 2013a, sections 9.2 and 9.3). Recent controlled
studies have focused on a variety of plant responses to O3
including the underlying mechanisms governing such responses. These
mechanisms include: (1) Reduced carbon dioxide uptake due to stomatal
closure (U.S. EPA, 2013a, section <IP_ADDRESS>); (2) the upregulation of
genes associated with plant defense, signaling, hormone synthesis and
secondary metabolism (U.S. EPA, 2013a, section <IP_ADDRESS>); (3) the down
regulation of genes related to photosynthesis and general metabolism
(U.S. EPA, 2013a, section <IP_ADDRESS>); (4) the loss of carbon assimilation
capacity due to declines in the quantity and activity of key proteins
and enzymes (U.S. EPA, 2013a, section <IP_ADDRESS>); and (5) the negative
impacts on the efficiency of the photosynthetic light reactions (U.S.
EPA, 2013a, section <IP_ADDRESS>). As described in the ISA, these new
studies ``have increased knowledge of the molecular, biochemical and
cellular mechanisms occurring in plants in response to O3'',
adding ``to the understanding of the basic biology of how plants are
affected by oxidative stress . . .'' (U.S. EPA, 2013a, p. 9-11). The
ISA further concludes that controlled studies ``have clearly shown that
exposure to O3 is causally linked to visible foliar injury,
decreased photosynthesis, changes in reproduction, and decreased
growth'' in many species of vegetation (U.S. EPA, 2013a, p. 1-15).
    Such effects at the plant scale can also be linked to an array of
effects at larger spatial scales. For example, recent field studies at
larger spatial scales, together with previously available evidence,
support the controlled exposure study results and indicate that
``ambient O3 exposures can affect ecosystem productivity,
crop yield, water cycling, and ecosystem community composition'' (U.S.
EPA, 2013a, p. 1-15; Chapter 9, section 9.4). The current body of
O3 welfare effects evidence confirms the conclusions reached
in the last review on the nature of O3-induced welfare
effects and is summarized in the ISA as follows (U.S. EPA, 2013a, p. 1-
8).

    The welfare effects of O3 can be observed across
spatial scales, starting at the subcellular and cellular level, then
the whole plant and finally, ecosystem-level processes. Ozone
effects at small spatial scales, such as the leaf of an individual
plant, can result in effects along a continuum of larger spatial
scales. These effects include altered rates of leaf gas exchange,
growth, and reproduction at the individual plant level, and can
result

in broad changes in ecosystems, such as productivity, carbon
storage, water cycling, nutrient cycling, and community composition.

    Based on its assessment of this extensive body of science, the ISA
determines that, with respect to vegetation and ecosystems, a causal
relationship exists between exposure to O3 in ambient air
and visible foliar injury effects on vegetation, reduced vegetation
growth, reduced productivity in terrestrial ecosystems, reduced yield
and quality of agricultural crops and alteration of below-ground
biogeochemical cycles \181\ (U.S. EPA, 2013a, Table 1-2). In
consideration of the evidence of O3 exposure and alterations
in stomatal performance, ``which may affect plant and stand
transpiration and therefore possibly affecting hydrological cycling,''
the ISA concludes that ``[a]lthough the direction of the response
differed among studies,'' the evidence is sufficient to conclude a
likely causal relationship between O3 exposure and the
alteration of ecosystem water cycling (U.S. EPA, 2013a, section 2.6.3).
The ISA also concludes that the evidence is sufficient to conclude a
likely causal relationship between O3 exposure and the
alteration of community composition of some terrestrial ecosystems
(U.S. EPA, 2013a, section 2.6.5). Related to the effects on vegetation
growth, productivity and, to some extent, below-ground biogeochemical
cycles, the ISA additionally determines that a likely causal
relationship exists between exposures to O3 in ambient air
and reduced carbon sequestration (also termed carbon storage) \182\ in
terrestrial ecosystems (U.S. EPA, 2013a, p. 1-10 and section 2.6.2).
Modeling studies available in this review consistently found negative
impacts of O3 on carbon sequestration, although the severity
of impact was influenced by ``multiple interactions of biological and
environmental factors'' (U.S. EPA, 2013a, p. 2-39).
---------------------------------------------------------------------------

    \181\ Based on studies focused on O3-associated
alterations in quality and quantity of carbon input to soil,
microbial community composition, and carbon and nutrient cycling,
the ISA concludes that the evidence is sufficient ``to infer that
there is a causal relationship between O3 exposure and
the alteration of below-ground biogeochemical cycles'' (U.S. EPA,
2013a, pp. 2-41 to 2-42).
    \182\ The terms sequestration and storage are used somewhat
interchangeably in the ISA and other documents in this review.
---------------------------------------------------------------------------

    The ISA notes that ``[t]he suppression of ecosystem [carbon] sinks
results in more [carbon dioxide] accumulation in the atmosphere'' and
that a recent study has suggested that ``the indirect radiative forcing
caused by O3 exposure through lowering the ecosystem
[carbon] sink could have an even greater impact on global warming than
the direct radiative forcing of O3'' (U.S. EPA, 2013a, p. 2-
39). With regard to direct radiative forcing, however, the ISA makes a
stronger causality conclusion that the evidence supports a causal
relationship between changes in tropospheric O3
concentrations and radiative forcing \183\ (U.S. EPA, 2013a, section
2.7.1). There are, however, ``large uncertainties in the magnitude of
the radiative forcing estimate attributed to tropospheric
O3, making the impact of tropospheric O3 on
climate more uncertain than the effect of the longer-lived greenhouse
gases'' (U.S. EPA, 2013a, p. 2-47). In this regard, the ISA observes
that ``radiative forcing does not take into account the climate
feedbacks that could amplify or dampen the actual surface temperature
response,'' that ``[q]uantifying the change in surface temperature
requires a complex climate simulation in which all important feedbacks
and interactions are accounted for'' and that ``[t]he modeled surface
temperature response to a given radiative forcing is highly uncertain
and can vary greatly among models and from region to region within the
same model'' (U.S. EPA, 2013a, p. 2-47). Even with these uncertainties,
the ISA notes that ``global climate models indicate that tropospheric
O3 has contributed to observed changes in global mean and
regional surface temperatures'' and as a result of such evidence
presented in climate modeling studies, concludes that there is likely
to be a causal relationship between changes in tropospheric
O3 concentrations and effects on climate (U.S. EPA, 2013a,
p. 2-47). The ISA additionally notes, however, that ``[i]mportant
uncertainties remain regarding the effect of tropospheric O3
on future climate change'' (U.S. EPA, 2013a, p. 10-31).
---------------------------------------------------------------------------

    \183\ Radiative forcing by a greenhouse gas or aerosol is a
metric used to quantify the change in balance between radiation
coming into and going out of the atmosphere caused by the presence
of that substance. For example, a reduction in outgoing infrared
radiation has been associated with O3 by satellite data
(U.S. EPA, 2013a, p. 2-47).
---------------------------------------------------------------------------

    Given the strong evidence base, and findings of causal or likely
causal relationships with O3 in ambient air, including the
quantitative assessments of relationships between O3
exposure and occurrence and magnitude of effects, we give a primary
focus to three main areas of effects. The three main areas, for which
the evidence is summarized in more detail below, are: 1) impacts on
tree growth, productivity and carbon storage (section IV.B.1.b); 2)
crop yield loss (section IV.B.1.c); and 3) visible foliar injury
(section IV.B.1.a). Consideration of these three areas includes, as
appropriate, consideration of evidence of associated effects at larger
scales, including ecosystems, and on associated ecosystem services.
    With regard to biologically based indices of exposure pertinent to
O3 effects on vegetation, the ISA states the following (U.S.
EPA, 2013a, p. 2-44).

    The main conclusions from the 1996 and 2006 O3 AQCDs
[Air Quality Criteria Documents] regarding indices based on ambient
exposure remain valid. These key conclusions can be restated as
follows: ozone effects in plants are cumulative; higher
O3 concentrations appear to be more important than lower
concentrations in eliciting a response; plant sensitivity to
O3 varies with time of day and plant development stage;
[and] quantifying exposure with indices that cumulate hourly
O3 concentrations and preferentially weight the higher
concentrations improves the explanatory power of exposure/response
models for growth and yield, over using indices based on mean and
peak exposure values.

    The long-standing body of available evidence upon which these
conclusions are based provides a wealth of information on aspects of
O3 exposure that are important in influencing plant
response. Specifically, a variety of ``factors with known or suspected
bearing on the exposure-response relationship, including concentration,
time of day, respite time, frequency of peak occurrence, plant
phenology, predisposition, etc.,'' have been identified (U.S. EPA,
2013a, section 9.5.2). In addition, the importance of the duration of
the exposure and the relatively greater importance of higher
concentrations over lower concentrations in determining plant response
to O3 have been consistently well documented (U.S. EPA,
2013a, section 9.5.3). Much of this evidence was assessed in the 1996
AQCD (U.S. EPA, 1996a), while more recent work substantiating this
evidence is assessed in the subsequent 2006 AQCD and 2013 ISA.
    Understanding of the biological basis for plant response to
O3 exposure led to the development of a large number of
``mathematical approaches for summarizing ambient air quality
information in biologically meaningful forms for O3
vegetation effects assessment purposes'' (U.S. EPA, 2013a, section
9.5.3), including those that cumulate exposures over some specified
period while weighting higher concentrations more than lower (U.S. EPA,
2013a, section 9.5.2). As with any summary statistic, these exposure
indices retain information on some, but not all, characteristics of the
original observations. The 1996 AQCD contained an extensive review of
the published

literature on different types of exposure-response metrics, including
comparisons between metrics, from which the 1996 Staff Paper built its
assessment of forms appropriate to consider in the context of the
secondary NAAQS review. The result of these assessments was a decision
by the EPA to focus on cumulative, concentration-weighted indices,
which were recognized as the most appropriate biologically based
metrics to consider in this context, with attention given primarily to
two cumulative, concentration-weighted index forms: SUM06 and
W126.\184\
---------------------------------------------------------------------------

    \184\ The SUM06 index is a threshold-based approach described as
the sum of all hourly O3 concentrations greater or equal
to 0.06 ppm observed during a specified daily and seasonal time
window (U.S. EPA, 2013a, section 9.5.2). The W126 index is a non-
threshold approach described as the sigmoidally weighted sum of all
hourly O3 concentrations observed during a specified
daily and seasonal time window, where each hourly O3
concentration is given a weight that increases from zero to one with
increasing concentration (U.S. EPA, 2013a, section 9.5.2).
---------------------------------------------------------------------------

    In both the 1997 and 2008 reviews, the EPA concluded that the risk
to vegetation comes primarily from cumulative exposures to
O3 over a season or seasons \185\ and focused on metrics
intended to characterize such exposures: SUM06 (61 FR 65716, December
13, 1996) and W126 (72 FR 37818, July 11, 2007) in the 1997 and 2008
reviews, respectively. Although in both reviews the policy decision was
made to set the secondary standard to be identical to a revised primary
standard (with an 8-hour averaging time), the Administrator, in both
cases, also concluded, consistent with CASAC advice, that a cumulative,
seasonal index was the most biologically relevant way to relate
exposure to plant growth response (62 FR 38856, July 18, 1997; 73 FR
16436, March 27, 2008; 75 FR 2938, January 19, 2010). This approach for
characterizing O3 exposure concentrations that are
biologically relevant with regard to potential vegetation effects
received strong support from CASAC in the last review and again in this
review, including strong support for use of such a metric as the form
for the secondary standard (Henderson, 2006, 2008; Samet, 2010; Frey,
2014c).
---------------------------------------------------------------------------

    \185\ In describing the form as ``seasonal'', the EPA is
referring generally to the growing season of O3-sensitive
vegetation, not to the seasons of the year (i.e., spring, summer,
fall, winter).
---------------------------------------------------------------------------

