Tree Encroachment in Forest Openings: a Case Study From Buffalo Mountain, Virginia

Castanea, Dec 2004 by Copenheaver, Carolyn A, Fuhrman, Nicholas E, Gellerstedt, Laura Stephens, Gellerstedt, Paul A

ABSTRACT

In eastern forests, openings dominated by grasses, forbs, or shrubs are areas of conservation concern because they typically contain endemic, threatened, and rare plants. Understanding the ecology and mechanisms of tree encroachment would be valuable for conservation managers and would add to a substantial body of literature on forest openings. In this study, we worked in grass-dominated forest openings on Buffalo Mountain, Virginia using a method that combined dendrochronology and belt transects to assess tree encroachment. We discovered both stable ecotones and areas where trees were invading the formerly grass-dominated openings. Both gradual and episodic patterns of tree encroachment were identified; however, successful tree establishment always initiated from the edge of the forest-grass ecotone and progressed towards the center of the opening rather than occurring across the entire forest opening. This spatial pattern of recruitment implies that successional facilitation is necessary for tree encroachment in forested openings at Buffalo Mountain.

INTRODUCTION

Forests of eastern North America have a number of naturally occurring, treeless openings that are dominated by vegetation consisting of grass, forbs, or shrubs. The eight forest openings commonly discussed in the literature include: balds, barrens, flat rocks, frost pockets, glades, rock outcrops, xeric limestone prairies, and scrub gaps (Table 1). The semantics associated with these terms are full of dissention because many of the definitions differ only slightly and there exist many variations of each term (Baskin et al. 1997). For example, the term "glade" encompasses cedar glades (Baskin and Baskin 2003), diabase glades (LeGrand 1988), dolomitic glades (Erickson et al. 1942), limestone glades (Baskin et al. 1995), and sandstone glades (Jeffries 1985) depending on their specific environmental or vegetative characteristics. Regardless of semantic disagreements, research has identified that the invasion of trees into these openings has increased during the past half-century (the major exception being some of the openings described as edaphic climaxes [i.e., Baskin and Baskin 1988, 2003; Baskin et al. 1995]). The closure of these openings has caused concern among conservationists because openings frequently serve as habitat for endemic, rare, or threatened plants and insects (Platt 1951, Wyatt 1977, Bartgis 1993, Ludwig 1999). As part of the conservation of forest openings, researchers have attempted to identify the origins of these openings and discuss management options for maintaining them. The historical development of forest openings has been attributed to grazing by wild herbivores (Billings and Mark 1957), warmer temperatures and increased aridity during the hypisthermal period (Baskin, Chester, and Baskin 1997), a post-hypsithermal cooling period (Mark 1958), clearing by European settlers followed by cattle grazing (Lindsay 1976), Native American burning (Baskin et al. 1994), natural fire regimes coupled with xeric soils (Batek et al. 1999), and/or extreme radiational cooling (Motzkin et al. 2002). The proposed conservation management plans generally involve some type of anthropogenic disturbance such as controlled burns, herbiciding unwanted saplings, bushhogging, or implementing grazing programs to prevent tree invasion (Baskin et al. 1994). The conservation maintenance of forest openings can be expensive in terms of workforce, equipment, and liability (especially if controlled burns are implemented). Therefore, an understanding of the ecology and mechanisms of tree encroachment into these forested openings would benefit conservation managers and potentially allow them to differentiate quickly, easily, and inexpensively between forest openings that are edaphic climaxes and those that are at risk of converting to forest.

Three main patterns of tree and shrub encroachment into forest openings have been observed. Several studies have found that small microenvironments within the openings favor seedling and shrub establishment which creates "islands" of woody plants within the openings that may (or may not, depending on the disturbance regime) eventually fill the entire opening (Oosting and Anderson 1939, Erickson et al. 1942, Keever et al. 1951). Other studies have identified a spatially uniform distribution of woody plants throughout the forest openings that will develop to mature trees only if environmental conditions are favorable. This pattern results in small patches of even-aged trees (Lindsay and Bratton 1980, McClenahen and Houston 1998). The third pattern involves a gradual invasion of trees from the forest-grass ecotone towards the center of the opening. This pattern often includes a change in species composition, with some species able to establish along the ecotone and encroach into the opening while others were limited to the forest interior (Matlack 1994, Arévalo 2002).

One method to assess the patterns of tree establishment within forest openings would be belt transects. Belt transects or strip transects have traditionally been used to study transitions between community types, such as the ecotone between a forested community and the grassy vegetation in a forest opening. The width of the belt transect depends on the density (#/ha) and size of the vegetation, but typically varies from 2 m to 10 m in width (Mueller-Dombois and Ellenberg 1974). Arévalo (2002) in his study of the boundary between tall grass prairie and woodland established a belt transect 100 m in length (50 m in the forest community and 50 m in the prairie community) and 10 m in width to sample the tree density and canopy cover across the prairie-woodland ecotone. Knoepp et al. (1998) sampled soil phytolith content from 1 m-wide quadrats along a belt transect to assess historical changes in the boundary between forest and grassy balds in the southern Appalachians (grasses and trees produce morphologically different phytoliths or silicaceous formations). Most studies that employ belt transect sampling quantify structural differences across an ecotone.