Forests at the urban-rural interface are often subject to decades of over-browsing by white-tail deer and the invasion of numerous exotic plant species. The combination of invasion and over-browsing can potentially create forests where native forbs are extirpated or at very low abundance, and native trees fail to regenerate. These processes can lead to the loss of native trees, shrubs, and wildflowers. Even if browsers and exotic species are excluded or removed, long-term legacy effects may ensue where the native community is restricted by the regional species pool. Here, we test the hypothesis that the exclusion of deer and the removal of exotics will lead to increases in the abundance and diversity of the native community. However, we predict that only native species that persisted locally within the community will be able to recover when released from overbrowsing and exotic plant pressures. Thus, we also hypothesize that full community restoration will be propagule-limited and will require the addition of seeds of native species. We tested these hypotheses in a 250ha forest in western, Pennsylvania, by experimentally excluding deer using five large exclosures and paired control plots and by removing all exotic plant species from half of all plots.
Results/Conclusions
The exclusion of deer led to increases in the total cover of native species as well as a subset of exotic species, suggesting that restoration of biodiversity will require both the exclusion deer and the removal of exotics. Our surveys of the flora of this large forest fragment suggest a forest nearly devoid of many native wildflowers and shrubs that were likely present prior to over-browsing and the subsequent invasion of exotic plant species. We suggest the restoration of the native flora will require the introduction of native plant species that have become locally extirpated. Thus, many urban forests and those forests at the rural-urban interface may be subject to very long legacy effects because of propagule limitation. Consequently, even reductions in the size of deer herds and the elimination of exotics alone will often fail to lead to recovery or require decades for native species to recolonize these sites via rare dispersal events.