Influence of deer herbivory on regeneration dynamics and gap capture in experimental gaps, 18 years post-harvest

Declines in species diversity have been linked to shifting disturbance regimes and increases in ungulate herbivory. Successful forest regeneration depends on silvicultural methods that can address stand-specific recruitment limitations. This study revisits a long-term experimental gap study in a hemlock-hardwood forest. Our objective was to determine if the treatment effects of deer exclosures and gap size would alter regeneration trajectories. We examined cohort development using permanent plots and gap-wide surveys of likely gap-capture trees. After 18 years, deer herbivory was a strong filter for tree regeneration and appeared to pose an insurmountable bottleneck to the recruitment of some species. Overall, 90% of unprotected stems >0.5 to <= 2 m showed evidence of being browsed by white-tailed deer (Odocoileus virginianus Zimmerman). Our results indicate that browse sensitive species were unable to recruit even in resource-rich gap environments. Eastern hemlock (Tsuga canadensis L.) and yellow birch (Betula alleghaniensis Britton) were particularly sensitive to herbivory and absent in heights >1 m outside exclosures. However, inside exclosures in larger gaps, yellow birch saplings often co-dominated the tallest cohorts and eastern hemlock was well established at heights >1 m. Sugar maple (Acer saccharum Marsh.) was the dominant tree species across all treatments and ironwood (Ostrya virginiana Mill.) was frequently among the tallest trees in gaps outside of deer exclosures. Gap size only impacted unprotected seedlings which suggests regeneration is more sensitive to gap size when deer are present. The installation of relatively small (similar to 3.5% of total gap area) deer exclosures, at year two post-harvest, provided opportunities for the recruitment of browse-sensitive species. Collectively, our results highlight the pervasive role of ungulates in structuring forest composition.

Automated summary

The study finds that deer herbivory significantly impacts tree regeneration in the forest, particularly for browse-sensitive species. The treatment effects of deer exclosures and gap size can alter regeneration trajectories.