Reduced Seedling Height of Hard Mast Species in Areas with Low Deer Densities in an Oak-Dominated Forest

Reduced regeneration of oak and hickory species is of significant concern to scientists and land managers in the Central Hardwood Forest Region. Much of the decline has been attributed to a shift in land management practices (e.g. fire suppression, reduced harvest intensity) that have favored mesophytic species at the expense of hardwood mast species (Nowacki and Abrams 2008, Fralish and McArdle 2009, Holzmueller et al. 2011). Further complicating the issue is the increased abundance of white-tailed deer (Odocoileus virginianus) in the United States over the past 100 years (VerCauteren and Hygnstrom 2011). When present in large numbers, deer have been documented to cause a shift in plant species composition because of their browsing preferences (Russell et al. 2001, Rooney et al. 2002, Adams and Hamilton 2011). In the eastern U.S., states with dense deer populations (> 12 deer/km²; e.g., Pennsylvania, Wisconsin, and West Virginia) have been reported to have a negative impact on forest regeneration by reducing seedling density and height of desirable hard-mast species (Tilghman 1989, Rooney and Waller 2003, Miller et al. 2009). These impacts reduce the efforts of land managers in the restoration of oak and hickory species.

While the effects of deer have been documented in areas with high deer populations, the objective of this study was to investigate the longterm impact of how deer affect forest regeneration with low to moderate deer populations (6–12 deer/km2) in combination with a relatively strong hunting pressure (2–3 annually reported harvested deer/km2) (Adams and Ross 2013, IL DNR 2014a).

This study was conducted within the Shawnee Hills region of southeastern Illinois in Pope County. The region has an annual precipitation rate of 145 cm, a growing season of 170–220 days per year, and an annual high and low temperature of 20.61°C and 8.77°C, respectively. Soils on the sites are dominated by well-drained silt loams (Wellson-Berks complex, mesic ultic hapludalfs and mesic typic dystrudepts, respectively) and site index ranges from 18–24 m for upland oak (base age 50).

The study site consisted of a 10 ha block of upland hard-wood forest and was primarily comprised of white oak (Quercus alba), red oak (Q. rubra), post oak (Q. stellata), black oak (Q. velutina), shagbark hickory (Carya ovata), pignut hickory (Carya glabra) in the overstory and white ash (Fraxinus americana), black cherry (Prunus serotina), sassafras (Sassafras albidum), American elm (Ulmus americana), and sugar maple (Acer saccharum) in the under-story. To avoid edge effects, eleven deer exclosure plots (DEP) were placed within a 1.6 ha section in the center of the larger forested block in 2003 (at least 100 m from the forest edge). The exclosures were 3 m × 3 m in area, 1.5 m tall, and wrapped with wire fencing to keep deer out of the plots. Within each DEP, we recorded the height (cm) of the tallest woody seedling (> 30 cm in height and less than 2.54 cm in DBH) for four species groups: white oak, red oak, hickory, and ‘other’ (any tree species not falling into the aforementioned groups, Table 1). We also recorded woody seedling density (by species group), leaf area index (LAI), and overstory basal area. Leaf area index was recorded using a concave spherical densitometer at one meter above ground level. We measured overstory basal area with a forestry angle gauge. This information was also collected in a plot five meters from the north side of each DEP that was the same area, but did not have an exclosure surrounding it (control plot). Differences in woody species density and height were tested using a paired t-test using SigmaPlot 12.5 (Systat Software, INC, San Jose, CA). We tested relationships between woody species density and size and LAI and basal area using regression analysis.

Table 1.

Relative frequency, by species, of the tallest seedling classified as other (i.e., not classified as white oak...