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.
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
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
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 are testing whether grazing
is also an important predictor of ecosystem source/sink strength in
In 1997 and 1998, in collaboration with the Centre for Climate and Global Change
Research at McGill University, we measured ecosystem CO2
in 9 sites that varied in the amount of grazing they received in Panamanian
pastures (Wilsey et al. 2002). 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. We are continuing studies
(Wilsey and Polley 2004) with
tallgrass prairies in Iowa and Texas (in collaboration with the Grassland, Soil and Water Research Lab).