Wetland determination relies on assumptions that site hydrologic and edaphic conditions limit plant species to certain environments. For example, using species' wetland indicator status for wetland determination assumes that tolerance of wetland conditions best explains distributional patterns. However, abiotic and biotic factors often interact to create complex plant responses across different environments. To evaluate these interactions, we used a hydrologic gradient in the coastal temperate rainforest of southeast Alaska to (1) quantify the primary determinants of conifer distributions, (2) identify thresholds in environmental factors limiting species' success and (3) assess current wetland indicator status of local conifers (Pinus contorta, Picea sitchensis, and Tsuga heterophylla). Data were collected using a hierarchical sampling scheme and analyzed within a Bayesian framework. Topography and hydrologic regime were the primary determinants of distributional patterns, but species were limited by specific microsite factors. Competitively dominant P. sitchensis occurred where hydrology, pH, and nitrogen were most favorable for establishment, while stress-tolerant P. contorta was competitively excluded from these sites. Tsuga heterophylla occurred across the gradient but took advantage of drier conditions, which promoted biomass accumulation. Tree distributions were limited by the interaction between abiotic and biotic factors rather than by abiotic tolerance alone. This knowledge of local and regional drivers of species' distributions and the relative importance of interacting abiotic and biotic drivers provide critical information for land management and regulation. Wetland delineation procedures can be improved through application of the regional empirical limits identified for plant species, as implemented and addressed in this study.
Bisbing, Sarah M.; Cooper, David J.; D'Amore, David V.; Marshall, Kristin N. 2016. Determinants of conifer distributions across peatland to forest gradients in the coastal temperate rainforest of southeast Alaska. Ecohydrology. 9(2): 354-367. https://doi.org/10.1002/eco.1640.