Variation in Soil and Forest Floor Characteristics Along Gradients of Ericaceous, Evergreen Shrub Cover in the Southern Appalachians

Ericaceous shrubs can influence soil properties in many ecosystems. In this study, we examined how soil and forest floor properties vary among sites with different ericaceous evergreen shrub basal area in the southern Appalachian mountains. We randomly located plots along transects that included open understories and understories with varying amounts of Rhododendron maximum (rosebay rhododendron) and Kalmia latifolia (mountain laurel) at three sites. The three sites were a mid-elevation ridge, a low-elevation cove, and a high-elevation southwest-facing slope. Basal area of R. maximum was more correlated with soil properties of the forest floor than was K. latifolia. Increasing R. maximum basal area was correlated with increasing mass of lower quality litter and humus as indicated by higher C:N ratios. Moreover, this correlation supports our prediction that understory evergreen shrubs may have considerable effect on forest floor resource heterogeneity in mature stands. INTRODUCTION Vegetation is one of the primary factors contributing to soil genesis (Boettcher and Kalisz 1990). Many studies have shown the effects of individual tree species on soil chemical and physical properties (Zinke 1962, Challinor 1968, Chastain et al. 2006, Boerner and Koslowsky 1989, Boettcher and Kalisz 1990, Pelletier et al. 1999). Woody species may affect soil properties by redistributing nutrients within the rooting zone (Boettcher and Kalisz 1990) and by the synthesis and input of organic material in the form of root exudates and decomposing litter (Boerner and Koslowsky 1989). Additionally, litter quality may influence decomposition and nutrient turnover rates. For example, litter of species with higher lignin content has slower decomposition rates (Hobbie et al. 2006). Ericaceous plants, in particular, influence soil properties by reducing soil enzyme activities and slowing nutrient cycling (Bloom and Mallik 2006, Chastain et al. 2006, Joanisse et al. 2007). This influence is primarily a result of litter quality and a large concentration of polyphenolic compounds (Wurzburger and Hendrick 2007). These phenolic compounds often bind with organic materials in the soil preventing or slowing their decomposition (Joanisse et al. 2007, Wurzberger and Hendrick 2007), reducing rates of nutrient mineralization (Straker 1996, Northup et al. 1998). Many ericaceous plants influence nutrient availability in cold-temperate or boreal regions that are nitrogen limited (Nilsson and Wardle 2005); however, ericaceous vegetation can also alter nutrient cycling in warm *email address: [email protected] **Present address: Department of Biology, Appalachian State University, Boone, North Carolina 28608. ***Present address: Adirondack Ecosystem Research Center, State University of New York, and College of Environmental Science and Forestry, Syracuse, New York 12852. Received February 26, 2008; Accepted May 10, 2009. CASTANEA 74(4): 340–352. DECEMBER 2009