    An alternative to using ambient exposure durations and
concentrations to predict plant response has been developed in recent
years, primarily in Europe, i.e., flux models. While ``some researchers
have claimed that using flux models can be used {sic{time}  to better
predict vegetation responses to O3 than exposure-based
approaches'' because flux models estimate the ambient O3
concentration that actually enters the leaf (i.e., flux or deposition)
(U.S. EPA, 2013a, p. 9-114), it is important to note that ``[f]lux
calculations are data intensive and must be carefully implemented''
(U.S. EPA, 2013a, p. 9-114). Further, the ISA states, ``[t]his uptake-
based approach to quantify the vegetation impact of O3
requires inclusion of those factors that control the diurnal and
seasonal O3 flux to vegetation (e.g., climate patterns,
species and/or vegetation-type factors and site-specific factors)''
(U.S. EPA, 2013a, p. 9-114). In addition to these data requirements,
each species has different amounts of internal detoxification potential
that may protect species to differing degrees. The lack of detailed
species- and site-specific data required for flux modeling in the U.S.
and the lack of understanding of detoxification processes have
continued to make this technique less viable for use in vulnerability
and risk assessments at the national scale in the U.S. (U.S. EPA,
2013a, section 9.5.4).
    Therefore, consistent with the ISA conclusions regarding the
appropriateness of considering cumulative exposure indices that
preferentially weight higher concentrations over lower for predicting
O3 effects of concern based on the long-established
conclusions and long-standing supporting evidence described above, and
in light of continued CASAC support, we continue to focus on cumulative
concentration-weighted indices as the most biologically relevant
metrics for consideration of O3 exposures eliciting
vegetation-related effects. Such a metric has an ``explanatory power''
that is improved ``over using indices based on mean and peak exposure
values'' (U.S. EPA, 2013a, section <IP_ADDRESS>, p. 2-44). In this review as
in the last review, we use the W126 cumulative, seasonal metric (U.S.
EPA, 2013a, sections <IP_ADDRESS> and 9.5.2) for consideration of the
effects evidence and in the exposure and risk analyses in the WREA.
    The subsections below summarize key aspects of the welfare effects
information for O3-elicited visible foliar injury (section
IV.B.1.a), effects on forest tree growth, productivity and carbon
storage (section IV.B.1.b) and reductions in crop yield (section
IV.B.1.c), as well as associated effects.
a. Visible Foliar Injury
    Visible foliar injury resulting from exposure to O3 has
been well characterized and documented over several decades of research
on many tree, shrub, herbaceous, and crop species (U.S. EPA, 2013a, p.
1-10; U.S. EPA, 2006a, 1996a, 1986, 1978). Additionally, O3-
induced visible foliar injury symptoms on certain plant species, such
as black cherry, yellow-poplar and common milkweed, are considered
diagnostic of exposure to O3 based on the consistent
association established with experimental evidence (U.S. EPA, 2013a, p.
1-10). The significance of O3 injury at the leaf and whole
plant levels depends on an array of factors, and therefore, it is
difficult to quantitatively relate visible foliar injury symptoms to
vegetation effects such as individual tree growth, or effects at
population or ecosystem levels (U.S. EPA, 2013a, p. 9-39). The ISA
notes that visible foliar injury ``is not always a reliable indicator
of other negative effects on vegetation'' (U.S. EPA, 2013a, p. 9-39).
Factors that influence the significance to the leaf and whole plant
include the amount of total leaf area affected, age of plant, size,
developmental stage, and degree of functional redundancy among the
existing leaf area (U.S. EPA, 2013a, section 9.4.2). Visible foliar
injury by itself is an indication of phytotoxicity due to O3
exposure, which occurs only when sensitive plants are exposed to
elevated O3 concentrations in a predisposing environment, a
major aspect of which is the lack of drought conditions during the year
such injury is assessed (U.S. EPA, 2013a, section 9.4.2).
    Recent research is consistent with previous conclusions and that
O3-induced visible foliar injury symptoms are well
characterized and considered diagnostic on certain bioindicator plant
species. Diagnostic usage for these plants has been verified
experimentally in exposure-response studies, using exposure
methodologies such as continuous stirred tank reactors, open-top
chambers (OTCs), and free-air carbon dioxide (and ozone) enrichment
(FACE). Although there remains a lack of robust exposure-response
functions that would allow prediction of visible foliar injury severity
and incidence under varying air quality and environmental conditions,
``experimental evidence has clearly established a consistent
association of the presence of visible foliar injury symptoms with
O3 exposure, with greater exposure often resulting in
greater and more prevalent injury'' (U.S. EPA, 2013a, section 9.4.2, p.
9-41). The research newly available in this review includes: 1)
controlled exposure studies conducted to test and verify the
O3

sensitivity and response of potential new bioindicator plant species;
2) multi-year field surveys in several National Wildlife Refuges (NWR)
documenting the presence of foliar injury in valued areas; and 3)
ongoing data collection and assessment by the U.S. Forest Service's
Forest Health Monitoring Forest Inventory and Analysis (USFS FHM/FIA)
program, including multi-year trend analysis (U.S. EPA, 2013a, section
9.4.2). These recent studies, in combination with the entire body of
available evidence, thus form the basis for the ISA determinations of a
causal relationship between ambient O3 exposure and the
occurrence of O3-induced visible foliar injury on sensitive
vegetation across the U.S. (U.S. EPA, 2013a, p. 9-42).
    Recently available evidence confirms the evidence available in
previous reviews that visible foliar injury can occur when sensitive
plants are exposed to elevated O3 concentrations in a
predisposing environment (i.e., adequate soil moisture) (U.S. EPA,
2013a, section 9.4.2). Recent evidence also continues to support
previous findings that indicated the occurrence of visible foliar
injury at cumulative ambient O3 exposures previously
examined.
    With regard to evidence from controlled exposure studies, a recent
study, using continuously stirred tank reactor chambers, evaluated the
occurrence of O3 characteristic visible foliar injury
symptoms on 28 species of plants that were suspected of being
O3 sensitive and most of which grow naturally throughout the
northeast and midwest U.S., including in national parks and wilderness
areas (U.S. EPA, 2013a, section <IP_ADDRESS>; Kline et al., 2008). Across
the 28 tested species, the study reported O3-induced
responses in 12, 20, 28 and 28 species at the 30, 60, 90 and 120 ppb
exposure concentrations,\186\ respectively; the plants were exposed for
7 hours per each weekday over 21 to 29 summer days (Kline et al.,
2008).
---------------------------------------------------------------------------

    \186\ Two of the target exposure levels, 30 and 60 ppb, fall
below the level of the current standard (75 ppb), although the
exposures were average concentrations for 7-hour exposures across
durations shorter than a month. Because the form of the current
standard targets peak concentrations in a season, an area that just
meets the current standard can be expected to have mean
concentrations well below that level due to variability in ambient
O3 concentrations.
---------------------------------------------------------------------------

    A string of recently published multi-year field studies provide a
complementary line of field-based evidence by documenting the incidence
of visible foliar injury symptoms on a variety of O3-
sensitive species over multiple years and across a range of cumulative,
seasonal exposure values in several eastern and midwestern NWRs (U.S.
EPA, 2013a, section <IP_ADDRESS>; Davis and Orendovici, 2006; Davis, 2007a,
b; Davis, 2009). Some of these studies also included information
regarding soil moisture stress using the Palmer Drought Severity Index
(PDSI). While environmental conditions and species varied across the
four NWRs, visible foliar injury was documented to a varying degree at
each site.
    By far the most extensive field-based dataset of visible foliar
injury incidence is that obtained by the USFS FHM/FIA biomonitoring
network program. A trend analysis of data from the sites located in the
Northeast and North Central U.S. for the 16 year period from 1994
through 2009 (Smith, 2012) describes evidence of visible foliar injury
occurrence in the field as well as some insight into the influence of
changes in air quality and soil moisture on visible foliar injury and
the difficulty inherent in predicting foliar injury response under
different air quality/soil moisture scenarios (Smith, 2012; U.S. EPA,
2013a, section <IP_ADDRESS>). Study results showed that incidence and
severity of foliar injury were dependent on local site conditions for
soil moisture availability and O3 exposure. Overall, there
was a declining trend in the incidence of visible foliar injury as peak
O3 concentrations declined, although the study additionally
indicated that moderate O3 exposures continued to cause
visible foliar injury at sites throughout the region (U.S. EPA, 2013a,
p. 9-40). In a similar assessment of the USFS FHM/FIA data in the West,
six years (2000 to 2005) of biomonitoring data, during a period where a
large proportion of California sites did not meet the current standard,
indicated O3-related visible foliar injury in 25-37% of
biosites in California (Campbell et al., 2007; U.S. EPA, 2013a, section
<IP_ADDRESS>).\187\ These recent studies provide additional evidence of
O3-induced visible foliar injury in many areas across the
U.S. and augment the EPA's understanding of O3-related
visible foliar injury and of factors, such as soil moisture, that
influence associations between O3 exposures or
concentrations and visible foliar injury.
---------------------------------------------------------------------------

    \187\ See: http://www.epa.gov/airtrends/values.html.
---------------------------------------------------------------------------

b. Effects on Forest Tree Growth, Productivity and Carbon Storage
    Ozone has been shown to affect a number of important U.S. tree
species with respect to growth, productivity, and carbon storage.
Ambient O3 concentrations have long been known to cause
decreases in photosynthetic rates and plant growth. As discussed in the
ISA, research published since the 2006 AQCD substantiates prior
conclusions regarding O3-related effects on forest tree
growth, productivity and carbon storage. The ISA states, ``previous
O3 AQCDs concluded that there is strong evidence that
exposure to O3 decreases photosynthesis and growth in
numerous plant species'' and that ``[s]tudies published since the 2008
review support those conclusions'' (U.S. EPA, 2013a, p. 9-42). The
available studies come from a variety of different study types that
cover an array of different species, effects endpoints, levels of
biological organization and exposure methods and durations. The
O3-induced effects at the plant scale may translate to the
ecosystem scale, with changes in productivity and carbon storage. As
stated in the ISA, ``[s]tudies conducted during the past four decades
have demonstrated unequivocally that O3 alters biomass
allocation and plant reproduction'' (U.S. EPA, 2013a, p. 1-10).
    The previously available strong evidence for trees includes robust
E-R functions for seedling relative biomass loss (RBL) \188\ in 11
species developed under the National Health and Environmental Effects
Research Laboratory-Western Ecology Division program. This series of
experiments used OTCs to study seedling growth response for a single
growing season under a variety of O3 exposures (ranging from
near background to well above current ambient concentrations) and
growing conditions (U.S. EPA, 2013a, section 9.6.2; Lee and Hogsett,
1996). The evidence from these studies shows that there is a wide range
in sensitivity across the studied species in the seedling growth stage
over the course of a single growing season, with some species being
extremely sensitive and others being very insensitive over the range of
cumulative O3 exposures studied (U.S. EPA, 2014c, Figure 5-
1). At the other end of the organizational spectrum, field-based
studies of species growing in natural stands have compared observed
plant response across a number of different sites and/or years when
exposed to varying ambient O3 exposure conditions. For
example, a study conducted in forest stands in the southern Appalachian
Mountains during a period when O3 concentrations exceeded
the current standard found that the cumulative

effects of O3 decreased seasonal stem growth (measured as a
change in circumference) by 30-50 percent for most of the examined tree
species (i.e., tulip poplar, black cherry, red maple, sugar maple) in a
high O3 year in comparison to a low O3 year (U.S.
EPA, 2013a, section <IP_ADDRESS>; McLaughlin et al., 2007a). The study also
reported that high ambient O3 concentrations can increase
whole-tree water use and in turn reduce late-season streamflow
(McLaughlin et al., 2007b; U.S. EPA, 2013a, p. 9-43).
---------------------------------------------------------------------------

    \188\ These functions for RBL estimate reduction in a year's
growth as a percentage of that expected in the absence of
O3 (U.S. EPA, 2013a, section 9.6.2; U.S. EPA, 2014b,
section 6.2).
---------------------------------------------------------------------------

