Avoid, tolerate, or escape? Native vegetation responses to invasion vary between functional groups

Biological invasions are one of the greatest threats to biodiversity and ecosystem functioning. However, the constraints imposed by the invaders on native organisms and their associated response, remain poorly understood. Native species can survive invasion through multiple resistance strategies (avoidance, tolerance, or escape), but the relative importance of each strategy and how they vary among functional groups have been little explored. In this study, we examined the resistance strategies of native forbs and graminoids facing invasion by Solidago canadensis . First, we characterized the general impacts of invader density on native plant biomass production and diversity. Then, we investigated specific constraints linked to the invasion (competition for light, nutrients and mycorrhizal fungi), and the associated resistance strategies of native species. S. canadensis had different negative impacts on native vegetation biomass production and diversity—depending on functional groups—due to increased competition for light, nutrients, and mycorrhizal interactions. The increased competition for light was partially (1) avoided (tall forbs and graminoids) or (2) tolerated (small, shade-resistant graminoids). The effects of (3) allelopathic compounds and (4) increased competition for nutrients were avoided by some forbs (high mycorrhizal infection rates). Finally, some forbs and graminoids (5) escaped all constraints by completing their cycle early in the season. Our results highlight the diversity of non-exclusive strategies (avoidance, tolerance, escape) by which different functional groups can respond to invasion-induced constraints. They suggest that to improve understanding of the mechanisms underlying invasion, the native community responses should be decomposed into strategies specific to functional groups.