Forest regeneration unaffected by severe understory plant invasion

Background/Question/Methods

Numerous ecological impacts have been attributed to invasive species, but one that has been little studied is the possible inhibition of forest regeneration by alien plants. If invasions lead to a suppression of native tree seedlings in forest understories, the short-term effects may be subtle, but dramatic consequences may manifest over decades or centuries. To address current knowledge gaps, we investigated the effects of alien understory plants on advance regeneration in a heavily invaded urban forest fragment in the mid-Atlantic region of the United States. We also compared the effects of alien plants to those of native plants, and examined the role of canopy cover.

Results/Conclusions

We found that alien understory cover and native tree seedling density were completely unrelated, both when all native species were pooled and when each native species was analyzed independently. Similarly, our data revealed that tree regeneration was unaffected by native understory cover. However, we found that native plants (tree seedlings as well as other native understory plants) were negatively associated with higher canopy cover; in contrast, alien understory cover was unaffected by canopy cover. Collectively, our findings suggest that invaders may be able to occupy a unique niche in deeply shaded understories, but that native tree seedlings are largely unaffected by understory vegetation, regardless of whether it is alien or native. As such, our work provides hope that even heavily invaded forests may be able to regenerate adequately, and that understory invasions may therefore have limited implications for the forest canopy.