    The magnitude of O3 impact on ecosystem productivity and
on forest composition can vary among plant communities based on several
factors including: the type of stand or community in which the
sensitive species occurs (e.g., single species versus mixed canopy),
the role or position of the species in the stand (e.g., dominant, sub-
dominant, canopy, understory), the sensitivity of co-occurring species
and environmental factors (e.g., drought and other factors). For
example, O3 has been found to have little impact on white
fir, but to greatly reduce growth of ponderosa pine in southern
California, and cause decreased net primary production of most forest
types in the Mid-Atlantic region, although only small impacts on
spruce-fir forest (U.S. EPA, 2013a, section <IP_ADDRESS>).
    As noted above, long-standing evidence has demonstrated that
O3 alters biomass allocation and plant reproduction (U.S.
EPA, 2013a, section 9.4.3). Several studies published since the 2006
O3 AQCD further demonstrate that O3 can alter
reproductive processes in herbaceous and woody plant species, such as
the timing of flowering and the number of flowers, fruits and seeds
(U.S. EPA, 2013a, section <IP_ADDRESS>). Further, limited evidence in
previous reviews reported that vegetation effects from a single year of
exposure to elevated O3 could be observed in the following
year. For example, growth affected by a reduction in carbohydrate
storage in one year may result in the limitation of growth in the
following year. Such ``carry-over'' effects have been documented in the
growth of some tree seedlings and in roots (U.S. EPA, 2013a, section
9.4.8; Andersen, et al., 1997). In the current review, additional
field-based evidence expands the EPA's understanding of the
consequences of single and multi-year O3 exposures in
subsequent years. A number of studies were conducted at a planted
forest at the Aspen FACE site in Wisconsin. These studies, which
occurred in a field setting (more similar to natural forest stands than
OTC studies), observed tree growth responses when grown in single or
two species stands within 30-m diameter rings and exposed over a period
of ten years to existing ambient conditions and elevated O3
concentrations. Some studies indicate the potential for carry-over
effects, such as those showing that the effects of O3 on
birch seeds (reduced weight, germination, and starch levels) could lead
to a negative impact on species regeneration in subsequent years, and
that the effect of reduced aspen bud size might have been related to
the observed delay in spring leaf development. These effects suggest
that elevated O3 exposures have the potential to alter
carbon metabolism of overwintering buds, which may have subsequent
effects in the following year (Darbah, et al., 2008, 2007; Riikonen et
al., 2008; U.S. EPA, 2013a, section 9.4.3). Other studies found that,
in addition to affecting tree heights, diameters, and main stem volumes
in the aspen community, elevated O3 over a 7-year study
period was reported to increase the rate of conversion from a mixed
aspen-birch community to a community dominated by the more tolerant
birch, leading the authors to conclude that elevated O3 may
alter intra- and inter-species competition within a forest stand (U.S.
EPA, 2013a, section 9.4.3; Kubiske et al., 2006; Kubiske et al., 2007).
These studies confirm earlier FACE results of aspen growth reductions
from a 6-7 year exposure to elevated O3 and of cumulative
biomass impacts associated with changes in annual production in studied
tree communities (U.S. EPA, 2013a, section 9.4.3; King et al., 2005).
    In addition to individual studies, recent meta-analyses have
quantitatively analyzed the effect of O3 on trees across
large numbers of studies. In particular, a recent meta-analysis of 55
peer reviewed studies from the past 40 years indicates a negative
relationship between O3 concentrations in the northern
hemisphere during that period and stomatal conductance and
photosynthesis, which decreases growth (U.S. EPA, 2013a, section
<IP_ADDRESS>; Wittig et al., 2007). In this analysis, younger trees (less
than 4 years) were affected less by O3 than older trees
(U.S. EPA, 2013a, section <IP_ADDRESS>; Wittig, et al., 2007). A second
meta-analysis that quantitatively compiled 263 peer-reviewed studies
``demonstrates the coherence of O3 effects across numerous
studies and species that used a variety of experimental techniques, and
these results support the conclusion of the previous AQCD that exposure
to O3 decreases plant growth'' (U.S. EPA, 2013a, p. 9-43).
Other meta-analyses have examined the effect of O3 exposure
on root growth and generally found that O3 exposure reduced
carbon allocated to roots (U.S. EPA, 2013a, pp. 9-45 to 9-46).
    As noted above, robust E-R functions have been developed for 11
tree species (black cherry, Douglas fir, loblolly pine, ponderosa pine,
quaking aspen, red alder, red maple, sugar maple, tulip poplar,
Virginia pine, white pine) from the extensive evidence base of
O3-induced growth effects that was also available and relied
upon in the previous review. While the species for which robust E-R
functions have been developed represent only a small fraction (0.8
percent) of the total number of native tree species in the contiguous
U.S. (1,497), this small subset includes eastern and western species,
deciduous and coniferous species, and species that grow in a variety of
ecosystems and represent a range of tolerance to O3 (U.S.
EPA, 2013a, section 9.6.2; U.S. EPA, 2014b, section 6.2, Figure 6-2,
Table 6-1). Each of these species were studied in OTCs, with most
species studied multiple times under a wide range of exposure and/or
growing conditions, with separate E-R functions developed for each
combination of species, exposure condition and growing condition
scenario. These species-specific composite E-R functions have been
successfully used to predict the biomass loss response from tree
seedling species over a range of cumulative exposure conditions (U.S.
EPA, 2013a, section 9.6.2). The 11 robust composite E-R functions
available in the last review, as well as the E-R for eastern cottonwood
(derived from a field study in which O3 and climate
conditions were not controlled), are described in the ISA and graphed
in the WREA to illustrate the predicted responses of these species over
a wide range of cumulative exposures (U.S. EPA, 2014b, section 6.2,
Table 6-1 and Figure 6-2; U.S. EPA, 2013a, section 9.6.2). For some of
these species, the E-R function is based on a single study (e.g., red
maple), while for other species there were as many as 11 studies
available (ponderosa pine). In total, the E-R functions developed for
these 12 species (the 11 with robust composite E-R functions plus
eastern cottonwood) reflect 52 tree seedling studies. A stochastic
analysis in WREA, summarized in section IV.C below, indicates the
potential for within species variability to contribute appreciably to
estimates for each species. Consideration of biomass loss estimates in
the PA and in discussions

below, however, is based on conventional method and focuses on
estimates for the 11 species for which the robust datasets from OTC
experiments are available, in consideration of CASAC advice.\189\
---------------------------------------------------------------------------

    \189\ The CASAC cautioned EPA against placing too much emphasis
on the eastern cottonwood data, stating that while the cottonwood
data are important results, they are not as strong as those from
other experiments that developed E-R functions based on controlled
O3 exposure in OTCs; they are from a single gradient
study that did not control for O3 and climatic conditions
and they show extreme sensitivity to O3 compared to other
studies (Frey, 2014c, p. 10).
---------------------------------------------------------------------------

c. Crop Yield Loss
    The ``detrimental effect of O3 on crop production has
been recognized since the 1960s'' (U.S. EPA, 2013a, p. 1-10, section
9.4.4). On the whole, the newly available evidence supports previous
conclusions that exposure to O3 decreases growth and yield
of crops. The ISA describes average crop yield loss reported across a
number of recently published meta-analyses and identifies several new
exposure studies that support prior findings for a variety of crops of
decreased yield and biomass with increased O3 exposure (U.S.
EPA, 2013a, section <IP_ADDRESS>, Table 9-17). Studies have also ``linked
increasing O3 concentration to decreased photosynthetic
rates and accelerated aging in leaves, which are related to yield'' and
described effects of O3 on crop quality, such as nutritive
quality of grasses, macro- and micronutrient concentrations in fruits
and vegetable crops and cotton fiber quality (U.S. EPA, 2013a, p. 1-10,
section 9.4.4). The findings of the newly available studies do not
change the basic understanding of O3-related crop yield loss
since the last review and little additional information is available in
this review on factors that influence associations between
O3 levels and crop yield loss (U.S. EPA, 2013a, section
9.4.4.).
    The new evidence has strengthened support for previously
established E-R functions for 10 crops (barley, field corn, cotton,
kidney bean, lettuce, peanut, potato, grain sorghum, soybean and winter
wheat), reducing two important areas of uncertainty, especially for
soybean. The established E-R functions were developed from OTC-type
experiments (U.S. EPA, 2013a, section 9.6.3; U.S. EPA, 2014b, section
6.2; U.S. EPA, 2014c, Figure 5-4). In this review, the ISA included an
analysis comparing OTC data for soybean from the National Crop Loss
Assessment Network (NCLAN) with field-based data from SoyFACE (Soybean
Free Air Concentration Enrichment) studies (U.S. EPA, 2013a, section
<IP_ADDRESS>).\190\ Yield loss in soybean from O3 exposure at the
SoyFACE field experiment was reliably predicted by soybean E-R
functions developed from NCLAN data, demonstrating a robustness of the
E-R functions developed with NCLAN data to predict relative yield loss
from O3 exposure. A second area of uncertainty that was
reduced is that regarding the application of the NCLAN E-R functions,
developed in the 1980s, to more recent cultivars currently growing in
the field. Recent studies, especially those focused on soybean, provide
little evidence that crops are becoming more tolerant of O3
(U.S. EPA, 2006a; U.S. EPA, 2013a). A meta-analysis of 53 studies found
consistent deleterious effects of O3 exposures on soybean
from studies published between 1973 and 2001 (Morgan et al., 2003).
Further, Betzelberger et al. (2010) utilized the SoyFACE facility to
compare the impact of elevated O3 concentrations across 10
soybean cultivars to investigate intraspecific variability of the
O3 response, finding that the E-R functions derived for
these 10 current cultivars were similar to the response functions
derived from the NCLAN studies conducted in the 1980s (Heagle, 1989),
``suggesting there has not been any selection for increased tolerance
to O3 in more recent cultivars'' (U.S. EPA, 2013a, p. 9-59).
Additionally, the ISA comparisons of NCLAN and SoyFACE data referenced
above ``confirm that the response of soybean yield to O3
exposure has not changed in current cultivars'' (U.S. EPA, 2013a, p. 9-
59; section <IP_ADDRESS>). Thus, the evidence available in the current
review has reduced uncertainties in two areas with regard to the use of
E-R functions for soybean crop yield loss.
---------------------------------------------------------------------------

    \190\ The NCLAN program, which was undertaken in the early to
mid-1980s, assessed multiple U.S. crops, locations, and
O3 exposure levels, using consistent methods, to provide
the largest, most uniform database on the effects of O3
on agricultural crop yields (U.S. EPA 1996a; U.S. EPA, 2006a; U.S.
EPA, 2013a, sections 9.2, 9.4, and 9.6, Frey, 2014c, p. 9). The
SoyFACE experiment was a chamberless (or free-air) field-based
exposure study conducted in Illinois from 2001-2009 (U.S. EPA,
2013a, section 9.2.4).
---------------------------------------------------------------------------

    During past O3 NAAQS reviews, there were very few
studies that estimated O3 impacts on crop yields at large
geographical scales (i.e., regional, national or global). Recent
modeling studies of the impact of O3 concentrations
historically found that increased O3 in the past generally
reduced crop yield, but the impacts varied across regions and crop
species, with the largest O3-induced crop yield losses
estimated to have occurred in high-production areas that had been
exposed to elevated O3 concentrations, such as the Midwest
and the Mississippi Valley regions of the U.S. (U.S. EPA, 2013a,
Section <IP_ADDRESS>). Among affected crop species, the estimated yield loss
for wheat and soybean were higher than rice and maize (i.e., field
corn). Additionally, satellite and ground-based O3
measurements have assessed soybean yield loss estimated to result from
O3 over the continuous area of Illinois, Iowa, and
Wisconsin, finding a relationship which correlates well with the
previous results from FACE- and OTC-type experiments (U.S. EPA, 2013a,
section <IP_ADDRESS>).
    Thus, consistent with the conclusions of the 1996 and 2006 CDs, the
current ISA concludes that O3 concentrations in ambient air
can reduce the yield of major commodity crops in the U.S. and focuses
on use of crop E-R functions based on OTC experiments to characterize
the quantitative relationship between ambient O3
concentrations and yield loss (U.S. EPA, 2013a, section 9.4.4). In the
PA, as summarized in sections IV.D and IV.E below, relative yield loss
(RYL) is estimated for 10 different crops using the individual E-R
functions described in the WREA \191\ (U.S. EPA, 2014b, section 6.2;
U.S. EPA, 2014c, section 6.3).
---------------------------------------------------------------------------

    \191\ These functions for RYL estimate reduction in a year's
growth as a percentage of that expected in the absence of
O3 (U.S. EPA, 2013a, section 9.6.2; U.S. EPA, 2014b,
section 6.2).
---------------------------------------------------------------------------

2. Potential Impacts on Public Welfare
    The magnitude of a public welfare impact or the degree to which it
may be considered adverse is dependent upon the nature and severity of
the specific welfare or ecological effect, the use or service (and
value) of the affected ecosystem and the relevance and significance of
that use \192\ to the public welfare. In the preamble of the 2012 final
notice of rulemaking on the secondary standards for oxides of nitrogen
and sulfur (NOx/SOx), the EPA stated that ``[a]n evaluation of
adversity to the public welfare might consider the likelihood, type,
magnitude, and spatial scale of the effect, as well as the potential
for recovery and any uncertainties relating to these conditions'' (77
FR 20232, April 3,

2012). The EPA additionally stated that ``[c]onceptually, changes in
ecosystem services may be used to aid in characterizing a known or
anticipated adverse effect to public welfare'' (77 FR 20232, April 3,
2012).\193\
---------------------------------------------------------------------------

