Invader soil conditioning impacts invader and native plant performance

Plant invaders can promote invasion success through interactions with soil-biota (i.e., soil-conditioning), forming feedback, which can change in strength and direction over time. Thus, native plant responses to invader soil-conditioning dynamics may be dependent on the degree of invasion and could additionally be mediated by plant functional traits. To investigate the temporal dynamics of invader-soil-conditioning and the role of traits in mediating plant responses, we conducted a greenhouse experiment focusing on Oncosiphon pilulifer, an invasive annual forb spreading across Southwestern United States and Western Australia. We grew Oncosiphon and six native plants in live whole soil vs sterilized whole soil inocula from an existing Oncosiphon invasion gradient, with four levels of invasion ranging from uninvaded, small patches, large monocultures, and the origin point of invasion resulting in a space-for-time substitution. We measured plant biomass, mycorrhizal root colonization, and leaf and root traits. We found native plant growth was reduced with soil from patchily invaded soils, while mycorrhizal root colonization rates were reduced with Oncosiphon monoculture soil. Oncosiphon itself experienced reduced growth over the course of invasion, with consistently low root colonization. Our trait analysis suggests an interaction between root and leaf traits can mediate plant vulnerability to invader impacts on soil-biota.