Introgression underlies adaptively significant variation and range boundaries in forest trees

• Introgression can be an important source of adaptive phenotypes, although conversely it can have deleterious effects. Evidence for adaptive introgression is accumulating but information on the genetic architecture of introgressed traits lags behind. • Here we determine trait architecture in Populus trichocarpa under introgression from P. balsamifera using admixture mapping and phenotypic analyses. • Our results reveal that admixture is a key driver of clinal adaptation and suggest that the northern range extension of P. trichocarpa depends, at least in part, on introgression from P. balsamifera. However, admixture with P. balsamifera can lead to potentially maladaptive early phenology and a reduction in growth and disease resistance in P. trichocarpa. Strikingly, an introgressed chromosome 9 haplotype block from P. balsamifera restores the late phenology and high growth parental phenotype in admixed P. trichocarpa. This epistatic restorer block may be strongly advantageous in maximizing carbon assimilation and disease resistance in the southernmost populations where admixture has been detected. We also confirm a previously demonstrated case of adaptive introgression in chromosome 15 and show that introgression generates a transgressive chlorophyll–content phenotype. • We provide strong support that introgression provides a reservoir of genetic variation associated with adaptive characters that allows improved survival in new environments.