Indirect and interactive effects drive impacts of prescribed fire and invasive grass on a native wetland herb

Understanding the indirect and interactive effects of environmental stressors is critical to planning conservation interventions, but such effects are poorly understood. For example, invasive species may modify fire effects by altering fire intensity or frequency, increasing or decreasing their abundance in response to fire, and/or changing the trajectory of post-fire recovery. Without a clear understanding of the direct, indirect, and interactive effects of prescribed fire and invasive species on native plants, managers cannot design effective conservation measures and risk exacerbating invasion through fire or wasting resources on approaches that do not yield desired results. In this study, researchers worked directly with the manager of a wet meadow in southern Idaho to explore how prescribed fire would directly and indirectly impact an iconic native herb (Camassia quamash) in areas invaded by a perennial pasture grass (Alopecurus arundinaceus). We found that spring prescribed fire increased the abundance of invasive A. arundinaceus, which indirectly strengthened its suppression of C. quamash growth and reproduction. In contrast, fire reversed the negative influence of A. arundinaceus on C. quamash survival. Survival rates of C. quamash were higher after fire in areas with greater invasive grass abundance. This study points to the importance of understanding the indirect and interactive effects of prescribed fire and invasives on native plants across their life cycle for restoration projects and suggests fire, at least in spring, is not an appropriate management strategy for reducing A. arundinaceus invasion at this site.