Geographic variations in fine-scale vegetation patterns: aspect preferences of montane pine stands over Southern Appalachian landscapes

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ABSTRACTLandscape mosaics commonly reflect local terrain interactions with broad-scale processes. In the northern hemisphere, insolation interacts with terrain such that south-facing slopes are warmer, drier, and have sparser and more flammable vegetation than north-facing slopes. These vegetation differences are reinforced through positive feedbacks. In the southern Appalachian Mountains, USA, south-facing slopes harbor xerophytic, fire-dependent pine stands within a hardwood-forest matrix. On certain landscapes, however, pines prefer west- and northwest-facing slopes. We examine pine distribution in the southern Appalachian Mountains (Virginia through Georgia), finding that pines prefer south- and southwest-facing slopes in the southern section of this region but west-, southwest-, and northwest-facing slopes in the northern section, suggesting that broad-scale processes interact differently with terrain in the two sections. To investigate these differences, we analyze three topo-climatic factors (topographic wetness, insolation, and wind) that may influence pine distributions, and discuss other potential influences (bedrock dip, soils, and ice storms). Insolation receipt can straightforwardly explain pine distribution on southern landscapes. No single explanation accounts fully for the anomalous northern pattern, but several mechanisms (especially wind and disturbances) may contribute. We present a conceptual model of these processes and the longer-term coevolution of pine forests with microclimates, fire regimes, soils, and landforms.