COS 137-9
Living on “leftovers”: Effects of simulated grazing on the soil moisture niche of two late season grassland weeds
Domestic livestock grazing in many North American grasslands has long been implicated as a factor potentially promoting the establishment and spread of invasive plant species. However, we still do not have a good understanding of how grazing modulates the competitive relationships between existing vegetation and newly introduced invaders. Within the California annual grassland, most ranchers practice spring grazing in order to take advantage of the pulse of primary production that occurs typically during March and April. Field observations indicate that this spring grazing practice appears to promote the invasion and spread of two late season annual grass invaders, Aegilops triuncialis (barbed goatgrass) and Elymus caput-medusae (medusahead). By reducing leaf area of early spring season annuals, grazing can reduce water use by this dominant plant group and increase the amount of residual soil water available for use by late season invaders. We used field grazing-exclosure experiments and planted plots to examine the potential interactive effects of simulated grazing (i.e. clipping) and competition on soil moisture availability as well as physiological, phenological, and demographic responses in these two grass invaders. We predicted that simulated spring grazing should increase residual soil moisture availability and thus the performance of late season invaders.
Results/Conclusions
When compared to unclipped controls, spring clipping significantly increased late season volumetric soil moisture at both 7 cm and 20 cm soil depths. This higher soil moisture availability in clipped plots allowed significantly higher rates of stomatal conductance for both invader species. In the clipped plots, flowering was significantly later for both species and suggests that the growing season for these invaders was extended under clipping because of larger reserves of residual soil moisture. In the planted plots, the effects of clipping on the demographic response of both invaders depended on whether competition was between the two invaders or with other plant groups such as early spring annuals. When the invaders grew together, there was not a significant effect of clipping on survival or reproduction in either invader. However, when growing in mixtures of early spring annuals or native species, clipping strongly increased both survival and reproductive output in both invaders. Spring grazing modulates competition between these late season invaders and the rest of the grassland community by affecting the availability of residual soil moisture. Delaying grazing until after this residual moisture has been used by the annual community may reduce the population growth and spread of late season invaders.