SYMP 19-4 - Ecological transitions in a changing sagebrush steppe

Invasive annual grasses are increasingly transforming plant community composition and fire cycles across the sagebrush steppe ecosystems of the Great Basin in Western North America. In many ecosystems throughout the sagebrush biome, exotic annual grass dominance has long been recognized in state-and-transition models as an undesirable possible state. Fire feedbacks and plant-soil feedbacks can make these transitions durable and difficult to reverse, motivating efforts to protect core healthy ecosystems before they are transformed. Management strategies rely on an existing understanding of how resistance and resilience of sagebrush ecosystems vary across environmental gradients, yet these relationships are not static: as exotic annual grasses rapidly increase in dominance, our expectations of the probability and causes of transition may need updating. Here, we share preliminary results from an extensive field survey of vegetation communities across the Northern Great Basin in which we evaluated the prevalence of exotic annual grasses in relation to environmental drivers at both regional and local scales. While overall vegetation patterns with respect to climatic gradients were consistent with expectations that hotter, drier conditions would favor annual grass dominance, we were surprised to find a weaker link between fire and annual grass invasion than previously expected. Annual grass cover at unburned sites was on average just as high as burned sites, suggesting that annual grasses may be becoming less dependent on fire under ongoing global change and increasingly ubiquitous invasion. Changes in the abundance of different invasive species may also alter patterns of transitions, and we examined how major species differed in their distributions across environmental gradients. We will consider what these preliminary findings may mean for applied concepts of ecosystem states, such as ecological site descriptions and state-and-transition models, and the potential for further transitions under future global change.