Living on leftovers: biomass management in annual grasslands may shift functional group dominance

Livestock grazing in North American rangelands has the capacity both to promote and control the spread of undesirable plant species. Within California annual grasslands, desirable forage peaks in spring and supports considerable livestock grazing. However, spring grazing appears to promote the invasion and spread of two late-season and unpalatable non-native annual grasses, Aegilops triuncialis and Elymus caput-medusae. We tested the hypothesis that grazing reduces the leaf area and water use of early-spring annuals, thus increasing residual soil water availability for the late-season species. We used grazing-exclosure experiments to examine the interactive effects of simulated grazing (i.e., clipping) and competition on soil moisture availability, and on physiological, phenological, and demographic responses. When compared to unclipped controls, spring clipping significantly increased late-season volumetric soil moisture by 13–24% in the top 7 cm of soil, and 8–11% in the top 20 cm of soil (p < 0.05, all sites), which supported significantly higher rates of stomatal conductance (73–100% increase) in both late-season invading species (p < 0.01, all sites). Flowering was significantly delayed in clipped plots for both invader species suggesting these species experienced a longer growing period (p < 0.0001 in all cases). In competition plots, the effects of clipping on the demographic response depended on neighborhood composition. When invaders were grown together, no significant effect of clipping on survival or reproduction was detected in either invader. However, when growing in mixtures with early-spring forage annuals or native species, clipping increased survival and reproductive output in late-season invader species by 3-fold. We suggest that strategies for arresting or reversing the dominance of these late-season invasive annuals must recognize the influence of current biomass management strategies on late-season resource availability.