    \192\ Ecosystem services have been defined as ``the benefits
that people obtain from ecosystems'' (U.S. EPA, 2013a, Preamble, p.
1xxii; UNEP, 2003) and thus are an aspect of the use of a type of
vegetation or ecosystem. Similarly, a definition used for the
purposes of EPA benefits assessments states that ecological goods
and services are the ``outputs of ecological functions or processes
that directly or indirectly contribute to social welfare or have the
potential to do so in the future'' and that ``[s]ome outputs may be
bought and sold, but most are not marketed'' (U.S. EPA, 2006b).
    \193\ Ecosystem services analyses were one of the tools used in
that review to inform the decisions made with regard to adequacy and
as such, were used in conjunction with other considerations in the
discussion of adversity to public welfare (77 FR 20241).
---------------------------------------------------------------------------

    Potential public welfare impacts associated with ecosystems and
associated services have a range of dimensions, including spatial,
temporal, and social, and these likely will vary depending on the type
of effect being characterized. For example, ecosystems can cover a
range of spatial scales, and the services they provide can accrue
locally or be distributed more broadly, such as when crops are sold and
eaten locally and/or also sold in regional, national and world markets.
Accordingly, impacts can be localized or more widely distributed.
Further, ecosystem services can be realized over a range of temporal
scales from immediate up to longer term. The size of the societal unit
receiving benefits from ecosystem services can also vary dramatically.
For example, a national park can provide direct recreational services
to the thousands of visitors that come each year, but also provide an
indirect value to the millions who may not visit but receive
satisfaction from knowing it exists and is preserved for the future
(U.S. EPA, 2014b, chapter 5, section 5.5.1).
    As recognized in the last review, judgments regarding adverse
effects to the public welfare depend on the intended use for and
significance of the affected vegetation, ecological receptors,
ecosystems and resources to the public welfare (73 FR 16496, March 27,
2008).\194\ For example, a number of different types of locations
provide services of special significance to the public welfare. As
emphasized in previous O3 NAAQS decisions, and summarized in
section IV.A above, Class I areas and other parks have been afforded
special federal protection to preserve services that provide for the
enjoyment of these resources for current and future generations.
Surveys have indicated that Americans rank as very important the
existence of the resource, the option or availability of the resource
and the ability to bequest or pass on to future generations (Cordell et
al., 2008). These and other services provided by Class I areas and
other areas that have been afforded special protection can flow in part
or entirely from the vegetation that grows there. Aesthetic value and
outdoor recreation depend on the perceived scenic beauty of the
environment. Many outdoor recreation activities directly depend on the
scenic value of the area, in particular scenic viewing, wildlife-
watching, hiking, and camping (U.S. EPA, 2014b, chapters 5 and 7).
Further, analyses have reported that the American public values--in
monetary as well as nonmonetary ways--the protection of forests from
air pollution damage. In fact, studies that have assessed willingness-
to-pay for spruce-fir forest protection in the southeastern U.S. from
air pollution and insect damage have found that values held by the
survey respondents for the more abstract services (existence, option
and bequest) were greater than those for recreation or other services
(U.S. EPA, 2014b, Table 5-6; Haefele et al., 1991; Holmes and Kramer,
1995).
---------------------------------------------------------------------------

    \194\ As noted in section IV.A above, in judgments regarding
public welfare significance in the last review, emphasis was placed
on vegetation and ecosystem effects to sensitive species that are
known to or are likely to occur in federally protected areas such as
national parks and other Class I areas, or on lands set aside by
states, tribes and public interest groups to provide similar
benefits to the public welfare (73 FR 16496, March 27, 2008; 75 FR
3023-24, January 19, 2010).
---------------------------------------------------------------------------

    There are several potential public welfare impacts related to the
three main categories of O3 effects on vegetation (i.e.,
effects on tree growth, productivity and carbon storage; crop yield
loss; and, visible foliar injury, as described in section IV.B.1 above)
and their associated ecosystem services. At the same time, these three
categories of effects differ with regard to aspects important to
judging their public welfare significance. Judgments regarding crop
yield loss, for example, depend on considerations related to the heavy
management of agriculture in the U.S., while judgments regarding the
other categories of effects generally relate to considerations
regarding forested areas. For example, while both tree growth-related
effects and visible foliar injury have the potential to be significant
to the public welfare through impacts in Class I and other protected
areas, they differ in how they might be significant and with regard to
the clarity of the data which describes the relationship between the
effect and the services potentially affected.
    With regard to effects on tree growth, reduced growth is associated
with effects on an array of ecosystem services including reduced
productivity, altered forest and forest community (plant, insect and
microbe) composition, reduced carbon storage and altered water cycling
(U.S. EPA, 2013a, Figure 9-1, sections <IP_ADDRESS> and <IP_ADDRESS>; U.S. EPA,
2014b, section 6.1). For example, forest or forest community
composition can be affected through O3 effects on growth and
reproductive success of sensitive species in the community, with the
extent of compositional changes dependent on factors such as
competitive interactions (U.S. EPA, 2013a, sections 9.4.3 and <IP_ADDRESS>).
Depending on the type and location of the affected ecosystem, services
benefitting the public in other ways can be affected as well. For
example, other services valued by people that can be affected by
reduced tree growth, productivity and carbon storage include aesthetic
value, food, fiber, timber, other forest products, habitat,
recreational opportunities, climate and water regulation, erosion
control, air pollution removal, hydrologic and fire regime
stabilization (U.S. EPA 2013a, sections <IP_ADDRESS> and <IP_ADDRESS>; U.S. EPA,
2014b, section 6.1, Figure 6-1, section 6.4, Table 6-13). Further,
impacts on some of these services (e.g., forest or forest community
composition) may be considered of greater public welfare significance
when occurring in Class I or other protected areas.
    Consideration of the magnitude of tree seedling growth effects that
might cause or contribute to adverse effects for trees, forests,
forested ecosystems or the public welfare is complicated by aspects of,
or limitations in, the available information. For example, the evidence
on tree seedling growth effects, deriving from the E-R functions for 11
species, provides no clear threshold or breakpoint in the response to
O3 exposure. Additionally, there are no established
relationships between magnitude of tree seedling growth reduction and
forest ecosystem impacts and, as noted in section IV.B.1.b above, other
factors can influence the degree to which O3-induced growth
effects in a sensitive species affect forest and forest community
composition and other ecosystem service flows from forested ecosystems.
These include: 1) the type of stand or community in which the sensitive
species is found (i.e., single species versus mixed canopy); 2) the
role or position the species has in the stand (i.e., dominant, sub-
dominant, canopy, understory); 3) the O3 sensitivity of the
other co-occurring species (O3 sensitive or tolerant); and
4) environmental factors, such as soil moisture and others. The lack of
such established relationships complicates judgments as to the extent
to which different amounts of tree seedling growth would be significant
to the public welfare and thus an important consideration in the level
of protection for the secondary standard.
    During the 1997 review of the secondary standard, views related to

this issue were provided by a 1996 workshop of 16 then-leading
scientists in the context of discussing their views for a secondary
O3 standard (Heck and Cowling, 1997). In their consideration
of tree growth effects as an indicator for forest ecosystems and crop
yield reduction as an indicator of agricultural systems, the workshop
participants identified annual percentages, of RBL for forest tree
seedlings and RYL for agricultural crops, considered important to their
judgments on the standard. With regard to forest ecosystems and
seedling growth effects as an indicator, the participants selected a
range of 1-2% RBL per year ``to avoid cumulative effects of yearly
reductions of 2%.'' With regard to crops, they indicated an interest in
protecting against crop yield reductions of 5% RYL yet noted
uncertainties surrounding such a percentage which led them to
identifying 10% RYL for the crop yield endpoint (Heck and Cowling,
1997). The workshop report provides no explicit rationale for the
percentages identified (2% RBL and 5% or 10% RYL); nor does it describe
their connection to ecosystem impacts of a specific magnitude or type
and judgments on significance of the effects for public welfare, e.g.,
taking into consideration the intended use and significance of the
affected vegetation (Heck and Cowling, 1997). In recognition of the
complexity of assessing the adversity of tree growth effects and
effects on crop yield in the broader context of public welfare, the
EPA's consideration of those effects in both the 1997 and 2008 reviews
extended beyond the consideration of various benchmark responses for
the studied species, and with regard to crops, additionally took note
of their extensive management (62 FR 38856, July 18, 1997; 73 FR 16436,
March 27, 2008).
    While, as noted above, public welfare benefits of forested lands
can be particular to the type of area in which the forest occurs, some
of the potential public welfare benefits associated with forest
ecosystems are not location dependent. A potentially extremely valuable
ecosystem service provided by forested lands and for which the ISA
concludes a likely causal relationship with O3 in ambient
air is carbon storage, a regulating service that is ``of paramount
importance for human society'' (U.S. EPA, 2013a, section <IP_ADDRESS> and p.
9-37). The service of carbon storage is potentially important to the
public welfare no matter in what location the sensitive trees are
growing, or what their intended current or future use. In other words,
the benefit exists as long as the tree is growing, regardless of what
additional functions and services it provides.
    Another example of locations potentially vulnerable to
O3-related impacts but not necessarily identified for such
protection might be forested lands, both public and private, where
trees are grown for timber production, particularly where they are
dominated by a single timber species stand that is sensitive to
O3, such as ponderosa pine. Further, forests in urbanized
areas provide a number of services that are important to the public in
those areas, including air pollution removal, cooling of the heat
island effect, and beautification (U.S. EPA, 2014b, section 6.6.2 and
Appendix 6D; Akbari, 2002).\195\ The presence of O3-
sensitive trees in such areas may place them at risk from elevated
O3 exposures, contributing to potential impacts on important
services provided by urban forests (U.S. EPA, 2014b, sections 6.6.2 and
6.7). There are many other tree species, such as species used in the
USFS biomonitoring network, and various ornamental and agricultural
species (i.e., Christmas trees, fruit and nut trees) that provide
ecosystem services that may be judged important to the public welfare
but whose vulnerability to impacts from O3 on tree growth,
productivity and carbon storage has not been quantitatively
characterized (U.S. EPA, 2014b, Chapter 6; Abt Associates, 1995).
---------------------------------------------------------------------------

    \195\ For example, see http://www.fs.fed.us/research/urban/environmental-justice.php.
---------------------------------------------------------------------------

    As noted above, in addition to tree growth-related effects,
O3-induced visible foliar injury also has the potential to
be significant to the public welfare through impacts in Class I and
other similarly protected areas. Visible foliar injury is a visible
bioindicator of O3 exposure in species sensitive to this
effect, with the injury affecting the physical appearance of the plant.
Accordingly visible foliar injury surveys are used by federal land
managers as tools in assessing potential air quality impacts in Class I
areas. These surveys may focus on plant species that have been
identified as potentially sensitive air quality related values (AQRVs)
due to their sensitivity to O3-induced foliar injury (USFS,
NPS, FWS, 2010). An AQRV is defined by the National Park Services as a
``resource, as identified by the FLM for one or more Federal areas that
may be adversely affected by a change in air quality'' and the resource
``may include visibility or a specific scenic, cultural, physical,
biological, ecological, or recreational resource identified by the FLM
for a particular area'' (USFS, NPS, USFWS, 2010).\196\ No criteria have
been established, however, regarding a level or prevalence of visible
foliar injury considered to be adverse to the affected vegetation, and,
as noted in section IV.B.1.a above, there is not a clear relationship
between visible foliar injury and other effects, such as reduced growth
and productivity. Thus, key considerations with regard to public
welfare significance of this endpoint have related to qualitative
consideration of the plant's aesthetic value in protected forested
areas. Depending on the extent and severity, O3-induced
visible foliar injury might be expected to have the potential to impact
the public welfare in scenic and/or recreational areas during the
growing season, particularly in areas with special protection, such as
Class I areas.
---------------------------------------------------------------------------

    \196\ The identification, monitoring and assessment of AQRVs
with regard to an adverse effect is an approach used for assessing
the potential for air pollution impacts from pending permit actions
in Class I areas (USFS, NPS, USFWS, 2010). An adverse impact is
recognized by the National Park Service as one that results in
diminishment of the Class I areas's national significance or the
impairment of the ecosystem structure or functioning, as well as
impairment of the quality of the visitor experience (USFS, NPS,
USFWS, 2010). Federal land managers (FLMs) make such adverse impact
determinations on a case-by-case basis, using technical and other
information which they provide for consideration by permitting
authorities. The National Park Services has developed is a document
describing an overview of approaches related to assessing projects
under the National Environmental Policy Act and other planning
initiatives affecting the National Park System (http://www.nature.nps.gov/air/Pubs/pdf/AQGuidance_2011-01-14.pdf).
---------------------------------------------------------------------------

