The sagebrush biome of the intermountain west, has been extensively disturbed by cattle grazing and other human activities. In some areas this has resulted in a dominance of an annual grass, Bromus tectorum, with a negative impact on native shrubs, bunchgrasses and forbs. Under such conditions a positive fire feedback, between B. tectorum and fire, has exacerbated the negative impact. A recent survey of the biome showed B. tectorum dominates in areas with dry, hot summers. In cooler, moister areas, B. tectorum is a component of the community but does not dominate; however, climate predictions suggest that such areas of the biome are likely to become warmer and drier, making them more suitable for B. tectorum.
We evaluated if a changing climate and fire would alter the plant community, giving an advantage to B. tectorum in the cooler, moister areas of the biome. We used open-top-chambers alone and in combination with rain-shelters to warm (+2 oC), and warm and dry (~20C and 55% less precipitation) experimental plots, half of which were also subjected to fire. The experiment was performed for three summers, in southwest Montana. Data were analysed, in R, using a mixed-effects model with year as a random effect.
Results/Conclusions
Bromus tectorum showed a pattern of increased cover under the fire treatment (p=0.06). Also, unburnt warmer and drier conditions resulted in the lowest B. tectorum cover when compared with the ambient and warmed unburnt treatments (p<0.01), and the warmer and drier burn treatment (p=0.06), due to an interaction between climate and fire (p<0.01). In addition, B. tectorum abundance showed a negative response to cover of the native grasses (p<0.001). Native grasses were negatively affected by fire (p=0.01) across all climate treatments. Individual fecundity of B. tectorum was significantly increased (p=0.04) and dominant grass, Pseudoroegnaria spicata, significantly decreased (p=0.04) by fire, also cross all climate treatments.
To conclude, our results suggest that a warmer and drier spring (April/May) will not directly increase the climate suitability for B. tectorum in southwest Montana sagebrush. However, warmer and drier climate conditions plus fire will likely advantage B. tectorum cover and fecundity but not the native plant community, suggesting the native community may have reduced resilience under this scenario. Overall, our results support previous research that suggests maintaining a diverse native grass community helps to mitigate dominance by B. tectorum.