Drylands are defined by water scarcity, and concerns regarding how land-use activities may modify ecosystem productivity and ecological processes are paramount. Grazing by domestic livestock is the most widespread land-use in drylands globally. Of particular concern is potentially irreversible ecosystem state changes brought about by improper grazing on sensitive soils and plant communities. These concerns are especially salient in the American Southwest given the increased future risk of multi-year droughts brought on by climate change, the high vulnerability of important forage species, and the demonstrated low resilience of many sensitive dryland ecosystems to the combined impacts of grazing and drought.
Using a multi-study experimental process, we are asking what effects drought and grazing have on plant communities. In one long-term study, we are imposing a chronic but subtle drought (35% precipitation reduction) using passive removal shelters, across a wide geographic region. In another, we imposed extreme seasonal drought (66% reductions in summer or winter) in a grassland system over 2 years. And in a new experiment, our objective is to look at how different drought mitigation grazing strategies may influence the structure and function of dryland ecosystems of the Colorado Plateau during a more extreme drought (66% yearly reduction).
Results/Conclusions
Here we present the results from two experiments that have informed new experimental design and potential takeaways. Results from the long-term study show varying resistance and resilience to chronic drying among dominant functional types, with cool season grasses showing lowest resistance and resilience. Results from the extreme seasonal drought study show decreases in soil moisture in both seasonal drought plots followed by subsequent decreases in overall cover of grasses and forbs. However, the dominant warm-season grass, Pleuraphis jamesii (James’ galleta), exhibited an unexpected increase in above-ground biomass in the cool-season drought treatment relative to the control. These results suggest that seasonal changes in water availability can alter the structure of Colorado Plateau ecosystems by differentially impacting plant species. Taken together these results suggest drought mitigating grazing strategies are critical in maintaining desired plant communities for continued grazing.
Our conclusion was to design a new experiment, collaborating with local ranchers, in which we impose a yearly drought and test different grazing strategies on currently grazed grassland systems on the Colorado Plateau. We will present the establishment of the new experiment as well as methods and reasoning behind chosen grazing treatments in the context of being implemented in a practical framework.