Topographic Factors Affecting the Tree Species Composition of Forests in the Upper Piedmont of Virginia

Abstract

There are many factors that influence forest species composition and many are linked to topographical features. This study, conducted on the Ferrum College campus in the Upper Piedmont Physiographic Province of Virginia revealed three major forest types associated with topographic factors using cluster analysis and detrended correspondence analysis . The first type of forest occurred mostly on northeastern slopes on toe slope topographic positions and was mainly composed of tulip tree (Liriodendron tulipifera) and red maple (Acer rub rum). The second type of forest was found on shoulder and side slope positions and was composed mostly of high densities of sourwood ( Oxydendrum arboreum ), red maple and chestnut oak (Quercus prinus) species. The final forest type was located mostly on ridgetops and shoulder slope positions with a southwestern aspect and was composed mostly of white pine (Pinus strobus), sourwood, chestnut oak and scarlet oak (Quercus coccinea). In general, tree density increased with ascending slope position while DBH decreased. Species richness did not differ significantly by topographic position or aspect. INTRODUCTION There are many variables that influence forest species composition including soil moisture and nutrients, air temperature, light and disturbance regime. These variables are often strongly linked to topographic features such as aspect, slope position, inclination and elevation (Desta et al. 2004). Edaphic and topographic factors exert important influences along the upper Piedmont and Blue Ridge physiographic provinces of Virginia (Stephenson 1982, Harrison et al. 1989, Farrell and Ware 1991, Copenheaver et al. 2006). These forests, however, also have a long and complex disturbance history that has affected forest species composition. The forests in this region of Virginia were once dominated by American chestnut (Castanea dentata) until the invasion of the chestnut blight fungus (Endothia parasitica) in the 1920s (Johnson and Ware 1982). Following this event, highest rankings of density and basal area have been shared by a number of tree species, 1 corresponding author; tfredericksen@ferrum.edu 4 VIRGINIA JOURNAL OF SCIENCE predominately oaks (Quercus) and hickories (Carya) (Johnson and Ware 1982). A wide range of other natural and anthropogenic factors including recent ice storms (Stueve et al. 2007) and gypsy moth defoliation (Whitmire and Tobin 2006) also influence the species composition of Appalachian and Piedmont forests in Virginia. In addition, selective logging, deer browsing and the spread of invasive plant species, particularly ailanthus (Ailanthus altissima), continue to impact the structure and composition of these forests (Carter and Fredericksen 2007). This study characterized the species composition of the forests on the property of Ferrum College located on the Upper Piedmont Physiographic Province close to the Blue Ridge Escarpment in Franklin County, Virginia. Data were collected on topographic position and aspect in order to interpret the species composition in relation to topographic variables. MATERIALS AND METHODS During 2006 and 2007, 19 permanent plots were established in forested areas of the 700-acre Ferrum College campus (N 36.5°, W 80.1°). Plantation forests were not included in this study. Plots were 20 x 20 m in size and were initially established randomly from a topographical map. After selection of the first eight plots, however, an effort was made to select plot locations based on representation of possible aspect and slope position combinations, with each individual slope position and aspect being represented at least twice, except for northwest slope positions which had n = 1. The stands chosen for this study had not been subjected to recent logging; however, all of the stands had most likely been subjected to light selective logging during the 1970s, mostly for oak species and tulip tree (Liriodendron tulipifera ). The elevation of the plots ranged from approximately 300-400 m. Each tree in the plot with diameter at breast height (DBH) of~ 10cm was identified to species, tagged, and evaluated for crown class, crown condition, stem quality and stem condition. Each plot location was marked and recorded with a GPS mapping system, and aspect and slope position in the topography were recorded. Aspect was measured with a compass and slope position was categorically determined as ridge top, slope shoulder, side slope, toe slope, or valley bottom. Average DBH, average basal area, average density and species richness were calculated by site according to aspect and also by slope position. Cluster analysis and detrended correspondence analysis (DCA) were used to determine similarities among plots with respect to species composition. Rare species were down-weighted in the analyses because they can exert an effect on ordinations that is disproportionate to their abundance. All ordination analyses