The seasonal timing of precipitation and temperature mediate rangeland vegetation response to grazing
Climate and grazing are both strong drivers of grassland community structure and function. We know that plant communities can be very sensitive to temporal changes in precipitation and temperature, but little is known about how seasonality interacts with grazing. In light of climate models that predict seasonal changes to temperature and precipitation regimes in many places worldwide (IPCC, 2007), we investigated the importance of precipitation seasonality and growing season duration on broad-scale patterns of plant community response to grazing. We used data from grazing exclusion studies worldwide (n=222) to conduct a meta-regression, determining the relative importance of selected climatic factors (seasonality of precipitation, growing season duration, and mean annual precipitation) and grazing variables (grazing intensity and grazing duration) in influencing changes in plant species composition and primary production due to grazing. In order to take into account how systems that evolved with the presence of grazers may vary in their response to grazing, we used a hierarchical Bayesian approach, allowing evolutionary history of grazing to modify the effect of each of the other variables.
Grazing intensity, duration of grazing exclusion, mean annual precipitation, and seasonality of precipitation all influenced grazing effects on plant communities. Wetter locations and sites with a greater proportion of annual precipitation occurring in the summer experienced greater grazing-induced changes in species composition. The magnitude of species composition response to precipitation seasonality was greatest in sites with a long evolutionary history of grazing. Change in production due to grazing was most responsive to the duration of the growing season, but was also influenced by duration of grazing exclusion and mean annual precipitation. Wetter locations and those with shorter growing seasons experienced greater decreases in production due to grazing. The effect of growing season duration on production response was greater in systems with longer evolutionary histories of grazing. Results from this study indicate that in addition to factors already known to control grazing effects, responsiveness to grazing can vary depending on the seasonal timing of precipitation and temperature, particularly in systems with longer evolutionary histories of grazing. This study highlights the complexity of the interactions at play in rangeland systems, which will be important to take into consideration when assessing vulnerability to climate change.