    The ecosystem services most likely to be affected by O3-
induced visible foliar injury (some of which are also recognized above
for tree growth-related effects) are cultural services, including
aesthetic value and outdoor recreation. In addition, several tribes
have indicated that many of the species identified as O3-
sensitive (including bioindicator species) are culturally significant
(U.S. EPA, 2014c, Table 5-1). The geographic extent of protected areas
that may be vulnerable to such public welfare effects of O3
is potentially appreciable. Sixty six species that occur on U.S.
National Park Service (NPS) and U.S. Fish and Wildlife Service lands
\197\ have been identified as sensitive to O3-induced
visible foliar injury and some also have particular cultural importance
to some tribes (U.S. EPA, 2014c, Table 5-1 and Appendix 5-A; U.S. EPA,
2014b, section 6.4.2). Not all species are equally sensitive to
O3, however, and quantitative relationships between
O3 exposure and other important effects, such as seedling
growth reduction, are

only available for a subset of the 66, as described in section IV.B.1.
above. A diverse array of ecosystem services has been identified for
these twelve species (U.S. EPA, 2014c, Table 5-1). Two of the species
in this group that are relatively more sensitive with regard to effects
on growth are the ponderosa pine and quaking aspen (U.S. EPA, 2014b,
section 6.2), the ranges for which overlap with many lands that are
protected or preserved for enjoyment of current and future generations
(consistent with the discussion above on Class I and other protected
areas), including such lands located in the west and southwest regions
of the U.S. where ambient O3 concentrations and associated
cumulative seasonal exposures can be highest (U.S. EPA, 2014c, Appendix
2B).\198\
---------------------------------------------------------------------------

    \197\ See http://www2.nature.nps.gov/air/Pubs/pdf/flag/NPSozonesensppFLAG06.pdf.
    \198\ Basal area for resident species in national forests and
parks are available in files accessible at: http://www.fs.fed.us/foresthealth/technology/nidrm2012.shtml.
---------------------------------------------------------------------------

    With regard to agriculture-related effects, the EPA has recognized
other complexities, stating that the degree to which O3
impacts on vegetation that could occur in areas and on species that are
already heavily managed to obtain a particular output (such as
commodity crops or commercial timber production) would impair the
intended use at a level that might be judged adverse to the public
welfare has been less clear (73 FR 16497, March 27, 2008; 75 FR 3024;
January 19, 2010). We note that while having sufficient crop yields is
of high public welfare value, important commodity crops are typically
heavily managed to produce optimum yields. In light of all of the
inputs that go into achieving these yields, such as fertilizer,
herbicides, pesticides, and irrigation, it is difficult to determine at
what point O3-induced yield loss creates an adverse impact
for the producer in the way of requiring increased inputs in order to
maintain the desired yields. Moreover, based on the economic theory of
supply and demand, increases in crop yields would be expected to result
in lower prices for affected crops and their associated goods, which
would primarily benefit consumers. Given these competing impacts on
producers and consumers, it is unclear how to consider these effects in
terms of potential adversity to the public welfare (U.S. EPA, 2014c,
sections 5.3.2 and 5.7).
    When agricultural impacts or vegetation effects in other areas are
contrasted with the emphasis on forest ecosystem effects in Class I and
similarly protected areas, it can be seen that the Administrator has in
past reviews judged the significance to the public welfare of
O3-induced effects on sensitive vegetation growing within
the U.S. to differ depending on the nature of the effect, the intended
use of the sensitive plants or ecosystems, and the types of
environments in which the sensitive vegetation and ecosystems are
located, with greater significance ascribed to areas identified for
specific uses and benefits to the public welfare, such as Class I
areas, than to areas for which such uses have not been established. In
summary, several considerations are recognized as important to
judgments on the public welfare significance of the array of effects of
different O3 exposure conditions on vegetation. While there
are complexities associated with the consideration of the magnitude of
key vegetation effects that might be concluded to be adverse to
ecosystems and associated services, there are numerous locations where
O3-sensitive tree species are present that may be vulnerable
to impacts from O3 on tree growth, productivity and carbon
storage and their associated ecosystems and services. It is not
possible to generalize across all studied species regarding which
cumulative exposures are of greatest concern, however, as this can vary
by situation due to differences in exposed species sensitivity, the
importance of the observed or predicted O3-induced effect,
the role that the species plays in the ecosystem, the intended use of
the affected species and its associated ecosystem and services, the
presence of other co-occurring predisposing or mitigating factors, and
associated uncertainties and limitations. These factors contribute to
the complexity of the Administrator's judgments regarding the adversity
of known and anticipated effects to the public welfare.

C. Exposure and Risk Assessment Information

    The WREA characterized ambient O3 exposure and its
relationship to tree biomass loss, crop yield loss, and visible foliar
injury and the associated ecosystem services \199\ in national-scale
and case study analyses. The WREA also qualitatively assessed impacts
to some ecosystem services, including impacts on the hydrologic cycle,
pollination regulation, and fire regulation; commercial non-timber
forest products and insect damage; and aesthetic and non-use values. In
the quantitative analyses, the WREA characterized effects associated
with exposures to O3 in ambient air using the W126 metric.
---------------------------------------------------------------------------

    \199\ In its review of drafts of the WREA and PA, the CASAC
conveyed support for analyses and considerations of ecosystem
services that may be affected by O3 exposures (Frey,
2014b, 2014c).
---------------------------------------------------------------------------

    The following sections summarize the analyses and adjustment
approach used to develop the O3 concentrations used as
inputs to the vegetation risk analyses for tree biomass and crop yield
loss, and the analyses, including key results and uncertainties, for
tree seedling growth, productivity, carbon storage and associated
ecosystem services (section IV.C.2); crop yield loss (section IV.C.3);
and visible foliar injury (section IV.C.4).
1. Air Quality Analyses
    The WREA evaluated O3 exposure and risks for several
national-scale air quality scenarios: recent conditions (2006 to
2008),\200\ the current secondary standard, and W126 index values of 15
ppm-hrs, 11 ppm-hrs, and 7 ppm-hrs, using three-year averages (U.S.
EPA, 2014b, chapter 4). For each of these scenarios, three-year average
W126 index values were estimated at each 12 km by 12 km grid cell in a
national-scale spatial surface. Additionally, some analyses were based
on single-year surfaces.\201\ The method for creating the five
scenarios generally involved two steps (summarized in Table 5-4 of the
PA). The first is derivation of the average W126 index value (across
the three years) at each monitor location. This value is based on
unadjusted O3 concentrations from monitoring data for recent
conditions and adjusted concentrations for the four other scenarios.
Concentrations were adjusted based on model predicted relationships
between O3 and U.S.-wide emissions reductions in oxides of
nitrogen (NOx). The adjusted air quality does not represent an
optimized control scenario that just meets the current standard (or
target W126 index values for other scenarios), but rather characterizes
one potential distribution of air quality across a region when all
monitor locations meet the standard (U.S. EPA, 2014b, section <IP_ADDRESS>).
The development of adjusted concentrations was done for each of nine
regions independently (see U.S. EPA, 2014b, section <IP_ADDRESS>). In the
second step, national-scale spatial surfaces (W126 index values for
each 12 km x 12 km

grid cell used in the air quality model) were created using the
monitor-location values and the Voronoi Neighbor Averaging (VNA)
spatial interpolation technique (details on the VNA technique are
presented in U.S. EPA, 2014b, Appendix 4A).
---------------------------------------------------------------------------

    \200\ Certain visible foliar injury analyses assessed recent
conditions from 2006 to 2010 on an annual basis.
    \201\ An analysis using data from USFS FHM/FIA O3
biomonitoring sampling sites (``biosites'') and a screening-level
assessment in 214 national parks were done using national-scale
spatial surfaces of unadjusted O3 concentrations (in
terms of the W126 index) created for each year from 2006 through
2010 using the VNA interpolation technique (U.S. EPA, 2014b, section
4.3.2, Appendix 4A).
---------------------------------------------------------------------------

    In the dataset used to create the recent conditions scenario, the
three-year average W126 index values at the monitor locations (before
application of the VNA technique) ranged from below 5 ppm-hrs to 48.6
ppm-hrs (U.S. EPA, 2014b, Figure 4-4 and Table 4-3). In the nine
modeling regions, the maximum three-year average W126 index values at
monitor locations ranged from 48.6 ppm-hrs in the West region down to
6.6 ppm-hrs in the Northwest region.\202\ After adjustment of the
monitor location concentrations to just meet the current standard in
each region (using relationships described above), the region-specific
maximum three-year average W126 values ranged from 18.9 ppm-hrs in the
West region to 2.6 ppm-hrs in the Northeast region (U.S. EPA, 2014b,
Table 4-3). With the next step, creation of the national surface of air
quality values at grid cell centroids, the highest values were reduced,
such that all the three-year average W126 index values were below 15
ppm-hrs across the national surface with the exception of a very small
area of the Southwest region (near Phoenix) where average W126 index
values were just above 15 ppm-hrs. Thus, it can be seen that
application of the VNA interpolation method to estimate W126 index
values at the centroid of every 12 x 12 km \2\ grid cell rather than
only at each monitor location results in a lowering of the highest
values.
---------------------------------------------------------------------------

    \202\ The regions referenced here and also with regard to
monitoring data analyses described in section IV.D.4 below are NOAA
climate regions, as shown in Figure 2B-1 of the PA.
---------------------------------------------------------------------------

    Because the W126 estimates generated for the different air quality
scenarios assessed are inputs to the vegetation risk analyses for tree
biomass and crop yield loss, and also used in the foliar injury
analyses, any uncertainties in the air quality analyses are propagated
into the those analyses (U.S. EPA, 2014b, section 8.5). The WREA
identified sources of uncertainty for the W126 estimates for each air
quality scenario and qualitatively characterized the magnitude of
uncertainty and potential for directional bias (U.S. EPA, 2014b, Table
4-5). As discussed in Chapter 4 and 8 of the WREA, an important large
uncertainty in the analyses is the assumed response of the W126
concentrations to emissions reductions needed to meet the existing
standard (U.S. EPA, 2014b, section 8.5.1). Any approach to
characterizing O3 air quality over broad geographic areas
based on concentrations at monitor locations will convey inherent
uncertainty. The model-based adjustments are based on U.S.-wide
emissions reductions in NOx and characterize only one potential
distribution of air quality across a region when all monitor locations
meet the standard (U.S. EPA, 2014b, section <IP_ADDRESS>).\203\
Additionally, the surface is created from the three-year average at the
monitor locations, rather than creating a surface for each year and
then averaging across years at each grid cell; the potential impact of
this on the resultant estimates is considered in the WREA (U.S. EPA,
2014b, Appendix 4A).
---------------------------------------------------------------------------

    \203\ The WREA analyses used U.S.-wide NOx emissions reductions
to simulate air quality that independently in each region would just
meet the existing standard and the three W126 scenarios. The NOx
emissions reductions were determined such that the highest monitor
within each region would just meet the target level. In this way,
the adjustment results in broad regional reductions in O3
and includes reductions in O3 at some monitors that were
already meeting or below the target level. Thus, the adjustments
performed to develop a scenario meeting a target level at the
highest monitor in each region did result in substantial reduction
below the target level in some areas of the region. This result at
the monitors already well below the target indicates an uncertainty
with regard to air quality expected from specific control strategies
that might be implemented to meet a particular target level.
---------------------------------------------------------------------------

