Population Structure
We explored the population structure for both species using STRUCTURE (Pritchard et al. 2000) for possible K of 1 through 10. For Thuja plicata , we found three clusters (Fig. 3, Fig. S1). While two of the clusters are geographically restricted to the coast or the inland, the third has no clear geographic structure. At K = 2 (Fig. 3), we recover the geographically structured pattern observed in the first two clusters of K = 3 (albeit some coastal samples were sometimes present in the inland cluster).
For Tsuga heterophylla, we selected K = 2 as the optimalK value (Fig. S2) We do not see a geographic association between the coastal and inland samples with the two clusters (Fig. 3). However, when we investigate K = 3, we do observe a strong geographic structure, with one cluster mostly restricted to the inland and the other present along the coast. Because the justification of deltaK is based on simulations with no hierarchical population structure (which is almost always present in nature), interpretation of multiple clustering scenarios is critical.