Interactive effects of chronic deer browsing and canopy gap disturbance on forest successional dynamics

Jennifer A. Holm,Norman A. Bourg,Jonathan R. Thompson,William J. McShea

doi:10.1890/es13-00223.1

Abstract

The interaction of browsing by white‐tailed deer (Odocoileus virginianus) and canopy gap disturbances may affect long‐term tree composition and lead to significant changes in forest structure. We used an individual‐based forest gap model (ZELIG) to better understand the aggregate and interactive impacts of these processes on the long‐term (200 years) successional dynamics of a mesic deciduous forest. We parameterized ZELIG to: (1) simulate successional dynamics within a temperate deciduous secondary forest typical of eastern North America; (2) simulate browsing impacts by white‐tailed deer; and (3) simulate gap‐scale disturbance of variable size and frequency. Our estimates of browsing impacts by species were derived from a 20‐year, four‐hectare deer exclusion study. Model calibration matched observed tree species composition, density by size class, and total basal area (39.92 m2 ha−1 vs. 37.13 m2 ha−1). Simulated deer browsing had little impact on total basal area over two centuries. However, deer browsing had substantial impacts on community composition, creating a less diverse understory, lower species richness, and decreased abundance of Quercus species, while retaining the dominance of Liriodendron tulipifera. Simulated gap disturbances exacerbated the impacts of chronic deer browsing and these impacts became stronger over time. Our analyses suggest that recent increases in white‐tailed deer density within many forests of eastern North America will result in altered community dynamics that persist beyond the sapling level, and that any increases in overstory disturbance frequency will exacerbate these impacts.

Highlights

In the secondary forests that cover much of eastern North America, overstory gap-scale disturbance is a dominant mechanism driving succession
While our deer exclusion plot is among the longest running in the region, the observed impacts of deer browse are confined to the seedling and sapling age classes (McGarvey et al 2013)
Our study yielded three main outcomes: (1) total basal area was not affected by deer herbivory or gap disturbance; (2) forest tree species composition was altered, and the forest became less diverse, in the presence of deer and common rates of gap disturbance; and (3) gap disturbances exacerbated deer browsing impacts and affected overstory species composition

Summary

Introduction

In the secondary forests that cover much of eastern North America, overstory gap-scale disturbance is a dominant mechanism driving succession. The role of biological drivers in successional processes is not as well understood, but is potentially important. Herbivory by whitetailed deer (Odocoileus virginianus), for example, can reduce the survival and growth of several woody species and change the dominance rank of tree species at the sapling stage In and of itself, may affect long-term successional processes is not well understood. The potential for deer herbivory to affect succession has increased dramatically in recent decades as white-tailed deer populations have increased—in some cases to densities .50 deer kmÀ2—throughout many forests of eastern North America (McShea et al 1997, McShea 2012)

Methods

Results

Discussion

Conclusion