Reduction of invasive plants is a common management goal; however, restoration success is often significantly constrained by legacies of invaders. These legacies are often formed through interactions with soil biota over the course of invasion and are often strongly spatially structured. Invasive plants may interact with beneficial soil symbionts to enhance their own fitness by selecting for a generalist beneficial symbiont to enhance their own performance. Conversely, they may reduce the abundance and diversity of soil symbionts that native plants are differentially dependent upon and thus indirectly increase performance differences. Lastly, invasive plants may act as a naive host to generalist pathogens and increase the abundance of generalist pathogen populations to indirectly alter plant-plant interactions and directly inhibit performance of pathogen sensitive native species. Here, we ask how these plant soil interactions develop spatially among reserves and temporally over the course of invasion of a recently expanding invasive forb (Oncosiphon piluliferum) in Southern California grasslands to influence native plant performance. To answer this question we conducted a plant-soil feedback greenhouse experiment where we grew six native forbs and Oncosiphon piluliferum in sterile soil inoculated with soil from three reserves, across four invasion histories. We sampled plant biomass after 8 weeks.
Results/Conclusions
Our results indicate that Oncosiphon piluliferum soil conditioning significantly inhibits native forb performance as well as its own performance, reflected in negative biomass response ratios. The temporal development, with degree of invasion as a substitute for time, affects the intensity of conditioning where native plants grown in uninvaded and partially invaded soils had more positive biomass response ratios than those grown in later stages of invasion. Our results indicate that Oncosiphon piluliferum produces soil legacy effects that reduce the performance of native plants and reduces its own performance, suggesting potential reduction of soil symbionts as well as generalist pathogen accumulation. These mechanisms (reducing soil symbionts and increasing soil pathogens) reducing native plant performance are not mutually exclusive, where invasive plants may reduce the abundance and diversity of native soil symbionts while simultaneously increasing the abundance of pathogens in the soil. Our study provides evidence successful invader management must take into consideration the time since invasion and site characteristics to improve the efficacy of invader management and restoration.