Ecosystem transformations are difficult to reverse and often create novel ecosystems. These systems can be maintained by stabilizing feedbacks driven by external or internal mechanisms. Increasing our understanding of these feedback components and how they relate to each other is crucial to know if and when they can be reversed. We studied the sagebrush biome in the western United states. Here, cheatgrass invasion and fire are converting vast areas of shrublands into grasslands, which may be two alternative stable states. We aimed to understand the components of positive feedback mechanisms that maintain these two states, and the impacts that conversion from shrubland to grassland has on ecosystem function.
In chapter one the connectivity of fuel influenced the burn severity of fire, which favored fire-tolerant annuals in the seedbank. In chapter two invasion and the loss of shrubs and perennial grasses by fire changed the system from a sink to a source of soil carbon and nitrogen. In chapter three, we constructed a fire history atlas to isolate the effect of time since fire by removing the effect of repeated fires. There was little evidence of recovery towards the pre-fire state, and instead livestock grazing and cheatgrass abundance interacted to maintain the grass-dominated state. In chapter four we created an allometric equation to calculate biomass from cover estimates that can be applied to broad-scale models of cheatgrass abundance.
Overall, we document self-reinforcing mechanisms of an alternative stable state, over several decades. We demonstrate how interannual climate variability creates initial or sustaining conditions that can alter longer-term trends and interannual oscillations in ecosystem functions. Finally, there are important management implications of this work, as cheatgrass dominance can present a sustained and novel ecosystem type with few opportunities to restore to native shrubland. Cheatgrass-dominated landscapes in the U.S. Great Basin offer a critical view into how novel alternative stable vegetation states can be initiated through disturbance and sustained through self-reinforcing mechanisms. Global changes such as climate warming, expanding land use, and invasive species present novel disturbance regimes and species assemblages. This study provides key insights into what prompts and sustains ecosystem transformation in drylands.</p
