Where have all the wildflowers gone? The role of exotic grass thatch

Invasion by exotic plant species can profoundly affect native plant species performance and the inferred proximate cause is typically competition. We used invaded grasslands in the semi-arid Western USA to separate resource competition from structural interference of an exotic grass on native forb performance, specifically evaluating the role of competition from living vegetation versus litter accumulation (hereafter, thatch). We simultaneously tested whether a positive thatch-feedback exists for the dominant exotic grass species, Bromus diandrus. Thatch and B. diandrus density were manipulated and coupled with native seed addition to separate the effect of B. diandrus competition relative to thatch accumulation. To determine the response of native forb species and B. diandrus to varying thatch abundance, we created a gradient of thatch densities and measured species response, soil moisture and light availability. The thatch of B. diandrus greatly reduced native forb performance (number of individuals and biomass) and resulted in near complete exclusion of many species. The effect of living B. diandrus density on native forb performance was minimal and inconsistent. Forb performance and light availability both exhibited exponential declines with thatch build-up suggesting that light reduction is a primary mechanism through which thatch affects forbs. Simultaneous with forb suppression, B. diandrus performance was positively affected by thatch, consistent with a positive feedback initiated by invasion of this species. Our results demonstrate that thatch accumulation, rather than competition for resources, is the primary cause of native species decline in grasslands invaded by this exotic annual grass. In addition, the dominance of B. diandrus in many invaded grasslands may be reinforced through a positive thatch-feedback and require active management, such as grazing or fire, to be broken.