Plant community response to prescribed fire varies by pre-fire condition and season of burn in mountain big sagebrush ecosystems

Artemisia tridentata ssp. vaseyana ecosystems evolved with periodic fire, but invasive grasses, conifer encroachment, fire suppression, and climate change have resulted in altered fire regimes and plant communities. Post-fire increases in invasive annual grasses such as Bromus tectorum and reductions in native vegetation are common across the sagebrush steppe. Where fire has been excluded though, there are increases in the native tree Juniperus occidentalis, which outcompetes the native understory. We applied prescribed fire in spring and fall at three sites (native-dominated, B. tectorum-dominated, and J. occidentalis-dominated). We documented 65% survival of A. tridentata following fall burns and 33% survival following spring burns in native-dominated plots, with evidence of post-fire sprouting in Purshia tridentata and Tetradymia canescens. At the B. tectorum-dominated site, shrub cover was reduced to <1%. Fires at the J. occidentalis site were discontinuous, resulting in ∼50% mortality of trees and shrubs, with little resprouting. Native herbaceous vegetation persisted following fires, with no increases in B. tectorum. There were higher plant survival rates following fall fires and native-dominated sites than in spring burns or where exotics dominated. These results show that burn season and prefire condition are important considerations when evaluating management alternatives in A. tridentata ssp. vaseyana ecosystems.