Does white-tailed deer density affect tree stocking in forests of the Eastern United States?

Abstract

Background: White-tailed deer (Odocoileus virginianus) have increased during the past century in the USA. Greater deer densities may reduce tree regeneration, leading to forests that are understocked, where growing space is not filled completely by trees. Despite deer pressure, a major transition in eastern forests has resulted in increased tree densities. Methods: To reconcile conflicting trends, we applied generalized linear mixed models to compare deer densities during 1982 and then 1996 to tree stocking after about 30 years and 15 years of potential reductions of small trees by deer, for the entire eastern US and 11 ecological provinces. We also compiled deer browse preferences and compared preferred browse with trends in tree species composition from historical (1620-1900) and current tree surveys. Results: The forested area of the eastern US, including a prairie ecological province, was equally well-stocked (52%) and understocked (48%) during 2011-2017 tree surveys. For 1982 deer densities, 38% of area had deer densities > 5.8 deer/km 2 and for 1996, 66% of area had deer densities > 5.8 deer/km 2. Deer densities and tree stocking were not related significantly for the entire eastern US. Deer may reduce tree stocking in the Laurentian Mixed Forest; however, this province had both lower deer densities and greater tree stocking than other provinces. Furthermore, major tree species trends did not match tree browse preferences. Conclusions: Rather than too few trees, too many trees is an ecological problem where historical open oak and pine forests had herbaceous understories, and currently, trees have captured growing space. We attribute other drivers than deer to explain this transition.

Rapid digitization to reclaim thematic maps of white-tailed deer density from 1982 and 2003 in the conterminous US

“…The concurrence of increased tree recruitment and decreased deer populations during Euro-American settlement may suggest that relief from browsing pressure was influential in releasing tree growth because herbivores are potential drivers of vegetation structure. However, transition back to open forests is not occurring after resumption of deer pressure at or above thresholds of 3 to 9 deer/km 2 expected to cause change, even where deer densities exceed 10 deer/km 2 (e.g., in Mississippi, Hanberry et al, 2014;Hanberry, Coursey & Kush, 2018;Hanberry & Abrams, 2019). Overall, research indicates that deer reduce regeneration of tree seedlings (Habeck & Schultz, 2015;Ramirez, Jansen & Poorter, 2018), but most tree seedlings will not survive with or without herbivores due to density-dependent mortality.…”

“…Based on analysis using these digitized maps, deer densities do not appear to be correlated with tree stocking (i.e., percent occupied growing space accounting for both density and diameter) at landscape scales in the eastern US (Hanberry & Abrams, 2019; to account for time lag of effects, we used 1982 and 1996 deer densities and current tree stocking after about 30 years and 15 years of deer browsing). It may be that when tree regeneration is limited by fire, deer and other herbivores can help maintain open forests, grasslands, and shrublands.…”

“…It may be that when tree regeneration is limited by fire, deer and other herbivores can help maintain open forests, grasslands, and shrublands. In addition, despite variable browse preferences, almost all tree species, including species such as northern white cedar (Thuja occidentalis) identified as preferred browse reduced by deer browsing, have increased in relative abundance of trees (diameter ≥12.7 cm) in the eastern US between the 1800s and the current decade of 2010 (Hanberry, Palik & He, 2013;Hanberry & Abrams, 2019;Hanberry & Dey, 2019). Increases in most tree species probably preceded increases in deer densities, but recent trends during the past decades are similar to historical trends (Hanberry, 2019; northern white cedar increased slightly in northern mixed forests, B Hanberry, pers.…”

“…obs., 2019). Decreasing tree species include fire-tolerant oak and pines, some wetland species, and species that are affected by novel diseases perhaps in combination with forestry practices (Hanberry, Palik & He, 2013;Hanberry & Nowacki, 2016;Hanberry & Abrams, 2019). Although woody plants have benefitted from conditions during the past century, deer herbivory is an additional stressor on herbaceous plants that have become less abundant due to intense competition with woody species; with limited growing space, abundance of forbs and grasses necessarily decreases as tree densities increase (Hanberry, Bragg & Hutchinson, 2018).…”

Interspecific Differences in Canopy-Derived Water, Carbon, and Nitrogen in Upland Oak-Hickory Forest

“…Given the prevalence, longevity, and diversity of oak species in forest ecosystems, it is likely that these species strongly mediate nutrient cycling when present [6][7][8]. During the past century, the decline of upland oak species due to a myriad of issues (e.g., suppression of natural fire [7,9], excess nitrogen deposition [10,11], pressure from herbivore activity [12,13], climate change [14]) has led to a shift in species composition in temperate deciduous forests from shade-intolerant, xeric species (i.e., oak) to more shade-tolerant, mesic species (i.e., 'mesophytes'). Red maple (Acer rubrum L.) has garnered the most attention as a mesophyte across the Central Hardwoods region since the term's coinage in 2008 [15].…”