Nonindigenous plant species (NIS) can affect individuals, communities, and ecosystems through numerous direct and indirect mechanisms. To synthesize the current understanding of how NIS cause impacts, we reviewed experimental research from the past decade. We found alteration of the microenvironment, such as incident light and air and soil temperature, was much more often a mechanism underlying NIS impacts than competition for soil water and nutrients. NIS litter frequently caused the alteration of microenvironments, and litter effects were often of greater consequence than the effects of live NIS plants. Results supported altered soil microbial communities and mycorrhizal associations as mechanisms underlying NIS impacts on native plant growth, community structure, and nutrient cycling. Impacts often could not be attributed to a single mechanism, highlighting the need for multi-factor studies that identify and distinguish between multiple, concurrently operating mechanisms. Overall, our synthesis indicates that effective management will require attention to legacy effects of NIS, that removing live NIS may not ameliorate impacts, and that removal of dead NIS biomass may be necessary for native species' survival. Furthermore, rehabilitating soil microbial and mycorrhizal communities may be crucial for successful post-NIS management revegetation.
Management Implications: The primary objective of most nonindigenous plant species (NIS) management is to reduce or eradicate live NIS plants using herbicides and manual removal. While this often is a sensible approach, it does not always achieve the desired outcome of reestablishing native plant cover. Through a review of experimental NIS impact studies, we found that consideration of the mechanisms, or ways, by which NIS cause impacts, may improve general approaches to NIS management. It is often assumed that the primary way NIS impact native plants is by robbing them of necessary resources, such as water and nutrients. However, the experimental research we reviewed shows that this often is not the underlying cause and that NIS impacts are more often traced to alterations of microenvironments, belowground communities, and plant-pollinator interactions. Additionally, many NIS impacts are attributable to NIS litter rather than live plants. Reducing NIS impacts and reestablishing native plant cover may be more successful if some resources typically spent on direct NIS treatment (i.e. spraying, pulling) are diverted to removal of NIS litter and applying mycorrhizal and microbial inoculations to improve or rebuild belowground communities. These communities are thought to be an important component of revegetation success. Another alternative management approach based on impact mechanisms is NIS flower removal, particularly when whole plant removal or chemical treatment is not feasible. Removing NIS flowers prevents native plant-pollinator interactions from being altered, increasing the chances of successful native plant reproduction. NIS flower removal has the additional benefit of eliminating NIS seed production, thereby lowering or stabilizing population growth rate. Our synthesis shows that fine-tuning management to address the mechanisms underlying NIS impacts will likely improve NIS control and increase the success of native revegetation.