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Prefire grazing by cattle increases postfire resistance to exotic annual grass (Bromus tectorum) invasion and dominance for decades

Kirk W. Davies, Jon D. Bates, Chad S. Boyd, Tony J. Svejcar
Ecology and evolution 2016 v.6 no.10 pp. 3356-3366
Artemisia, Bromus tectorum, annual weeds, basins, biomass, burning, cattle, ecological invasion, ecological succession, ecosystems, environmental impact, evolution, grasses, grazing, habitat destruction, herbivores, introduced plants, invasive species, mortality, plant communities, soil crusts, species diversity, temperature, wildfires, United States
1. Fire, herbivory and their interaction influence plant community dynamics. However, little is known about the influence of pre-fire herbivory on post-fire plant community response, particularly long-term resilience to post-fire exotic plant invasion in areas that historically experienced limited large herbivore pressure and infrequent, periodic fires. 2. We investigated the long-term post-fire effects of pre-fire herbivory by cattle, an exotic herbivore, in Artemisia (sagebrush) plant communities in the northern Great Basin, USA. Study areas were grazed or not grazed by cattle since 1936 and then were burned in 1993. Plant community response was measured the 19th through the 22nd year post-fire. Prior to burning exotic annual grass presence was minimal (< 0.5% foliar cover) and plant community characteristics were similar between grazed and ungrazed treatments, with the exception of litter biomass being two times greater in the ungrazed treatment. 3. Two decades post-fire, Bromus tectorum L., an exotic annual grass, dominated the ungrazed treatment. Native bunchgrasses, species richness and soil biological crusts were greater in pre-fire grazed areas compared ungrazed areas. 4. These results suggest that pre-fire herbivory by an exotic herbivore increased the resistance of the plant community to post-fire invasion and dominance by an exotic annual grass. We presume that herbivory reduced mortality during the fire by reducing fine fuel (litter) and subsequently burn temperatures. 5. Synthesis: This research demonstrates that a moderate disturbance (herbivory) may mediate the effects of a subsequent disturbance (fire). The effects of disturbances are not independent therefore quantifying these interactions is critical to preventing oversimplification of complex plant community dynamics and guiding the conservation of endangered ecosystems.