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Climatic vs. non-climatic control of western hemlock distribution in its coastal and interior ranges.
Gavin, Daniel*,1, Hu, Feng Sheng1, 1 Department of Plant Biology, Urbana, IL
ABSTRACT- The role of climate in determining species distributions at large spatial scales is rarely tested rigorously. We used three bioclimatic models (climatic envelopes, sequential maximum a posteriori classification (SMAP), and response surfaces) to assess climatic vs. non-climatic controls of the range of western hemlock (Tsuga heterophylla), a late-successional species that occurs in disjunct coastal and interior ranges in northwestern North America. The three bioclimatic models differ in the level of generalization of the species-climate relationship. Each model was calibrated with both the observed coastal and interior ranges of hemlock and then applied to the entire region to predict the species distribution. All models predicted the coastal range more accurately than the interior range. The response surface model was most accurate in its calibration range but failed to predict the interior range using the coastal calibration. In contrast, the climatic envelope model was the least accurate in its calibration range but predicted a southern portion of interior range using the coastal calibration. The SMAP model was moderately accurate within its calibration range and could most accurately predict the interior range using the coastal calibration, suggesting that it has an appropriate level of generalization. The most suitable models predicted the potential interior range of hemlock to be ca. 40% larger than its observed range. The poorer fit of all models in the interior range than in the coastal range, and the agreement in substantial areas of overprediction in the interior, suggest that non-climatic factors affect range limits in the interior to a greater degree than near the coast. Dispersal limitation and/or competition following fire probably exert important constraints on the distributional limits of hemlock in the interior range.
Key words: range limits, Tsuga heterophylla, bioclimatic model, Pacific Northwest