Long-term patterns and mechanisms of plant invasions in forests: the role of forest age and land-use history

It has become increasingly apparent that even mature forests are susceptible to plant invasions. However, invasive plants are often more abundant in younger forest stands. It is difficult to disentangle possible mechanisms that would explain this pattern due to the scarcity of long-term studies in successional forests. Several mechanisms have been proposed to explain patterns of invasions as forests age, including biotic resistance, window of opportunity, historical legacies, and invasion debt. We explored patterns and potential mechanisms of plant invasions over 70 years in a regenerating forest with different land-use histories in the Bolleswood Natural Area, Connecticut, USA. We examined how environmental factors related to colonization patterns of invasive and non-invasive introduced species over time, and whether these patterns were consistent with the proposed mechanisms. Non-invasive introduced species declined rapidly with canopy closure, while many invasive plants persisted or even increased over time as the forest aged. Colonization was focused in younger post-agricultural areas, although this declined with time. Dispersal distance, soil conditions, and land-use history played important roles in patterns of colonization, while the effect of shading was less clear. There was some evidence for each mechanism, but the relative importance of each mechanism was species-dependent, making generalizations about how invasive plants invade forests difficult. We found that land-use history impacted invasion more strongly than forest age, but over time even mature forests were slowly being invaded by some species. Thus, invasive species management may be required even in mature forests.