March towards monodominance: Deer, fern, and legacy effects in the Allegheny hardwood forest type

Background/Question/Methods

In forests, empirical work has repeatedly demonstrated that either browsing or competition from dense understory plant layers can reduce tree seedling diversity and abundance, and favor shade-tolerant and browse-tolerant species.  It is likely that herbivory and competition act synergisticly, however, field studies evaluating the interaction of these two processes are still limited.  We conducted a long-term experiment in Pennsylvania to investigate the independent and synergistic effects of white-tailed deer (Odocoileus virginianus) herbivory and understory plant competition on forest regeneration and diversity.  Deer have been over abundant in these forests for decades and contain an understory dominated by hay-scented fern (Dennstaedtia punctilobula).  We tested two hypotheses: (1) deer browsing would decrease overall seedling densities and shift seedling composition towards browse tolerant or unpalatable species; and (2) the deeply shaded conditions found under a dense fern canopy would decrease seedling densities and limit species composition to only shade tolerant species.  To test these hypotheses we paired six, 14 × 20 meter deer exclosures with control plots at each of three sites across northwestern Pennsylvania and created gaps in the fern canopy of two different sizes (control and 6.25 m²) within each exclosure and adjacent unfenced control plot.  We monitored tree seedling diversity, density, and survival in 1 m²subplots centered within each fern gap for ten years.

Results/Conclusions

We found that fern gaps significantly increased species richness, Shannon diversity (H’), total plant density, and total recruitment.  Fern gaps also significantly enhanced the density, recruitment, and survival of Acer rubrum, as well as the height of Prunus serotina.  By contrast, we found that excluding deer for a decade had neither independent nor synergistic effects with fern cover on woody species richness or abundance at any of our sites.  We argue that a century of overbrowsing has created a depauperate forest community dominated by two unpalatable species, a fern and black cherry.  Thus, even excluding deer for a decade was not sufficient to overcome overbrowsing legacy effects.  Consequently, without a stand replacement disturbance, the future forests of this region will be subject to increasing monodominance in both the understory and overstory.  This study suggests that long-term forest dynamics in the Allegheny Plateau will lead to increasingly depauperate forest communities.  Consequently, forest management practices that promote the establishment and growth of diverse regeneration may not only be useful, but perhaps indispensable, in order to curb the trend toward increasing monodominance.