Vegetative growth drives the negative effects of an invasive species on resident community diversity and is not limited by plant–soil feedbacks: A temporal assessment

Many pathways of invasion have been posited, but ecologists lack an experimental framework to identify which mechanisms are dominant in a given invasion scenario. Plant–soil feedbacks (PSFs) are one such mechanism that tend to initially facilitate, but over time attenuate, invasive species' impacts on plant diversity and ecosystem function. PSFs are typically measured under greenhouse conditions and are often assumed to have significant effects under field conditions that change over time. However, direct tests of PSFs effects in natural settings and their change over time are rare. Here we compare the role of PSFs with the effects of biomass in limiting the dominance of an invasive species and impacts on resident species diversity. We characterized the effects of the invader Bromus inermis (Leyss.) on native plant communities over time and measured changes in its conspecific PSFs and vegetative growth to understand their integrated effects on community diversity. To do so, we combined data from a 6‐year field study documenting the rate and impacts of invasion with a short‐term greenhouse experiment quantifying PSF as a function of time since invasion in the field. We found that the nature and strength of B. inermis PSFs did not change over time and were not mediated by soil microbial communities. Though PSFs impacted B. inermis reproduction, they did not sufficiently limit vegetative growth to diminish the negative impacts of B. inermis biomass on native species. B. inermis experienced the full strength of its negative PSFs immediately upon invasion, but they were ineffective at reducing B. inermis vigor to facilitate the recovery of the native plant community. We recommend that conservation efforts focus on limiting B. inermis vegetative growth to facilitate community recovery. We investigated the role of plant–soil feedbacks (PSFs) and biomass in limiting the dominance of the invasive species Bromus inermis. We found that though PSFs affected B. inermis reproduction, they did not reduce its vegetative growth or its negative effects on native species diversity over time. We suggest conservation efforts should prioritize controlling B. inermis vegetative growth to aid native community recovery.