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Structure and Function of Peatland-Forest Ecotones in Southeastern Alaska

High-latitude warming could cause northern peatlands to become C sources. Where peatlands border boreal forests, strong differences in ecosystem C balances reflect drainage differences. Because local drainage conditions could be influenced by alterations in temperature and precipitation regimes, peatland-forest ecotones represent useful locations for monitoring potential impacts of global warming. We characterized the soils, hydrology, and forest structure along transects bracketing a peatland-forest ecotone in southeastern Alaska. We expected to find soil properties and processes at the peatland-forest edge that were intermediate between those from peatland and forest. Instead, we found that above- and belowground features of the ecotone did not coincide. Conifers grew on mineral soils, but also grew on Cryofibrists and Cryohemists, soils with high soil organic C (SOC) contents to 100 cm (57 kg m -2 ) that are significantly greater than the SOC contents of adjacent forested, non-Histosol pedons. Soil respiration rates (SRR) at peatland-forest edges (0.08 g CO 2 -C m -2 h -1 ), by contrast, were threefold lower than forest rates and did not differ significantly from peatland rates. Respiration rates were strongly influenced by water table height. Peatland and edge water tables were both significantly shallower than forest water tables. Our conceptual model suggests that if additional forest expansion and warmer summers enhance drainage of these edge soils and stimulate SRR to forest-like levels, 23 kg C m -2 could ultimately be mineralized from these extensive peatland-forest boundaries. Afforestation of peatland margins under this scenario could represent a transient positive feedback to rising atmospheric CO 2 levels.