Grazing effects on plant and ecosystem processes

Is species diversity and primary productivity limited by seed availability?

Is any possible seed limitation influenced by grazing history?

We tested whether species diversity and primary productivity in subhumid grasslands were limited by seed availability, and whether seedling emergance was affected by the grazing history of the site (Wilsey and Polley 2003). To test these hypotheses, we seeded 20 native species into fields with different grazing histories and then monitored seedling emergence over two growing seasons. Primary productivity and species diversity was measured during the second year of the study. Counter to our expectations, we found that release from grazing led to a linear increase in seedling emergence when litter was removed, and highest seedling emergence in fields with an intermediate amount of time since grazing when litter was left intact. Release from grazing led to lower light availability at the soil surface, as expected. But surprisingly, fields that had not been grazed for the longest time period had the highest soil moisture. Perhaps most importantly, seed additions had no affect on species diversity and no consistent effect on primary productivity (seed additions lowered productivity in most fields due to priority effects).

A second set of studies were designed to look for possible seed limitations in the largest planted prairie in the United States Neal Smith NWR (Martin and Wilsey 2006). Twenty-five species of rare forbs and grasses were added inside and outside of exclosures that exclude bison grazing to determine if species diversity and productivity of this system is limited by the availability of seed (i.e. larger scale dispersal limitation) or microsites created by disturbance (i.e. local scale processes). Establishment of rare prairie species was highest with both seed additions and bison grazing.

Are there relationships between level of disturbance and net ecosystem carbon exchange?

Out of the approximately 8 Pg (1015 g) of CO2 that are released annually by mankind, only about one half ends up in the atmosphere (Poisson 1998, Prentice and Lloyd 1998). The rest is uptaken by sinks on the land surface and in oceans. Recent research has indicated that terrestrial ecosystems of the northern hemisphere are, for the time being at least, acting as sinks for CO2. Numerous field sites to monitor CO2 exchange between the land surface and atmosphere have been set up across the US, including an eddy covariance flux network and the USDA-ARS rangelands flux network based on the Bowen ratio.

Both networks are based on measurements of carbon exchange that average over large spatial scales, and with little consideration of disturbance. For this reason, the overall effects of small-scale disturbance on ecosystem carbon exchange is "averaged out." We tested whether disturbance is modifying net ecosystem CO2 exchange by making measurements with clear polycarbonate chambers on grasslands, under varying amounts of disturbance from grazing mammals. Measurements with chambers complement larger scale eddy covariance and bowen ratio studies. Grazing, either by native or exotic mammals and insects, is ubiquitous in grasslands and savannahs around the world. Furthermore, the amount of grazing that occurs (grazing intensity), which can vary greatly from site to site and from patch to patch, is an extremely important predictor of plant growth and primary productivity. We tested whether grazing intensity is also an important predictor of ecosystem source/sink strength in grasslands. By measuring CO2 exchange under ambient, reduced, and no light, we were able to develop light response curves. We then compared associated derived variables such as respiration and uptake among plots with different grazing intensities, and with declines in plant species diversity (Wilsey and Polley 2004).