Why forests appear resistant to exotic plant invasions: intentional introductions, stand dynamics, and the role of shade tolerance

TLDR: This review reveals that in temperate and tropical regions around the world, at least 139 exotic plant species are known to have invaded deeply shaded forest understories that have not undergone substantial disturbance, and that anthropogenic processes can be expected to accelerate the rate of invasion.

Abstract: Invasion ecology has traditionally focused on exotic plant species with early successional life-history traits, adapted to colonize areas following disturbance. However, the ecological importance of these traits may be overstated, in part because most invasive plants originate from intentional introductions. Furthermore, this focus neglects the types of plants most likely to invade established communities, particularly forests – namely shade-tolerant, late-successional species. In invasion ecology, it is generally assumed that undisturbed forests are highly resistant to plant invasions. Our review reveals that this assumption is not justified: in temperate and tropical regions around the world, at least 139 exotic plant species are known to have invaded deeply shaded forest understories that have not undergone substantial disturbance. These exotics present a particular management challenge, as they often increase in abundance during succession. While forest invasions may develop comparatively slowly under natural disturbance regimes, anthropogenic processes, including the spread of exotic pests and pathogens, can be expected to accelerate the rate of invasion.