Aspect Induced Differences in Vegetation, Soil, and Microclimatic Characteristics of an Appalachian Watershed

Castanea, Jun 2004 by Desta, Fekedulegn, Colbert, J J, Rentch, James S, Gottschalk, Kurt W

ABSTRACT

This study evaluates and quantifies the variation in vegetation, plant nutrients, and microclimate across four topographic aspects in an Appalachian watershed (39�39'43''N, 79�45'28''W). The study found that the north and east aspects were 27-50% more productive than the west and southwest aspects. Species groups that showed strong aspect preference included yellow poplar (Liriodendron tulipifera), black cherry (Prunus serotina), chestnut oak (Quercus prinus), and white oak (Quercus alba); the former two being dominant on the north and east aspects while the latter two dominate the west and southwest aspects. Red oak (Quercus rubra) and red maple (Acer rubrum) showed mild aspect preference indicating their broad ecological amplitude. Although the north and east aspects had greater biomass, the west and southwest aspects had about 23% more stems per hectare.

There were large differences in microclimate among the four aspects. Air temperature during midday period averaged 25.2�C, 24.9�C, 30.5�C, and 29.4�C for the north, east, west and southwest aspects respectively. The maximum temperature difference between the mesic (north and east aspects) and xeric (west and southwest aspects) sites was 5.55�C and was observed at noon. The relative humidity at the xeric site was about 25% lower than that at the mesic site during midday periods. Plant water stress as measured by vapor pressure difference was about 37% higher on west and southwest aspects than on north and east aspects. Plant nutrients only showed minor differences with concentrations being higher on the north and east aspects except for phosphorus, which was higher on the west and southwest aspects.

INTRODUCTION

Growth rates of forest plant species are governed by numerous site factors such as soil moisture, air temperature, light, nutrients, response to competition and predation, and disturbance regime (Fritts 1976, Kramer and Kozlowski 1960, Oliver and Larson 1996, Hicks 1998). Variations in these environmental parameters across a landscape are associated with variations in site productivity, forest growth, and species composition. The spatial variability in these growth-influencing factors is closely tied to attributes of local topography such as aspect, elevation, and slope position and inclination. Differences in site quality, species composition, and forest productivity, due to differences in topographic characteristics have long been recognized by foresters in the eastern United States (Trimble and Weitzman 1956; Doolittle 1957, 1958; Olson and Della-Bianca 1959; Trimble 1964; Phillips 1966; Carmean 1967, 1975; Olson 1969; Lee and Sypolt 1974; Luxmoore et al. 1978; Auchmody and Smith 1979; Spurr and Barnes 1980; Knight 1980; Tajchman and Wiant 1983; Carmean and Kahn 1983; Frank et al. 1984; Hicks and Frank 1984; Boyles and Tajchman 1984; Tajchman and Lacey 1985; McNab 1989, 1993; Hicks 1998).

As a topographic variable, aspect affects the amount and daily cycle of solar radiation received at different times of the year, and has an influence on the microclimate, especially air temperature, humidity, and soil moisture (Rosenberg et al. 1983). It is generally believed that a southwestern slope is sunnier, hotter, and drier than a northeastern slope because the apex of the solar disk is perpendicular to south facing slopes. However, a study by Lee and Sypolt (1974) showed that, in some years, available soil moisture was not significantly different by aspect in West Virginia. Therefore, an alternative explanation for differential growth rates between forests on north and south-facing slopes relates to differences in energy exchange and thermal regimes rather than differences in soil moisture.

Soil-site studies involve measuring several soil and topographic variables and relating them through multiple regression analysis to site index, height growth, and biomass production. Because of its dissected topography, most soil-site studies in the central Appalachians identify aspect as the influential factor for explaining spatial variability in growth but the amount of variation in vegetation and microclimatic characteristics is not well quantified. The current study specifically examines the extent of variation in site characteristics across four aspects in an Appalachian watershed. In the study area, rainfall is plentiful and even the most xeric aspects could be characterized as moist by southwestern United States standards. However, it is hypothesized that significant differences in site characteristics are expected in response to the fine scale differences in microclimate due to changing topographic aspect. The specific objectives of this study are to examine the spatial variability in vegetation, soil-plant nutrients, and microclimate.

DESCRIPTION OF THE STUDY AREA AND METHODS

Study Area

The study area, Little Laurel Run watershed, is part of Coopers Rock State Forest located approximately 16 km northeast of Morgantown, West Virginia (39�39'43''N, 79�45'28''W). The watershed encompasses approximately 271 ha (670 acres) and is typical of many Appalachian V-shaped valleys with long, steep and rocky slopes. The watershed orientation is from northwest to southeast. The average relief of the watershed is roughly 224 m above sea level (Tajchman and Wiant 1983). The topography of the area is fairly rugged and the average slope inclination is 14� (25%). The average oak site index is 22.6 m (74 ft). The watershed is covered with an evenaged 60-70 year-old mixed hardwood forest of mostly sprout origin. Roughly 62% of the forest is composed of mixed oak cover types. The predominant species in these stands are white oak (Quercus alba L.), black oak (Quercus velutina Lam.), northern red oak (Quercus rubra L.), scarlet oak (Quercus coccinea Muench.), and chestnut oak (Quercus prinus L.). The remainder, 38% of the forest, is occupied by mesophytic-hardwood stands. These stands occur on the mesic sites and consist primarily of yellow-poplar (Liriodendron tulipifera L.), red maple (Acer rubrum L.), black cherry (Prunus serotina Ehrh.), and northern red oak (Knight 1980).