Drought, grazing, and reproduction: Plant strategies under future climate scenarios in a working landscape

Thomas Merchant, Institute of Arctic and Alpine Research; Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Boulder, CO, Julie E. Larson, Department of Ecology and Evolutionary Biology; Institute of Arctic and Alpine Research, University of Colorado, Boulder, Boulder, CO and Katharine N. Suding, Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO

Background/Question/Methods
Climate change is causing more frequent and severe droughts across the globe, impacting the functioning of working landscapes such as rangelands. While it is well established that both drought and grazing management can impact rangeland dynamics, less is known about how they interact to drive plant community turnover. Here we explore the mechanisms underlying plant community change, focusing on how these two disturbances impact the reproduction and growth of individual plant species. We compared these effects across species from different functional groups (annual grasses, perennial C4 and C3 grasses, and forbs) in a xeric tallgrass prairie in a manipulative rainfall and grazing experiment near Boulder, Colorado. Rainfall and grazing were manipulated for a total of nine treatments, three rain (-66%, ambient, +66%) and three grazing (spring graze in June, fall graze in October, and no grazing). We measured community level responses such as seedbank and plant community composition, as well as individual level measurements of plant size, inflorescences produced, and seed counts. We expected additive stressors to generally reduce growth of individuals in long-lived perennials, and shift resources from reproduction to survival. In contrast, we expected growth, and reproduction of early season annuals respond positively to drought and grazing manipulations because of a release from competition.
Results/Conclusions
Results from a variety of growth and reproductive indices over two years indicate that grazing and drought can have complex consequences for both plant reproduction and growth. Preliminary results from the seedbank (i.e., community-level metric of seed production) show that for some annual species seed production increases under drought conditions (e.g., Vulpia octoflora), particularly in combination with increased growth under grazing conditions. In contrast, a late season C4 grass (Andropogon gerardii) was negatively impacted by spring grazing, which reduced plant height several months after the grazing occurred, and positively affected by water additions without grazing. Interestingly, fall grazing (during the dormant season) enhanced flowering stalk production, potentially due to increased deep-water infiltration in the fall grazed plots. We saw less of an initial impact on the growth of relatively long-lived C3 grasses (e.g., Koelaria macrantha). However, ongoing seed production counts at the individual plant level will shed further light on the subtler impacts of interacting climate and management factors. These results illustrate that exploring how drought and grazing scale from the individual to community level is critical to understanding and mitigating the mechanisms of community turnover under global change.