Wetland research has described changes in plant communities along environmental gradients, however, little is known about the relationship between fine-scale hydrologic and abiotic factors and the relative abundances of individual, co-occurring species. Larix laricina (Eastern Larch) and Picea mariana (Black Spruce) are the 2 dominant tree species in open boreal peatlands in the northeastern US. In order to describe abiotic gradients that correlate with species abundances at local spatial scales, we collected data on Eastern Larch and Black Spruce stem abundances, groundwater pH, conductivity, depth to water table, water temperature, dissolved oxygen, and canopy closure from 42 plots along 6 transects in an Adirondack wetland. We correlated stem abundances with each of the abiotic variables and then used regression to explain variation in stem abundances of the 2 species along those abiotic gradients. Percent canopy closure explained 56% of the variability in Eastern Larch stem abundance, and depth to groundwater was also positively correlated with number of Eastern Larch stems. These 2 abiotic conditions covaried; thus, the best model to explain variability in Eastern Larch stem abundance included only canopy closure. Black Spruce stem abundance was significantly lower in plots with higher water temperatures (R2 = 0.31). In a multiple-regression model, depth to the water table explained an additional 6% of the variance and substantially reduced Mallows' Cp. Eastern Larch and Black Spruce appear to establish along different abiotic gradients at the scale of tens of meters within this study wetland. Although light levels, as mediated by canopy closure, would be predicted to influence the establishment of Eastern Larch based on its silvics, the strong negative relationship between Black Spruce stem abundance and water temperature has not been previously reported. Sampling other peatlands will allow us to determine the universality of these patterns and to better understand which environmental gradients operate at local spatial scales to structure patterns of tree distribution within peatlands.
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Vol. 23 • No. 3