    An additional uncertainty related to the W126 index value estimates
for each air quality scenario comes from the creation of a national
W126 surface using the VNA technique to interpolate recent air quality
measurements of O3. In general, spatial interpolation
techniques perform better in areas where the O3 monitoring
network is denser. Therefore, the W126 index values estimated in the
rural areas in the West, Northwest, Southwest, and West North Central
with few or no monitors (U.S. EPA, 2014b, Figure 2-1) are more
uncertain than those estimated for areas with denser monitoring.
Further, this interpolation method generally underpredicts higher 12-
hour W126 exposures. Due to the important influence of higher exposures
in determining risks to plants, the potential for the VNA interpolation
approach to underpredict higher W126 exposures could result in an
underestimation of risks to vegetation in some areas. Underestimation
of the highest W126 index values for the current standard scenario is
an additional impact of the interpolation method that is important to
consider.
2. Tree Seedling Growth, Productivity, Carbon Storage and Associated
Ecosystem Services
    For the WREA assessments related to tree growth, productivity,
carbon storage and associated ecosystem services, the sections below
provide an overview of the analyses along with the key results (section
IV.C.2.a) and summarize the key uncertainties (section IV.C.2.b).
    a. Overview and Summary of Key Results
    The assessments to estimate the exposures and risks for tree
seedling growth, productivity, and carbon storage reflect a range of
spatial scales ranging from the county scale up to the national park,
urban area, and national scales. For the air quality scenarios
described above, the WREA applied the species-specific E-R functions to
develop estimates of O3-associated RBL, productivity, carbon
storage and associated ecosystem services (U.S. EPA, 2014b, Chapter 6).
Some analyses also apply the median across species E-R functions.
    The WREA examined multiple approaches for characterizing the median
tree response to O3 exposure based on the 11 robust E-R
functions for tree seedlings from the OTC research and the E-R function
for eastern cottonwood (U.S. EPA, 2014b, section <IP_ADDRESS> and Figure 6-
5). For some species, only one study was available (e.g., red maple),
and for other species there were as many as 11 studies available (e.g.,
ponderosa pine). To illustrate the effect of within-species variability
associated with the E-R data available on estimates for a median
response across the 12 species, the WREA performed a stochastic
sampling analysis involving multiple iterations of random selection of
E-R functions from the studies available for each of the 12 species.
This analysis produced median values at each cumulative exposure level
that were higher than medians derived by two conventional,
deterministic methods (U.S. EPA, 2014b, section <IP_ADDRESS> and Figure 6-
5).\204\ For example, the median seasonal W126 index value for which a
two percent biomass loss is estimated in seedlings for the studied
species ranges from approximately 7 ppm-hrs using the conventional
methods up to 14 ppm-hrs when derived by the stochastic method.
Although the stochastic method provides some illustration of the effect
of within-species variability, we focus on the conventional approach
that gives equal weight to each studied species,

calculating the median response based on the composite E-R functions,
consistent with CASAC advice (Frey, 2014b).
---------------------------------------------------------------------------

    \204\ These methods were calculating a median using the
composite functions and calculating a median using all tree seedling
studies available.
---------------------------------------------------------------------------

    The WREA estimates indicate substantial heterogeneity in plant
responses to O3, both within species, between species, and
across regions of the U.S. The tree species known to be O3-
sensitive are different in the eastern and western U.S. and the eastern
U.S. has far more such species. Ozone exposure and risk is somewhat
easier to assess in the eastern U.S. because of the availability of
more data and the greater number of species to analyze. In addition,
there are more O3 monitors in the eastern U.S. but fewer
national parks (U.S. EPA, 2014b, chapter 8). In consideration of CASAC
advice, the WREA derived RBL and weighted RBL (wRBL) estimates
separately with and without the eastern cottonwood. The results
summarized here are for the analyses that exclude cottonwood.\205\ The
WREA reported RBL estimates relative to a benchmark of 2% RBL for tree
seedlings, as well as relative to other percent RBL values. The 2% RBL
benchmark was considered based on CASAC advice that stated that ``focus
on a 2% loss level for trees . . . is appropriate.'' (Frey, 2014b, p.
6). The main WREA analyses for effects related to tree growth,
productivity and carbon storage are summarized below, with the key
findings for each.
---------------------------------------------------------------------------

    \205\ The CASAC advised that the eastern cottonwood response
data ``receive too much emphasis'' in a draft version of the PA,
explaining that these ``results are from a gradient study that did
not control for ozone and climatic conditions and show extreme
sensitivity to ozone compared to other studies'' and that
``[a]lthough they are important results, they are not as strong as
those from other experiments that developed E-R functions based on
controlled ozone exposure'' (Frey, 2014b, p. 10).
---------------------------------------------------------------------------

    Relative biomass loss nationally was estimated for each of the 12
studied species from the composite E-R functions for each species
described above and information on the distribution of those species
across the U.S. (U.S. EPA, 2014b, section <IP_ADDRESS> and Appendix 6A). As
one example of a tree species near the median of the studied species,
relative biomass loss estimates (reduced growth) for ponderosa pine in
the current standard air quality scenario are below two percent for
most areas where this species is found but estimates of RBL for this
species in some areas of the southwest fall above two percent biomass
loss (U.S. EPA, 2014b, Figure 6-8). Maximum estimates of RBL for all
areas where ponderosa pine is found decrease to just over three percent
and just over two percent for the 15 and 7 ppm-hrs scenarios,
respectively (U.S. EPA, 2014b, Table 6-6).
    To provide an indication of ecosystem-level impacts, weighted
estimates of RBL (wRBL) were also developed for each grid cell
nationwide. This is estimated from the species-specific E-R functions
and a weighting approach based on information on prevalence of the
studied species across the U.S. (i.e., the proportion of the total
basal area modeled by USFS across all species for which data were
available). An overall wRBL value for each grid cell is generated by
summing the wRBL values for each studied tree species found within that
grid cell. The wRBL is intended to be an indication of the potential
magnitude of the ecological effect that could occur in some ecosystems.
In general, the higher the wRBL is in a given ecosystem, the larger the
potential ecological effect. (U.S. EPA, 2014b, section 6.8, Table 6-
25).
    For the national-scale analysis, the WREA presents the percent of
total basal area with wRBL greater than 2%. The estimates for the
weighted biomass loss analysis reflecting the 11 tree species with
robust E-R functions are as follows (U.S. EPA, 2014b, Table 6-25):
     For the current standard scenario, the percent of total
basal area that exceeds a two percent wRBL is 0.2 percent.
     For the W126 scenarios of 15, 11 and 7 ppm-hrs, the
percent of total basal area that exceeds a two percent wRBL is 0.2
percent, 0.1 percent, and less than 0.1 percent respectively (U.S. EPA,
2014b, Table 6-25).
    In the wRBL analysis for Class I areas, the number of Class I areas
with wRBL greater than 2% is estimated for the grid cells located in
the 145 of the 156 Class I areas for which data were available (U.S.
EPA, 2014b, Table 6-26).
     For the current standard scenario, two of the 145 assessed
Class I areas have weighted RBL values above two percent (U.S. EPA,
2014b, Table 6-26).
     For the W126 scenarios of 15, 11 and 7 ppm-hrs, there are
two, two and one Class I area with wRBL above two percent,
respectively.
    In the county analysis, the WREA estimated the number of U.S.
counties in which any of the studied tree species is estimated to
experience more than two percent RBL, the number of species affected,
and the number of counties for which the median of the species-specific
functions exceeds two percent RBL. In addition to the estimates based
on all 12 studied species and also the 11 species with the exclusion of
eastern cottonwood (in response to CASAC advice), additional estimates
were developed without black cherry to show contribution of that
sensitive species to the multi-species estimates (U.S. EPA, 2014b,
Table 6-7).
     In the current standard scenarios, 66% of the 3,109
assessed counties are estimated to have at least one of the 11 species
(excluding cottonwood) with an RBL greater than two percent, with three
counties having three species exceeding two percent. The median RBL
(across the species present) is above two percent in 239 counties. The
maximum number of species in any one county with an RBL greater than
two percent is three (excluding cottonwood). (U.S. EPA, 2014b, Table 6-
7).
     For the 15, 11 and 7 ppm-hrs scenarios, the proportion of
3,109 counties with one or more species with an RBL above two percent
decreases to 61 percent, 59 percent, and 58 percent, respectively. For
the 7 ppm-hrs scenario, the median RBL is above two percent in six
percent of the counties (U.S. EPA, 2014b, Table 6-7).
     The county RBL estimates are appreciably influenced by
black cherry, a very sensitive species that is widespread in the
Eastern U.S. For 1,805 of the 1,929 counties estimated to have at least
one species with an RBL greater than two percent when air quality is
meeting the current standard, only black cherry exceeds this level of
RBL. If black cherry is excluded, the median RBL for the 10 remaining
species decreases. For the median RBL values, 203 of the 239 counties
estimated to have a median RBL above two percent when air quality is
meeting the current standard are because of the presence of black
cherry (U.S. EPA, 2014b, Table 6-7).
    Additionally, the WREA estimated relative yield loss in timber
production and associated changes in consumer and producer/farmer
economic surplus using E-R functions for tree seedlings to calculate
relative yield loss (equivalent to relative biomass loss) across full
tree lifespans and through modeling of the resulting market-based
welfare effects. Because the forestry and agriculture sectors are
related and trade-offs occur between the sectors, the WREA calculated
the resulting market-based welfare effects of O3 exposure in
the forestry and agriculture sectors on consumer and producer
surplus.\206\

Because demand for most forestry and agricultural commodities is not
highly responsive to changes in price, producer surplus (i.e., producer
gains) often declines. These declines can be more than offset by
changes in consumer surplus gains from lower prices, but, in some
cases, lower prices reduce producer gains more than can be offset by
consumer surplus (U.S. EPA, 2014b, Appendix 6B, Table and B-9).
---------------------------------------------------------------------------

    \206\ The WREA used the Forest and Agricultural Sector
Optimization Model with Greenhouse Gases (FASOMGHG). FASOMGHG is a
national-scale model that provides a complete representation of the
impacts of meeting alternative standards on the U.S. forest and
agricultural sectors. FASOMGHG simulates the allocation of land over
time to competing activities in both the forest and agricultural
sectors. FASOMGHG results include multi-period, multi-commodity
results over 60 to 100 years in 5-year time intervals when running
the combined forest-agriculture version of the model.
---------------------------------------------------------------------------

     In the current standard scenario, estimates of the
relative yield loss for timber production are below one percent other
than in the Southwest, Southeast, Central, and South regions (U.S. EPA,
2014b, section 6.3, Table 6-9) (see U.S. EPA, 2014b, Table 6-8 for
clarification on region names). The highest yield loss occurs in upland
hardwood forests in the South Central and Southeast regions at over
three percent per year and in Corn Belt hardwoods at just over two
percent loss per year (U.S. EPA, 2014b, section 6.3, Table 6-9).
     For the 15 and 11 ppm-hrs scenarios, relative yield loss
estimates for timber production are above one percent in parts of the
Southeast, Central, and South regions and above two percent in parts of
the Southeast and Central U.S.
     For the 7 ppm-hrs scenario, relative yield loss estimates
for timber production are above one percent in the Southeast and South
regions (U.S. EPA, 2014b, section 6.3, Table 6-9).
    The WREA also estimated impacts on tree growth and two ecosystem
services provided by urban trees: removal of air pollutants and carbon
storage. The estimates of the tons of carbon monoxide, nitrogen
dioxide, ozone and sulfur dioxide removed are for a 25-year period in
five urban case study areas: Baltimore, Syracuse, the Chicago region,
Atlanta, and the urban areas of Tennessee (U.S. EPA, 2014b, section
6.7).\207\
---------------------------------------------------------------------------

    \207\ The WREA used the i-Tree model for the urban case studies.
i-Tree is a peer-reviewed suite of software tools provided by USFS.
---------------------------------------------------------------------------

     Estimates for all five urban case study areas indicate
increased pollutant removal of O3, nitrogen dioxide, carbon
monoxide, and sulfur dioxide in the current standard scenario (U.S.
EPA, 2014b, sections 6.7). The results for the 15 ppm-hrs scenario were
very similar to those for meeting the current standard. For the 11 and
7 ppm-hrs scenarios, all five case study areas indicate smaller
additional increases in air pollutant removal beyond moving from
current conditions to the current standard (U.S. EPA, 2014b, sections
6.7).
    The WREA estimated carbon storage related to O3-induced
biomass loss in forests and agricultural crops nationally and also in
forests in five urban areas using the FASOMGHG and i-Tree models noted
above (U.S. EPA, 2014b, section 6.6). Ozone effects on tree growth
affects the climate regulation service provided by ecosystems by
reducing carbon sequestration and storage (U.S. EPA, 2013a, section
<IP_ADDRESS>; U.S. EPA, 2014b, chapter 6, section 6.6). Because
O3 exposure affects photosynthesis and CO2 uptake
by trees, forests sequester less carbon and thus more carbon stays in
the atmosphere. In the model used to calculate national-level impacts
to forests and agriculture from O3-related biomass loss,
carbon sequestration reflects carbon in standing (live and dead) trees,
forest soils, the forest understory vegetation, forest floor including
litter and large woody debris, and wood products both in use and in
landfills (U.S. EPA, 2014b, chapter 6, Appendix 6B, section 2.7.1).
     Over 30 years for the national-scale analysis, carbon
storage in the forestry sector estimated for the current standard
scenario is just over 89,000 million metric tons of CO2
equivalents (MMtCO2e); this is 11,840 more
MMtCO2e storage associated with the reduced O3-
related growth impact from meeting the current standard as compared
with recent conditions.\208\ The estimates of carbon storage in the
agricultural sector are much smaller (i.e., 8,469 MMtCO2e
for the current standard scenario which is 606 MMtCO2e more
than the recent conditions scenario) (U.S. EPA, 2014b, section 6.6.1
and Appendix 6B). The forestry sector carbon storage estimated for each
of the three W126 scenarios is just slightly greater than that
estimated for the current standard. As a percentage of the current
standard estimate, the three scenario estimates are less than 0.1% (13
MMtCO2e), just under 1% (593 MMtCO2e) and under
2% (1,600 MMtCO2e) for the 15, 11 and 7 ppm-hrs scenarios,
respectively (U.S. EPA, 2014b, Tables 6-19 and B-10).
---------------------------------------------------------------------------

