Effects of chronic herbivory and historic land use on population structure of a forest perennial, Trillium catesbaei

Applied Vegetation Science, Dec, 2007 by Michael A. Jenkins, Christopher R. Webster, Janet H. Rock

To examine the effects of chronic herbivory and past land use on population structure, we determined the age distribution, stage structure, and height distribution of Trillium catesbaei populations at two sites: (1) an area of intensive historic land use that has been maintained as open fields and woodlots with a history of chronic deer herbivory, and (2) a reference area with similar land use history, geology, and soils that has been allowed to succeed to closed-canopy secondary forest and has a lower deer population. We also compared the age structures of these two sites to that of a primary forest that has never received significant anthropogenic disturbance. From these examinations, we will address four questions: 1. Does chronic herbivory lead to a truncated age structure or a full-aged population of stunted individuals? 2. Does chronic herbivory cause individual plants to be shorter in stature (stunted) at a given age than at sites with less herbivory? 3. Does chronic herbivory increase the probability of smaller and younger plants flowering? 4. Do populations in uncut primary forest contain individuals of greater age than forests with a history of more intense human disturbance?

Methods

This study was conducted in western Great Smoky Mountains National Park (GSMNP), which is located in the southern Appalachian Mountains of Tennessee and North Carolina, USA. We examined T. catesbaei population age structure at three sites in GSMNP with contrasting histories of deer browsing and anthropogenic disturbance. All sites were within 10 km of each other in Sevier County, Tennessee. The minimum distance between sites was 4.8 km, which is greater than the mean radius of published maximum home-ranges of white-tailed deer in the Appalachian Mountains (Kammermeyer & Marchinton 1976; McShea & Schwede 1993; Campbell et al. 2004). Consequently, it is unlikely that there was any overlap among deer populations at the three sites.

Cades Cove (CC) is a former settlement area comprised of open fields and woodlots that are surrounded by closed-canopy secondary forest. Due to the abundance of edge habitat and the lack of hunting since the creation of GSMNP, CC has had chronically high levels of deer herbivory. Because CC is surrounded by mountains and embedded within GSMNP, it offers a unique opportunity to study the effects of chronic herbivory without the influence of surrounding land use and ownership. However, due to its unique history, topography, and geology (discussed in detail below), we were unable to replicate CC with similar sites. Whiteoak Sink (WOS) is an area with a similar geology and disturbance history that has been allowed to succeed into closed canopy forest and has a lower deer population. Similar to CC, we were unable to replicate WOS with similar sites.

Leadbetter Ridge (LBR) is an area of primary forest that has never received large-scale human disturbance. This site was less intensively sampled to provide a baseline of potential population structure of T. catesbaei in the absence of intensive anthropogenic disturbance. Data from this site are presented as a benchmark for the other sites, but were not analysed statistically.