Effects of canopy gap size and deer herbivory on tree seedling dynamics and diversity in a north temperate deciduous forest

Background/Question/Methods

Spatial and temporal variation in forest canopy structure can function to maintain tree species diversity when regenerating seedlings partition associated environmental and resource gradients. However, canopy gaps can also affect patterns of herbivory and the productivity of associated species in the ground-layer.  We examined the near-term effects of factorial treatments of canopy gap size and deer (Odocoileus virginanus) exclusion on tree seedling dynamics in a temperate deciduous forest in northern Wisconsin over the first four years following treatments. We examined whether 1) a range of canopy gap sizes increases diversity by increasing the abundance of rare species, 2) deer herbivory increases with canopy gap size, reducing the abundance of the dominant tree species, 3) effects of canopy gaps on tree seedling densities depend on initial densities, deer herbivory and interactions with associated species in the ground-layer.

Results/Conclusions

Rarefaction curves showed that tree species richness, standardized to seedling density, was higher in gap treatments than controls, and higher in unfenced than fenced treatments. Increases in richness were associated with shifts in species abundance distributions, and an increase in the abundance of rare species. The density of the most dominant species, Acer saccharum, increased to a greater extent in the deer exclusion treatments than the untreated control and gap treatments lacking exclosures. In contrast, deer herbivory favored increases in the density of Ostrya virginiana, a moderately abundant species, in the gap treatment lacking exclosures. These dynamics were associated with increases in browsing on A. saccharum and Fraxinus americana in gaps. However, tree seedling dynamics also depended strongly on initial density and interactions with associated species in the ground-layer. Our results show that variation in canopy gap size can affect tree regeneration processes and diversity directly through effects on resource and environmental conditions, as well as indirectly by increasing herbivory and competition with shrub species in the ground-layer.