    \208\ One MMtCO2e is equivalent to 208,000 passenger
vehicles or the electricity to run 138,000 homes for 1 year as
calculated by the EPA Greenhouse Gas Equivalencies Calculator
(updated September 2013 and available at http://www.epa.gov/cleanenergy/energy-resources/calculator.html).
---------------------------------------------------------------------------

     Estimates of the effects of avoided O3-related
biomass loss on carbon sequestration in forests in the five urban area
case studies indicate the potential for an increase in carbon
sequestration of somewhat more than one MMtCO2e for the
current standard scenario compared to the recent conditions estimate
(U.S. EPA, 2014b, section 6.6.2 and Appendix 6D). The additional
increases in O3-related carbon sequestration estimated
across the five case studies for the three W126 scenarios are
relatively small (U.S. EPA, 2014b, section 6.6.2 and Appendix 6D).
    Although not discussed in detail here, the WREA also describes
qualitative assessments for some ecosystem services that may be
affected by O3 effects on tree growth and productivity, such
as commercial non-timber forest products and recreation (U.S. EPA,
2014b, section 6.4), aesthetic and non-use values (U.S. EPA, 2014b,
section 6.4), increased susceptibility to insect attack and fire damage
(U.S. EPA, 2014b, sections 5.3 and 5.4, respectively). Other ecological
effects that are causally or likely causally associated with
O3 exposure, such as effects on terrestrial productivity,
the water cycle, the biogeochemical cycle, and community composition
(U.S. EPA, 2013a, Table 9-19), were not quantitatively addressed in the
WREA due to a lack of sufficient quantitative information.
b. Key Uncertainties
    The WREA identified several key limitations and uncertainties in
the biomass loss assessments for trees, which may have a large impact
on both overall confidence and confidence in individual analyses. Key
uncertainties that affect the assessment of impacts on ecosystem
services at the national and case-study scales, as well as across
species, U.S. geographic regions and future years, include those
associated with the interpolated and adjusted O3
concentrations used to estimate W126 exposures in the air quality
scenarios, the available seedling E-R functions, combining effects
across sensitive species, the effects of compounding over time, and
modeling impacts of biomass loss on timber harvesting and urban air
pollutant removal.
    With regard to the robust seedling E-R functions, the WREA provided
some characterization of the variability of individual study results
and the impact of that on estimates of W126 index values that might
elicit different percentages of biomass loss in tree seedlings (U.S.
EPA, 2014b, section <IP_ADDRESS>). Even though the evidence shows that there
are additional species affected by O3-related biomass loss,
the WREA only has E-R functions available to quantify this loss for 12
tree species. This limited information only allows a partial
characterization of the O3-related biomass loss impacts in
trees associated

with recent O3 index values and with just meeting the
existing and potential alternative secondary standards. In addition,
there are uncertainties inherent in these E-R functions, including the
extrapolation of relative biomass loss rates from tree seedlings to
adult trees and information regarding within-species variability. The
overall confidence in the E-R function varies by species based on the
number of studies available for that species. Some species have low
within-species variability (e.g., many agricultural crops) and high
seedling/adult comparability (e.g., aspen), while other species do not
(e.g., black cherry). The uncertainties in the E-R functions for
biomass loss and in the air quality analyses are propagated into the
analysis of the impact of biomass loss on ecosystem services, including
provisioning and regulating services (U.S. EPA, 2014b, Table 6-27). The
WREA characterizes the direction of potential influence of E-R function
uncertainty as unknown, yet its magnitude as high, concluding that
further studies are needed to determine how accurately the assessed
species reflect the larger suite of O3-sensitive tree
species in the U.S. (U.S. EPA, 2014b, Table 6-27).
    Another uncertainty associated with interpretation of the WREA
biomass loss-related estimates concerns the potential for
underestimation of compounding of growth effects across multiple years
of varying concentrations. Though tree biomass loss impacts were
estimated using air quality scenarios of three-year average W126 index
values, the WREA also conducted an analysis to compare the impact of
using a variable compounding rate based on yearly variations in W126
exposures to that of using a W126 index value averaged across three
years. The WREA compared the compounded values for an example species
occurring in the eastern U.S. and another example species occurring in
the western U.S. In both examples, one species (tulip polar and
ponderosa pine, respectively) and one climate region where that species
occurred (Southeast and Southwest regions, respectively) were chosen
and air quality values associated with just meeting the existing
standard of 75 ppb were used. Within each region, the WREA analysis
used both the W126 index value at each monitor in the region for each
year and the three-year average W126 index value using the method
described in Chapter 4 of the WREA. The results show that the use of
the three-year average W126 index value may underestimate RBL values
slightly (U.S. EPA, 2014b, section <IP_ADDRESS> and Figure 6-14). In both
regions, the three-year average W126 index value is sometimes above and
sometimes below the individual year W126 index value.
    The WREA recognizes uncertainty regarding the extent to which the
subset of studied tree species encompass the O3 sensitive
species in the U.S. and the extent to which it represents U.S.
vegetation as a whole (U.S. EPA, 2013a, pp. 9-123 to 9-125; U.S. EPA,
2014b, Table 6-27). There are also uncertainties associated with
estimating the national scale ecosystem-level impacts using wRBL. For
example the wRBL estimates are likely biased low as there may be other
unstudied O3-sensitive tree species in some areas that are
also being affected at those levels, although this analysis does not
take into account the effects of competition, which could further
affect forest biomass loss.
    Uncertainties are recognized in the national-scale analyses of
timber production, agricultural harvesting, and carbon sequestration,
for which the WREA used the FASOMGHG model. These uncertainties include
those associated with the functions for carbon sequestration, the
assumptions made regarding proxy species where there are insufficient
data, and the non-W126 E-R functions for three crops. The FASOMGHG
model does not include agriculture and forestry on public lands,
changes in exports due to O3 into international trade
projections, or forest adaptation. Despite the inherent limitations and
uncertainties, the WREA concludes that the FASOMGHG model reflects
reasonable and appropriate assumptions for a national-scale assessment
of changes in the agricultural and forestry sectors due to changes in
vegetation biomass associated with O3 exposure (U.S. EPA,
2014b, sections 6.3, 6.5, 6.6, and 8.5.2, and Table 6-27).
    In the case study analyses of five urban areas, the WREA used the
i-Tree model, which includes an urban tree inventory for each area and
species-specific pollution removal and carbon sequestration functions.
However, i-Tree does not account for the potential additional VOC
emissions from tree growth, which could contribute to O3
formation. Uncertainties are also recognized with regard to the base
inventory of city trees, the functions used for air pollutant removal
and for carbon storage (U.S. EPA, 2014b, sections 6.6.2 and 6.7, and
Table 6-27). Despite the inherent limitations and uncertainties, the
WREA concludes that the i-Tree model reflects reasonable and
appropriate assumptions for a case study assessment of pollution
removal and carbon sequestration for changes in biomass associated with
O3 exposure (U.S. EPA, 2014b, sections 6.6.2, 6.7, and
8.5.2).
3. Crop Yield
    Section IV.C.3.a below provides an overview of the assessments
performed in the WREA to estimate the exposures and risks for crop
yield, as well as the key results. Section IV.C.3.b summarizes the key
uncertainties.
a. Overview and Summary of Key Results
    The WREA conducted two analyses to estimate O3 impacts
related to crop yield, including annual yield losses estimated for 10
commodity crops grown in the U.S. with E-R functions and how these
losses affect producer and consumer economic surpluses (U.S. EPA,
2014b, sections 6.2, 6.5). Summary estimates for crop yield loss
related effects in the WREA are presented relative to a 5% yield loss
benchmark based on consideration of CASAC's recommendation to consider
a benchmark of 5% for median crop yield loss and to consider 5% yield
loss for individual crop species. In addition, other benchmarks levels
are considered in the WREA (e.g. 10% and 20%).
    The WREA derived estimates of crop RYL estimates nationally and in
a county-specific analysis. Crop-specific estimates of O3-
related RYL nationally were derived for each of the air quality
scenarios from the 10 E-R functions for crops described above combined
with information regarding crop distribution (U.S. EPA, 2014b, section
6.5). The WREA also reported crop RYL results at the county-level, as
well as the number of crop-producing counties with greater than five
percent RYL (U.S. EPA, 2014b, section 6.5.1, Appendix 6B).
     The largest reduction in O3-induced crop yield
loss and yield changes occurs when moving from the recent conditions
scenario to the current standard scenario (U.S. EPA, 2014b, section
6.5). Among the major commercial crops, winter wheat and soybeans are
more sensitive to ambient O3 levels than other crops.
     In the current standard scenario, no counties have RYL
estimates at or above 5% (U.S. EPA, 2014b, section 6.5).\209\
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    \209\ In the air quality scenario for the current standard, a
monitor that already met the current standard but was located within
the same region as another monitor that was above the current
standard would have had its concentration adjusted downward. This is
due to the fact that concentrations were adjusted independently for
each region, applying reductions to all monitors within the region,
such that all monitors located within a region meet the standard
(U.S. EPA, 2014b, section <IP_ADDRESS>).

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    The WREA also estimated O3-related crop impacts on
producer and consumer surplus.\210\ These are national-scale estimates
of the effects of yield loss on agricultural harvesting, which supply
provisioning services of food and fiber for each of the air quality
scenarios. Overall effect on agricultural yields and producer and
consumer surplus depends on (1) the ability of producers/farmers to
substitute other crops that are less O3 sensitive, and (2)
the responsiveness, or elasticity, of demand and supply (U.S. EPA,
2014b, section 6.5).
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    \210\ Welfare economics focuses on the optimal allocation of
resources and goods and how those allocations affect total social
welfare. Total welfare is also referred to as economic surplus,
which is the overall benefit a society, composed of consumers and
producers, receives when a good or service is bought or sold, given
a quantity provided and a market price. Economic surplus is divided
into two parts: Consumer and producer surplus (U.S. EPA, 2014b, p.
ES-6).
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     Estimates of consumer surplus, or consumer gains, were
generally higher in the current standard scenario in the agricultural
sector because higher productivity under lower O3
concentrations increased total yields and reduced market prices (U.S.
EPA, 2014b, Tables 6-17 and 6-18). Combined gains in producer and
consumer surplus for forestry and agriculture were essentially
unchanged for the 15 ppm-hrs scenario, but annualized gains increased
by $21 million beyond the current standard scenario for the 11 ppm-hrs
scenario and $231 million for the 7 ppm-hrs scenario. In some cases,
lower prices reduce producer gains more than can be offset by higher
yields (U.S. EPA, 2014b, Table 6-18).
     Because demand for most agricultural commodities is not
highly responsive to changes in price, producer surplus or producer
gains often declined. For agricultural welfare, annualized combined
consumer and producer surplus gains were estimated to be $2.6 trillion
in 2010 for the current standard scenario (U.S. EPA, 2014b, Table 6-
17).
b. Key Uncertainties
    The WREA discusses multiple areas of uncertainty associated with
the crop yield loss estimates, including those associated with the
model-based adjustment methodology as well as those associated with the
projection of yield loss using the FASOMGHG model at the estimated
O3 concentrations (U.S. EPA, 2014b, Table 6-27, section
8.5). Because the W126 estimates generated in the air quality analyses
are inputs to the vegetation risk analyses for crop yield loss, any
uncertainties in the air quality analyses are propagated into the those
analyses (U.S. EPA, 2014b, Table 6-27, section 8.5). Therefore, the air
quality scenarios in the crop yield analyses have the same
uncertainties and limitations as in the biomass loss analyses
(summarized above), including those associated with the model-based
adjustment methodology (U.S. EPA, 2014b, section 8.5).
4. Visible Foliar Injury
    Section IV.C.4.a below provides an overview of the assessment in
the WREA of O3-related visible foliar injury and associated
ecosystem services impacts, as well as the key results. Section
IV.C.4.b summarizes the key uncertainties.
a. Overview and Summary of Key Results
    The WREA presents a number of analyses of O3-related
visible foliar injury and associated ecosystem services impacts (U.S.
EPA, 2014b, Chapter 7). An initial analysis using USFS FHM/FIA biosite
data included the development of benchmark criteria reflecting
different prevalences of visible foliar injury at different W126
exposures and soil moisture conditions. These criteria were then used
in a screening-level characterization of the potential risk of foliar
injury incidence in 214 national parks and a case study assessment of
three national parks, which also provides limited characterization of
the associated ecosystem services.
    In the biosite data analysis, the WREA used the biomonitoring site
data from the USFS FHM/FIA Network (USFS, 2011),\211\ associated soil
moisture data during the sample years, and national surfaces of ambient
air O3 concentrations based on spatial interpolation of
monitoring data from 2006 to 2010 to calculate the proportion of
biosites with any visible foliar injury. The proportion of biosites
metric is derived by first ordering the data (across biosites and
sample years) by W126 index value estimated for that biosite and year.
Then for each W126 index value, the proportion of biosites is
calculated with any foliar injury for all observations at or below that
W126 index value. (U.S. EPA, 2014b, section 7.2). This analysis
indicates that the proportion of biosites showing the presence of any
foliar injury increases rapidly from zero to about 20 percent at
relatively low W126 index values. Specifically: (1) the proportion of
biosites exhibiting foliar injury rises rapidly with increasing W126
index values below approximately 10 ppm-hrs (W126 <10.46 ppm-hrs), and
(2) there is relatively little change in this proportion with
increasing W126 index values above approximately 10 ppm-hrs (W126
>10.46 ppm-hrs). The data for biosites during normal moisture years are
very similar to the dataset as a whole, with an overall proportion of
close to 18 percent for presence of any foliar injury. Among the
biosites with a relatively wet season, the proportion of biosites
showing injury is much higher and the relationship with annual W126
index value is much steeper. Much lower proportions of biosites show
injury with relatively dry seasons (U.S. EPA, 2014b, section 7.2.3,
Figures 7-10), consistent with the ISA finding that many studies have
shown that dry periods tend to decrease the incidence and severity of
O3-induced visible foliar injury (U.S. EPA, 2013a, section
9.4.2). While these analyses indicate the potential for foliar injury
to occur under conditions that meet the current standard, the extent of
foliar injury that might be expected under such conditions is unclear
from these analyses.
---------------------------------------------------------------------------

