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

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.