The legacy effects of grazing and precipitation variability on grassland productivity in a temperate steppe

Abstract

Livestock grazing have been observed to influence plant productivity and community composition in a variety of grassland ecosystems. However, it is still unclear how the influence of grazing might be exaggerated or mediated by the environment, and particularly by annual precipitation. We conducted a fourteen-year grazing experiment to compare the influence of traditional grazing regime against a no grazing treatment on previously grazed plots. Removing grazing pressure increased plant aboveground net primary productivity [ANPP] across the whole community. This increase was driven by increases in productivity in forbs, potentially due to relaxing herbivory pressure normally brought on by grazing. On average, net primary productivity increased across the entire plant community when grazing pressures were released, though there was not a significant increase in productivity of graminoids. We did observe, however, that a lack of grazing could decrease the ANPP of graminoids, potentially by allowing for a greater competitive influence of forbs. Removing grazing increased the logarithmic response ratios [LRRs] of both the whole community and forbs, regardless of precipitation. Current-year community-wide ANPP [ANPPn] was positively associated with current-year precipitation [PPTn] in both grazed and un-grazed fields, and we considered this to be largely driven by a positive relationship between rainfall and forb productivity. Interestingly, grazing alone did not influence the LRRs of graminoids over time, potentially suggesting that the long-term legacy influence of past grazing regimes is of greater importance than the acute grazing conducted during the experiment. We did find, however, that precipitation had variable influence on the relationship between grazing and LRR of graminoids. The removal of grazing was a more powerful influence on graminoid LRR during wetter years. We also found that the ANPPn of graminoids was positively associated with both PPTn and prior-year ANPP [ANPP n-1] of graminoids in both grazed and un-grazed fields, potentially due to its asexual reproduction via belowground meristems. Our results suggest that forbs are more sensitive to grazing and precipitation variability than graminoids, and cessation of grazing may be an appropriate management regime to increase community productivity and potentially associated ecosystem functions to prevent further grassland degradation.