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Effects of Deer Herbivory on Hardwood Forests in the Piedmont of South Carolina

Introduction

White-tailed deer (Odocoileus virginianus) populations have reached unprecedented numbers in parts of the United States.1 Elevated deer populations result in increased herbivory pressure (feeding on plants), which decreases plant species richness (number of species) and diversity among all types of plants.2,3 Prolonged herbivory can alter plant communities such that non-preferred plant species are all that remain in the forest.3 Reduced plant diversity in the understory and overstory can alter forest ecosystems, potentially reducing the density of small mammal populations, bird species, amphibians, and reptiles.4 Prolonged levels of herbivory can result in colonization by non-native/invasive plants, as reduction in the abundance of native plant species and loss of preferred browse species results in more open niches and unused resources.3,5 In addition to increased susceptibility to forest invasion by non-native/invasive plants, deer can also introduce less desirable species by transporting propagules into uninvaded forests and plant communities.5,6 Most studies on deer herbivory have been conducted in the northern United States (i.e., Wisconsin, Minnesota, and New York), which may not be applicable to southeastern ecosystems4,7,8 for reasons such as differences in climate, deer densities, soils, and vegetation.9,10,11

Members of the genus Quercus, referred to hereafter as oaks, are native to the Piedmont region. Oaks are an important hardwood species for both wildlife and timber production. Oaks are also especially important to white-tailed deer because they feed on the acorns, leaves, and young shoots.12 The impacts that deer have on oak regeneration are complex, and a number of studies have concluded that white-tailed deer reduce oak regeneration and recruitment.7,8,13 Several studies in the northern United States showed that deer herbivory shifted the composition of the overstory by eating most of the oaks while they were in the midstory and understory layers of forests.1,2,4,7,8,14 White-tailed deer browse impacts are not all negative, as the growth rate of mature oaks increased in their presence because competition for nutrients decreased, and deer waste acted as a fertilizer.11,15 This complex relationship is often summed up to deer eat all the oaks, resulting in negative impacts for both management of forests and of deer. To better understand the relationship between oak forests and deer and enhance management of both, studies should be conducted in distinct regions, ecosystems, and over long periods of time (i.e., >10 years).

This paper seeks to fill a gap in the literature concerning the impacts of deer herbivory on hardwood forests by presenting scientific information to determine if deer are responsible for the failure of oak regeneration in the Piedmont and to assist in determining deer herd management strategies. Foresters, wildlife biologists, and forest landowners can benefit from understanding the potential impacts of white-tailed deer on southeastern US forests.

The Piedmont Study

The study was conducted on the Clemson Experimental Forest (CEF) in the Keowee and Fants Grove Wildlife Management Areas. The overstory of the CEF primarily consists of several oak species (Quercus spp.), hickory (Carya spp.), and tulip-poplar (Liriodendron tulipifera), with shortleaf pine (Pinus echinata) and loblolly pine (Pinus taeda) also present.16 In 2004, six- 5.2- to 11.4-acre stands were harvested, three in each Wildlife Management Area and two plots were established within each stand. One plot was fenced to exclude deer; the other plot remained open, allowing deer free access. Deer densities were estimated to be between 20–24 deer per sq. mile.

In 2017, woody regeneration and plant cover were assessed following the modified Carolina Vegetation Survey level 2 and 3 protocols.17 Woody regeneration was split into three categories based on height and diameter breast height (DBH; at 54 inches): seedling (0–54 inch height), sapling (>54 inches in height, < 1-inch DBH), and tree (>54 inches in height, ≥1-inch DBH).

Species richness was greater outside the deer exclusion areas, and various plant populations were present at similar proportions both inside and outside the exclusion areas. Invasive and non-native species were present in similar numbers (typically two species present) within and outside the deer exclusion zone. This leads to the conclusion that deer herbivory did not alter the plant species diversity or distribution in upland hardwood forests at current deer densities in the Piedmont of South Carolina.

Deer Herbivory and Oak Regeneration in the Piedmont Region of South Carolina

At current densities (20–24 deer per sq. mile), white-tailed deer do not impact regeneration or recruitment of oaks in a thirteen-year-old regeneration hardwood forest in the Piedmont of South Carolina. 16 The long growing season in the region (200-day) may result in plants browsed by white-tailed deer having optimal conditions for a longer recovery time.16 Plant recovery can explain differences in findings between southern and northern US studies. The diverse nature of plant communities in the Piedmont region may also help reduce the impact of herbivory. White-tailed deer in the region have a wide array of plants on which to browse, reducing the impact of herbivory on any one species.

A positive aspect of white-tailed deer herbivory is that it helps control invasive plants such as Japanese honeysuckle (figures 1 and 2) that might otherwise dominate the forests. Controlling invasive plants allows for more resources (light, nutrients, and water) for and survival of less-competitive native plants. The result can be an increase in species richness.18 The common assumption of white-tailed deer reducing oak regeneration and recruitment is not supported. However, these results may differ with stand development and geographic regions in South Carolina and white-tailed deer densities.

An area in a forest with an exclosure showing fewer vines and honeysuckle outside the exclosure compared to inside the exclosure.

Figure 1. A visual comparison of the impact of white-tailed deer on the plant community within and outside an exclosure in the Southern Piedmont of South Carolina. Note the number of vines and the large Japanese honeysuckle inside the exclosure (right), compared to few vines and small honeysuckle outside the exclosure (left). Image credit: Calvin N. Norman, The Pennsylvania State University Cooperative Extension.

An area in a forest showing increased light penetration and fewer vines outside of the exclosure.

Figure 2. A photo of the exclosures, note the increased light penetration and fewer vines outside of the exclosure (left). Image credit: Calvin N. Norman, The Pennsylvania State University Cooperative Extension.

Conclusion

Previous studies in the northern United States have concluded that white-tailed deer may impact efforts to regenerate oaks in hardwood stands. However, at deer densities of 20–24 deer per sq. mile, oak regeneration was not negatively impacted due to deer browsing in a thirteen-year-old stand in the Piedmont of South Carolina. Deer may contribute positively to oak regeneration by controlling certain invasive species and subsequently decreasing competition for resources (i.e., water, sunlight, nutrients). Furthermore, constructing exclosures to prevent deer from browsing on oak seedlings and saplings is likely not a viable management strategy for private landowners due to costs.

Acknowledgements

We would like to thank Maria Akridge, and Gregg Chapman for data collection. We would also like to thank Dr. Rickie Davis and Wayne Carroll for exclosure construction and original data collection. Finally, we would like to thank Megan Pietruszewski, Bella Norman, Dr. John Rodgers Jr., Dr. Dara Park, and three anonymous reviewers for comments and contributions to this manuscript.

References Cited

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