- Spatial pattern and process in forest stands within the Virginia piedmont - 37 Journal of Vegetation Science 16: 37-48, 2005 © IAVS; Opulus Press Uppsala. Abstract Question: Underlying ecological processes have often been inferred from the analysis of spatial patterns in ecosystems. Using an individual-based model, we evaluate whether basic assumptions of species’ life-history, drought-susceptibility, and shade tolerance generate dynamics that replicate patterns between and within forest stands. Location: Virginia piedmont, USA. Method: Model verification examines the transition in forest composition and stand structure between mesic, intermediate and xeric sites. At each site, tree location, diameter, and status were recorded in square plots ranging from 0.25 to 1.0 ha. Model validation examines the simulated spatial pattern of individual trees at scales of 1-25 m within each forest site using a univariate Ripley’s K function. Results: 7512 live and dead trees were surveyed across all sites. All sites exhibit a consistent, significant shift in pattern for live trees by size, progressing from a clumped understorey (trees ≥ 0.1 m in diameter) to a uniform overstorey (trees > 0.25 m). Simulation results reflect not only the general shift in pattern of trees at appropriate scales within sites, but also the general transition in species composition and stand structure between sites. Conclusions: This shift has been observed in other forest ecosystems and interpreted as a result of competition; how- ever, this hypothesis has seldom been evaluated using simula- tion models. These results support the hypothesis that forest pattern in the Virginia piedmont results from competition in- volving species’ life-history attributes driven by soil moisture availability between sites and light availability within sites. Keywords: Individual-based model; MUSE; Point pattern analysis; Resource competition; Ripley’s K function; Soil moisture. Nomenclature: Harlow et al. (1996). Abbreviations: ALB = Above-ground live biomass; BA = Basal area; CSR = Complete spatial randomness; DBH = Diameter at breast height; LAI = Leaf area index; MUSE = Multistrata spatially explicit model; PET = Potential evapo- transpiration. Spatial pattern and process in forest stands within the Virginia piedmont Druckenbrod, Daniel L. 1* ; Shugart, Herman H. 2 & Davies, Ian 3 1 Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6036, USA; 2 Department of Environmental Sciences, University of Virginia, 291 McCormick Rd., Charlottesville, VA 22904-4123, USA; 3 Ecosystems Dynamics Group, Research School of Biological Sciences, GPO Box 475, Canberra ACT 2601, Australia; * Corresponding author; Fax +18655768543; E-mail druckenbrodd@ornl.gov Introduction The piedmont forests of eastern North America have a rich history of ecological research (Peet & Christensen 1987). Early research in the piedmont by Korstian & Coile (1938), Kramer & Decker (1944), and Kozlowski (1949) demonstrated the importance of competition for light and soil moisture among individual trees in struc- turing successional patterns in these forests. Similarly, at a larger scale, Oosting (1942) characterized three climax piedmont forest types along a moisture gradient from mesic to xeric: Quercus alba - Q. velutina - Q. rubra, Q. alba, and Q. alba - Q. stellata. These re- sources, light and soil moisture, have recently been shown to also influence the long-term temporal patterns of individual tree growth in the piedmont (Druckenbrod & Shugart 2004; Druckenbrod et al. 2003). Drawing upon these insights, we examine the concurrent influ- ence of light and soil moisture availability across scales of individual trees to forest stands in the piedmont. Using field survey data and individual-based modelling, we evaluate whether competition for light and soil mois- ture can reproduce piedmont forest patterns including tree distributions within stands and forest composition and structure among stands. In a survey of individual tree spatial pattern in the piedmont, Christensen (1977) reported a transition from a clumped distribution of understorey trees to a random or uniform distribution of overstorey trees. Field studies of individual-tree spatial pattern over a range of other forest types have generally revealed a similar transition and have often suggested that competition for light creates these spatial patterns (e.g. Cooper 1961; Laessle 1965; Whipple 1980; Good & Whipple 1982; Kenkel 1988; Moeur 1993; Ward et al. 1996; Busing 1998; Mast & Veblen 1999; Van Pelt & Franklin 2000). Watt (1947) first recognized the autogenic forma- tion of spatial pattern in vegetation by observing a landscape mosaic of regenerating forest patches. More recently, ecologists have sought to explain this patch-