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

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.

Impacts of biological invasions: what's what and the way forward

“…First is the need to shift attention further from dominant focus on the properties of invading organisms to how anthropogenic changes in ecosystems facilitate many invasions (e.g., [62,80] but see [81]). Such a shift can lead to new ways to prevent invasions or to mitigate consequences of ongoing ones, for example through grazing or water management policies.…”

Plant invasions are context‐dependent: multiscale effects of climate, human activity and habitat

“…For instance, woodlands showed high non-native species presence but extremely low richness and abundance. The few shade-tolerant non-native species that could overcome the limitation of light availability in woodlands are very likely to survive even if it is in low numbers (Martin et al, 2009). However, the species able to invade woodlands are generally rare or might be still in an incipient phase of expansion (Martin et al, 2009;González-Moreno et al, 2013a), reflecting low local richness and abundance.…”

A Comparison of Invasive Acer platanoides and Native A. saccharum First-Year Seedlings: Growth, Biomass Distribution and the Influence of Ecological Factors in a Forest Understory

“…Although a minority of invasive species are woody plants, a small proportion of these are tolerant to shade, which allows them to invade undisturbed, closed-canopy forests [3,4]. Shade-tolerant trees can endure periods of suppressed growth before reaching the canopy when gaps are created [5].…”

“…This low growth rate, in combination with the long generation time of trees, results in lag phases that slow the invasion process, making it difficult to perceive as it unfolds, and hence, difficult to manage in its early stages [4,[6][7][8]. However, these long-lived, competitive species, which may become dominant over time through succession, have the potential to deeply modify forest ecosystems and cause severe and pervasive impacts on plant communities [4,6,7].…”