Spatial variation of climatic and non-climatic controls on species distribution:: the range limit of Tsuga heterophylla

Aim: To assess which climatic variables control the distribution of western hemlock (Tsuga heterophylla), how climatic controls vary over latitude and between disjunct coastal and interior sub-distributions, and whether non-climatic factors, such as dispersal limitation and interspecific competition, affect range limits in areas of low climatic control. Location: North-western North America. Methods:We compared four bioclimatic variables [actual evapotranspiration (AET), water deficit (DEF), mean temperature of the coldest month (MTCO), and growing degree-days (GDD5)] with the distribution of T. heterophylla at a 2-km grid cell resolution. The distribution is based on a zonal ecosystem classification where T. heterophylla is the dominant late-successional species. For each bioclimatic variable and at each degree of latitude, we calculated the threshold that best defines the T. heterophylla distribution and assessed the extent to which T. heterophylla was segregated to one end of the bioclimatic gradient. We also fitted two forms of multivariate bioclimatic models to predict the T. heterophylla distribution: a simple threshold model and a complex Gaussian mixture model. Each model was trained separately on the coastal and interior distributions, and predicted areas outside of the T. heterophylla distribution (overprediction) were evaluated with respect to known outlier populations. Results: Actual evapotranspiration was the most accurate predictor across the T. heterophylla distribution; other variables were important only in certain areas. There was strong latitudinal variation in the thresholds of all variables except AET, and the interior distribution had wider bioclimatic thresholds than the coastal distribution. The coastal distribution was predicted accurately by both bioclimatic models; areas of overprediction rarely occurred > 10 km from the observed distribution and generally matched small outlier populations. In contrast, the interior distribution was poorly predicted by both models; areas of overprediction occurred up to 140 km from the observed distribution and did not match outlier populations. The greatest overprediction occurred in Idaho and Montana in areas supporting species that typically co-exist with T. heterophylla. Main conclusions: The high predictive capacity of AET is consistent with this species' physiological requirements for a mild and humid climate. Spatial variation of MTCO, GDD5 and DEF thresholds probably reflects both the correlation of these variables with AET and ecotypic variation. The level of overprediction in portions of the interior suggests that T. heterophylla has not completely expanded into its potential habitat. Tsuga heterophylla became common in the interior 2000-3500 years ago, compared with > 9000 years ago in the coastal region. The limited time for dispersal, coupled with frequent fires at the margins of the distribution and competition with disturbance-adapted species, may have retarded range expansion in the interior. This study demonstrates that bioclimatic modelling can help identify various climatic and non-climatic controls on species distributions.