Print PageClosed canopy, “intact” forests (i.e. with low anthropogenic disturbance or fragmentation) are traditionally viewed as highly resistant to invasion, mainly because the evidence of exotic invasions is much lower than other ecosystems. However, patterns of invasion dynamics in intact forests are obscured by the longevity of tree species and inherently slow dynamics in low-light environments. Combined with few long-term studies, this has left the long-term invasion dynamics of forests an open question. We used an empirically parameterized spatially-explicit forest simulator (SORTIE-ND) to explore the long-term invasibility of closed-canopy forests. The model is parameterized with species-specific field demography data on growth, mortality, allometry, shade cast, fecundity and dispersal to simulate invasion by two exotic trees, Ailanthus altissima and Acer platanoides. We also explored the role of disturbance in invasion, parameterizing observed patterns of natural (wind regimes) and human processes (logging). We asked two primary questions: (1) What is the long-term capacity of these two exotic species to invade closed canopy forests? (2) What life-history traits are key to invasiveness in forests?
Results/Conclusions
Both exotic species are successful invaders of forests, but in (mostly) different ways. With an equal initial abundance of natives and exotics, the exotics quickly dominate open forests, but without further disturbance, Ailanthus slowly declines through successional processes. Acer platanoides remains abundant with or without disturbance once its population numbers are sufficiently large. The primary trait behind the invasiveness of these exotic species is their extraordinary growth rates, but alone this does not create a strong invasive; additional life-history advantages are needed. Integrated metrics of life-history trade-offs (based on species-specific demography) show that the exotic species are not subject to the same trade-offs as native species: Ailanthus has both high growth and high fecundity, and A. platanoides combines high growth with high shade tolerance. Overall, invasion resistance is dependent on native species composition but not diversity—late successional natives maintain robust abundance throughout simulations, while early and mid-successional natives are much more impacted by the invasives. Resistance was also sensitive to initial stand structure and initial abundances of the exotic species. A frequent disturbance regime, either wind or logging, accelerates the rate and increases the eventual magnitude of invasions, showing that forest management should consider exotic species. Moreover, the diseases threatening late successional species like Tsuga canadensis and Fagus grandifolia are all the more urgent given their importance for the invasion resistance of the native forest.
