For three candidate genes in chromosome 15 (PRR5, Potri.015G002300; TTG1, Potri.015G002600; COMT1, Potri.015G003100), that showed signatures of selection in previous studies (Table S2, Supporting Information) (McKown et al. 2014; Geraldes et al. 2014), we estimated the FST value per SNP using Arlequin 3.5.1.2 (Excoffier et al. 2005), in 302 accessions from 28 drainages across much of the P. trichocarpa’s range. These FST values were compared to P. trichocarpa whole genome average FST (mean=0.05; 99th percentile = 0.18) following Geraldes et al. 2014. Using the coding regions of each gene, we inferred haplotypes using PHASE 2.1 (Stephens et al. 2001) and mapped haplotype distribution. For the COMT1 gene, we also estimated the allele frequencies across 46 P. balsamifera provenances (484 individuals). Finally, we analyzed a protein alignment of COMT2, a paralog of COMT1 encoded on chromosome 12, from P. balsamifera and P. trichocarpa genotypes, and a protein alignment of different homologous sequences retrieved from Phytozome (http://www.phytozome.net).