# Invader soil conditioning impacts invader and native plant performance

## TL;DR

This study shows how invasive plants like Oncosiphon pilulifer change soil conditions to suppress native plants and how plant traits influence vulnerability to these effects.

## Contribution

The study reveals how invader soil-conditioning dynamics and plant traits mediate native and invader plant performance over time.

## Key findings

- Native plant growth was reduced in soils from patchily invaded areas.
- Mycorrhizal root colonization decreased in soils from Oncosiphon monocultures.
- Plant traits like leaf and root thickness influence sensitivity to soil-mediated invasion effects.

## Abstract

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 the 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 that an interaction between root and leaf traits can mediate plant vulnerability to invader impacts on soil-biota.

Invasive plants often enhance invasion success by altering soil communities in ways that inhibit native species. We studied soil interactions associated with the emerging invader Oncosiphon pilulifer and found that it suppresses native plant growth while also limiting its own performance over time. We further show that plant traits, such as leaf and root thickness, influence how strongly species respond to these soil-mediated effects, highlighting trait-based sensitivity to invasion.