White-tailed deer (Odocoileus virginianus) overabundance has sparked dramatic changes in forests throughout North America. High levels of deer browse have caused large deviations in forest structure throughout the Eastern United States and have likely reduced forest ecosystem functions, such as wildlife habitat or carbon sequestration. However, intermediate browsing intensity from smaller deer populations could potentially increase stand diversity or structural complexity. In Pennsylvania’s Allegheny National Forest, we investigated how deer have altered canopy structural complexity, species diversity, and carbon stocks within four experimental deer enclosures. Four replicate 64 ha enclosures were maintained for ten years (1979-1989) and contained four deer density treatments (4, 8, 15, 25 deer/km2, n=16). Each treatment included a clear-cut region, where deer could selectively browse saplings over the course of the experiment. Nearly 30 years later, the residual effects of deer herbivory have had a significant impact on the diversity and structure of the resulting forest canopy. We established 3, 30 x 2.5 m belt transects in each treatment and recorded canopy complexity, species composition, and diameter at breast height (DBH) for all trees > 5 cm DBH. Canopy complexity, or rugosity, was measured using a ground-based portable canopy LiDAR, which records the three-dimensional arrangement of leaves and stems within a canopy using an upward-facing laser. We predicted that treatment effects on forest composition and structure would support the intermediate disturbance hypothesis and that stands experiencing moderate levels of deer browse during the early stages of regeneration would show increases in canopy complexity, carbon sequestration, and diversity 30 years later.
Results/Conclusions
Our findings indicated a significant increase in canopy structural complexity in the 15 deer/km2 treatment, as rugosity increased from 6.58 m (8 deer/km2) to 10.15 m (15 deer/km2) and then declined to 7.69 m (25 deer/km2) (p<0.05). There was a more variable treatment response in aboveground biomass and a significant decrease in Shannon Diversity (p<0.001, r2=21) with increasing deer density. Our results provide partial support for the intermediate disturbance hypothesis, with positive effects on canopy complexity from moderate deer herbivory during regeneration, but with negative effects on canopy species diversity. This study documents a novel forest canopy response to herbivory that may affect how deer browse intensity will alter forest functionality, such as carbon sequestration and wildlife habitat, over long periods of time.