were carried out using PC-ORD (Version 5, MJM Software, G leneden Beach, Oregon). Kruskal-Wallis (K-W) non-parametric tests were conducted to determine if species richness, tree density, or mean tree diameter varied among slope position, aspect, or plot groupings generated by DCA and cluster analysis. Differences were considered statistically significant at p ::_ 0.05. Analyses were carried out using SYST AT 10.2 (SYST AT Software, Inc., San Jose, CA). RESULTS The study plots contained 498 trees and 23 tree species. In these plots, the most abundant species were tulip tree, sourwood (Oxydendrum arboreum), and red maple TOPOGRAPHY AND FOREST COMPOSITION 5 TABLE 1. Number of plots (N), average diameter at breast height (DBH), tree density, species richness, and top three most abundant species by topographic position in forested plots on the property ofFerrum College, Franklin County, VA. Means with the same letter are not significantly different at p :::_ 0.05 . Li Tu = Liriodendron tulipifera, OxAr = Oxydendrum arboreum, AcRu = Acer rubrum, PiSt = Pinus strobus , QuPr = Quercus prinus , QuCo = Quercus coccinea, AiAl = Ailanthus altissima . Topographic N DBH Density Species Most abundant Position (cm) (#/ha) Richness species Valley 2 33 .7 a 238 b 3.0 a LiTu, AcRu, AiAl Toe 4 24 .6 b 600 ab 5.5 a LiTu, AcRu, OxAr Side 5 22 .0 be 725 a 6.4 a LiTu, AcRu, OxAr Shoulder 5 20.0 C 670 ab 5.4 a QuPr, OxAr, PiSt Ridge 3 20.0 C 800 a 5.7 a PiSt, OxAr, QuCo TABLE 2. Number of plots (N), average diameter at breast height (DBH), tree density, species richness, and top three most abundant species by aspect in forested plots on the property of Ferrum College, Franklin County, VA . Means with the same letter are not significantly different at p :::_ 0 .05 . LiTu = Liriodendron tulipifera, OxAr = Oxydendrum arboreum, AcRu = Acer rubrum , PiSt = Pinus strobus , QuPr = Quercus prinus, AiAl = Ailanthus altissima, QuAl = Quercus alba . Aspect N DBH Density Species Most abundant species (cm) (#Iha) Richness NE 7 21.6 a 685 a 6.3 a LiTu, OxAr, AcRu NW 1 22.4 a 650 a 4.0 a OxAr, PiSt, AcRu SE 3 23.4 a 633 a 6.0 a AcRu, LiTu, QuAl SW 3 21.6 a 675 a 5.0 a PiSt, QuPr, OxAr (Acer rub rum). Red maple was found in every plot and sourwood appeared in all but three of the plots. Tulip tree was found in slightly over half of the plots, but it was abundant in the plots in which it was located. White pine (Pinus strobus), and chestnut oak (Quercus prinus) were also common in plots of this study. Some trends were observed with respect to tree density and DBH by the topographic slope classification. In general, tree density increased with ascending topographic position while DBH decreased. Valley plots had a larger mean tree DBH than all other positions (Table 1) and toe slope positions had a significantly higher mean tree DBH than shoulder or ridge top topographic positions. Tree density tended to increase with topographic position (Table 1 ). Species richness did not differ significantly by topographic position (Table 1). No significant differences were observed for aspect in mean tree diameter, species richness, or tree density (Table 2). Cluster analysis (Figure 1 ), revealed three main types of forest tree communities. The attributes of these groups with respect to DBH, tree density, species richness, and 6 VIRGINIA JOURNAL OF SCIENCE p01 p11 pOS p03 pQ6 ---I p04 p07 p09 ~6g----I p12 p18 p1 3 ~1i --I I p19 p15 p16 r-----p02 FIGURE 1. Cluster analysis of the plots within fore sts on the property of Ferrum College , Franklin County, Virginia. The first cluster of plots 1, 11 , 5, 3 and 6 were plots with a modal tendency for toeslope positions and northeastern aspects . Cluster two of plots 4, 7, 8, 9 and 10 represented mostly shoulder and side slope plots. Plots 12-19 made up the third cluster occurring most often on southwestern aspects and ridgetop or shoulder. The main outlier plot identified from the cluster analysis was plot 2, a plot with a very high density (80% ) oftuliptree. TABLE 3. Number of plots (N) , average diameter at breast height (DBH) , tree density, species richness , highest mode(s) for topographic position (Mode TP) , and aspect (Mode aspect) , and top three most abundant species by major species groups identified in cluster analysis of forested plots on the property of Ferrum College, Franklin County, VA . Means with the same letter are not significantly different at p .::_ 0.05 . LiTu = L iriodendron tulip1fera , OxAr = Oxydendrum arboreum , Ac Ru = Acer rub rum , PiSt = Pin us strobus, QuPr = Quercus prinus, AiAl = A ilanthus altissima , QuCu = Quercus coccinea . Species N DBH Density Species Mode Mode Most abundant Group (cm) (#Iha) Richness TP Aspect species 5 26 .2 a 540 a 5.4 a Toes lope NE LiTu, AcRu , OxAr 2 5 24.4 ab 495 a 6.0 a Side slope NE OxAr, AcRu , QuPr Shoulder SE 3 8 20 .2 b 756 b 5.1 a Ridgetop SW PiSt, QuCo, QuPr Shoulder most abundant species is summarized in Table 3. According to cluster analysis (Figure 1 ), the first cluster of plots 1, 11, 5, 3 and