    \211\ Data were not available for several western states
(Montana, Idaho, Wyoming, Nevada, Utah, Colorado, Arizona, New
Mexico, Oklahoma, and portions of Texas).
---------------------------------------------------------------------------

    The national-scale screening-level assessment in 214 parks employed
benchmark criteria developed from the above analysis.212 213
For example, annual O3 concentrations corresponding to a
W126 index value of 10.46 ppm-hrs represents the O3 exposure
concentration where the slope of exposure-response relationship changes
for FHM biosites, with the percentage of biosites showing injury
remaining relatively constant for higher W126 index values. The WREA
refers to this as the ``base scenario'' benchmark. The

W126 benchmarks across this and the other four scenarios range from
3.05 ppm-hrs (foliar injury observed at five percent of biosites,
normal moisture) up to 24.61 ppm-hrs (foliar injury observed at 10
percent of biosites, dry). For the scenario of 10 percent biosites with
injury, W126 index values were approximately 4, 6, and 25 ppm-hrs for
wet, normal and dry years, respectively. The national-scale screening-
level assessment applied these benchmarks to 42 parks with
O3 monitors and a total of 214 parks with O3
exposure estimated from the interpolated national O3
surfaces for individual years from 2006 to 2010 (U.S. EPA, 2014b,
Appendix 7A and section 7.3).
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    \212\ The parks assessed in the WREA include lands managed by
the NPS in the continental U.S., which includes National Parks,
Monuments, Seashores, Scenic Rivers, Historic Parks, Battlefields,
Reservations, Recreation Areas, Memorials, Parkways, Military Parks,
Preserves, and Scenic Trails.
    \213\ This analysis considered the approach in Kohut (2007),
which assessed the risk of O3-induced visible foliar
injury on O3 bioindicators (i.e., O3-sensitive
vegetation) in 244 parks managed by the NPS. Consistent with advice
from CASAC (Frey and Samet, 2012a), however, the WREA modified the
approach used by Kohut (2007) to apply the W126 metric alone. The
WREA applied different foliar injury benchmarks in this assessment
after further investigation into the benchmarks applied in Kohut
(2007), which were derived from biomass loss rather than visible
foliar injury. Kohut cited a threshold of 5.9 ppm-hrs for highly
sensitive species from Lefohn (1997), which was based on the lowest
W126 estimate corresponding to a 10 percent growth loss for black
cherry. For soil moisture, Kohut (2007) qualitatively assessed
whether there appeared to be an inverse relationship between soil
moisture and high O3 exposure.
---------------------------------------------------------------------------

     Based on NPS lists, 95 percent of the 214 parks in this
screening-level assessment contain at least one vegetation species
sensitive to O3-induced foliar injury (U.S. NPS, 2003,
2006).
     In the current standard scenario, none of the 214 parks
had O3 concentrations estimated to exceed the annual
benchmark of a W126 index value above 10.46 ppm-hrs (U.S. EPA, 2014b,
section <IP_ADDRESS>).
    The case study analyses focused on Great Smoky Mountains National
Park (GRSM), Rocky Mountain National Park (ROMO), and Sequoia and Kings
Canyon National Parks (SEKI). Information on visitation patterns,
recreational activities and visitor expenditures was considered. For
example, visitor spending in 2011 exceeded $800 million, $170 million
and $97 million dollars in GRSM, ROMO and SEKI, respectively. In each
park, the percent cover of species sensitive to foliar injury was
estimated and the overlap between recreation areas within the park and
elevated W126 concentrations was described. (U.S. EPA, 2014b, section
7.4).
     In the current standard scenario, the three-year average
W126 index values were at or below 7 ppm-hrs in all areas of two of the
three parks (GRSM and SEKI). Three-year average W126 index values were
below 7 ppm-hrs in a little more than half of the area of the third
park (ROMO) and between 7 and 11 ppm-hrs in the remainder of the park
(U.S. EPA, 2014b, section 7.4).
     For the 15, 11 and 7 ppm-hrs scenarios, all areas of the
three specific national parks evaluated (GRSM, SEKI, and ROMO) had
three-year average W126 index values at or below 7 ppm-hrs, well below
the 10.46 ppm-hrs benchmark. However, the extent of foliar injury that
might be expected under these scenarios is unclear from these analyses.
    Although not discussed in detail here, the WREA also describes
qualitative assessments for some of the ecosystem services most likely
to be affected by O3-induced foliar injury such as cultural
services, including aesthetic value and outdoor recreation. Aesthetic
value and outdoor recreation depend on the perceived scenic beauty of
the environment. Many outdoor recreation activities directly depend on
the scenic value of the area, in particular scenic viewing, wildlife-
watching, hiking, and camping. These activities and services are of
significant importance to public welfare as they are enjoyed by
millions of Americans every year and generate millions of dollars in
economic value (U.S. EPA, 2014b, Chapters 5 and 7). Although data are
not available to explicitly quantify O3 effects on ecosystem
services, the WREA includes several qualitative analyses.
b. Key Uncertainties
    Uncertainties associated with these analyses are discussed in the
WREA, sections 7.5 and 8.5.3, and in WREA Table 7-24, and also
summarized in the PA (e.g., U.S. EPA, 2014c, section 6.3). As discussed
in the WREA (section 8.5.3), evaluating soil moisture is more
subjective than evaluating O3 exposure because of its high
spatial and temporal variability within the O3 season, and
there is considerable subjectivity in the categorization of relative
drought. The WREA generally concludes that the spatial and temporal
resolution for the soil moisture data is likely to underestimate the
potential for foliar injury to occur in some areas. In addition, there
is lack of a clear threshold for drought below which visible foliar
injury would not occur. In general, low soil moisture reduces the
potential for foliar injury, but injury could still occur, and the
degree of drought necessary to reduce potential injury is not clear.
Studies in the ISA provide additional information regarding the role of
soil moisture in influencing visible foliar injury response, (U.S. EPA,
2013a, section 9.4.2). These studies confirm that adequate soil
moisture creates an environment conducive to greater visible foliar
injury in the presence of O3 than drier conditions. As
stated in the ISA, ``[a] major modifying factor for O3-
induced visible foliar injury is the amount of soil moisture available
to a plant during the year that the visible foliar injury is being
assessed . . . because lack of soil moisture generally decreases
stomatal conductance of plants and, therefore, limits the amount of
O3 entering the leaf that can cause injury'' (U.S. EPA,
2013a, p. 9-39). As a result, ``many studies have shown that dry
periods in local areas tend to decrease the incidence and severity of
O3-induced visible foliar injury; therefore, the incidence
of visible foliar injury is not always higher in years and areas with
higher O3, especially with co-occurring drought (Smith,
2012; Smith et al., 2003)'' (U.S. EPA, 2013a, p. 9-39). This ``. . .
partial `protection' against the effects of O3 afforded by
drought has been observed in field experiments (Low et al., 2006) and
modeled in computer simulations (Broadmeadow and Jackson, 2000)'' (U.S.
EPA, 2013a, p. 9-87). In considering the extent of any protective role
of drought conditions, however, the ISA also notes that other studies
have shown that ``drought may exacerbate the effects of O3
on plants (Pollastrini et al., 2010; Grulke et al., 2003)'' and that
``[t]here is also some evidence that O3 can predispose
plants to drought stress (Maier-Maercker, 1998)''. Accordingly, the ISA
concludes that ``the nature of the response is largely species-specific
and will depend to some extent upon the sequence in which the stressors
occur'' (U.S. EPA, 2013a, p. 9-87).
    Due to the absence of biosite injury data in the Southwest region
and limited biosite data in the West and West North Central regions,
the W126 benchmarks for foliar injury that the WREA developed and
applied in the national park screening assessment may not be applicable
to these regions. The WREA applied the benchmarks from the national-
scale analysis to a screening-level assessment of 214 national parks
and case studies of three national parks. Therefore, uncertainties in
the foliar injury benchmarks are propagated into these analyses.
    Other uncertainties associated with these analyses include
uncertainty associated with our understanding of the number and
sensitivity of O3 sensitive species, uncertainties
associated with spatial assignment of foliar injury biosite data to 12
km x 12 km grid cells, and uncertainties associated with O3
exposure data of vegetation and recreational areas within parks (U.S.
EPA, 2014b, Table 7-22).
    There are also important uncertainties in the estimated
O3 concentrations for the different air quality scenarios
evaluated (U.S. EPA, 2014b, section 8.5), as discussed earlier in this
section. These uncertainties only apply to the national park case
studies because these are the only foliar injury analyses that rely on
the air quality scenarios, but any uncertainties in the air quality
analyses are propagated into those analyses. The WREA identifies
additional uncertainties that are associated with

the national park case studies. Specifically, there is uncertainty
inherent in survey estimates of participation rates, visitor spending/
economic impacts, and willingness-to-pay. These surveys potentially
double-count impacts based on the allocation of expenditures across
activities but also potentially exclude other activities with economic
value. In general, the national level surveys apply standard
approaches, which minimize potential bias. Other sources of uncertainty
are associated with the mapping, including park boundaries, vegetation
species cover, and park amenities, such as scenic overlooks and trails.
In general, the WREA concludes that there is high confidence in the
park mapping (U.S. EPA, 2014b, Table 7-24).

D. Conclusions on Adequacy of the Current Secondary Standard

    The initial issue to be addressed in the current review of the
secondary O3 standard is whether, in view of the currently
available scientific evidence, exposure and risk information and air
quality analyses, discussed in the PA, the existing standard should be
revised. In drawing conclusions on adequacy of the current
O3 secondary standard, the Administrator has taken into
account both evidence-based and quantitative exposure- and risk-based
considerations, and advice from CASAC. Evidence-based considerations
draw upon the EPA's assessment and integrated synthesis of the
scientific evidence from experimental and field studies evaluating
welfare effects related to O3 exposure, with a focus on
policy-relevant considerations, as discussed in the PA. Air quality
analyses inform these considerations with regard to cumulative,
seasonal exposures occurring in areas of the U.S. that meet the current