Much ado about deer? 5-8 years of deer exclusion does not impact the understory plant community in temperate forests in northeastern Ohio

Background/Question/Methods

In the eastern United States, changes in the abundance of herbivores, especially large ungulates, such as white tailed deer (Odocoileus virginianus), can have large effects on forest plant community composition and regeneration. Although many studies have found that high deer populations negatively affect the survival and growth of tree seedlings and herbaceous plants, others have found that excluding deer alone does not result in recovery of plant communities. To assess whether the effect of deer varies across the landscape and by site within individual forests, we utilized an existing network of deer exclusion/access plots in temperate hardwood forests at The Holden Arboretum where the managed deer population density is approximately 8 deer/km². One set of plots intensively sampled edge and interior zones at two sites within one sub-watershed using 40 plots randomly assigned to deer-access and deer-exclusion treatments in 2007. The other set examined forest interiors across three forest types in separate sub-watersheds using 22 paired deer-access/exclusion plots (44 sub-plots total) established between 2005 and 2007. In both studies, plots or sub-plots were 10 x 10-m in size, deer were excluded with ~2.5-m tall fences, and vegetation was sampled at exclosure establishment and periodically for up to eight years.

Results/Conclusions

After 5-8 years, no significant effects of deer exclusion were observed on tree regeneration and growth or herb abundance and diversity, regardless of the scale and design of the deer exclusion study. This is in dramatic contrast to other studies that have observed rapid plant community response to deer exclusion. However, edge environmental effects were detected in the intensive study. Further, herbaceous plant communities were impoverished (<10% cover, on average) in comparison to other similar types of forests. There are several possible explanations for the lack of effects of deer exclusion on these forest plant communities: 1. As a result of deer management, deer densities at Holden are only slightly above the recommended population for sustainable forest plant communities, and thus could currently be having little effect. 2. Previous chronic deer pressure combined with dispersal limitation could mean that understory plant communities have not recovered following deer exclusion. 3. There may be other environmental factors driving herb layer impoverishment such as chronic acid deposition, non-native worm invasion, and legacies of former land-use. Why understory communities at Holden are responding in this manner is uncertain, and requires additional investigation.