Grazing effects on plant and ecosystem processes
Is species diversity and primary productivity limited by seed
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
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
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
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
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
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
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
of the northern hemisphere are, for the time being at least, acting as sinks
CO2. Numerous field sites to monitor CO2 exchange
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
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 tested whether grazing
is also an important predictor of ecosystem source/sink strength in
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).