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

Authors: Davies, Kirk; Bates, Jonathan - Jon; Boyd, Chad; Svejcar, Anthony

Submitted to: Ecology and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2016
Publication Date: 5/20/2016
Publication URL: https://handle.nal.usda.gov/10113/62575
Citation: Davies, K.W., Bates, J.D., Boyd, C.S., Svejcar, A.J. 2016. Pre-fire grazing by cattle increases postfire resistance to exotic annual grass (Bromus tectorum) invasion and dominance for decades. Ecology and Evolution. 6(10):3356-3366. doi: 10.1002/ece3.2127.

Interpretive Summary: Grazing and fire occur across rangelands globally, however, little is known about the influence of pre-fire grazing on post-fire plant community response. This information is critically needed in dry sagebrush plant communities that did not recently evolve with high grazing pressure and are at risk of exotic annual grass invasion. We investigated long-term post-fire responses in areas that were grazed compared to not grazed prior to burning. The ungrazed treatment was applied by constructing exclosures in 1936 and then the grazed and ungrazed treatments were burning in 1993. Two decades post-fire, exotic annual grasses and native bunchgrasses dominated the ungrazed and grazed treatment, respectively. Pre-fire grazing reduced native bunchgrass mortality during the fire and this likely limited exotic annual grass invasion post-fire. This research suggests that moderate grazing may decrease the risk of post-fire exotic annual grass invasion and dominance.

Technical Abstract: 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.