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Abstract Population demographic changes, alongside landscape, geographic and climate heterogeneity, can influence the timing, stability and extent of introgression where species hybridise. Thus, quantifying interactions across diverged lineages, and the relative contributions of interspecific genetic exchange and selection to divergence at the genome‐wide level is needed to better understand the drivers of hybrid zone formation and maintenance. We used seven latitudinally arrayed transects to quantify the contributions of climate, geography and landscape features to broad patterns of genetic structure across the hybrid zone ofPopulus trichocarpaandP. balsamiferaand evaluated the demographic context of hybridisation over time. We found genetic structure differed among the seven transects. While ancestry was structured by climate, landscape features influenced gene flow dynamics. Demographic models indicated a secondary contact event may have influenced contemporary hybrid zone formation with the origin of a putative hybrid lineage that inhabits regions with higher aridity than either of the ancestral groups. Phylogenetic relationships based on chloroplast genomes support the origin of this hybrid lineage inferred from demographic models based on the nuclear data. Our results point towards the importance of climate and landscape patterns in structuring the contact zones betweenP. trichocarpaandP. balsamiferaand emphasise the value whole genome sequencing can have to advancing our understanding of how neutral processes influence divergence across space and time. 

PremiseBoreal and northern temperate forest trees possess finely tuned mechanisms of dormancy, which match bud phenology with local seasonality. After winter dormancy, the accumulation of chilling degree days (CDD) required for rest completion before the accumulation of growing degree days (GDD) during quiescence is an important step in the transition to spring bud flush. While bud flush timing is known to be genetically variable within species, few studies have investigated variation among genotypes from different climates in response to variable chilling duration. MethodsWe performed a controlled environment study using dormant cuttings from 10 genotypes ofPopulus balsamifera, representing a broad latitudinal gradient (43–58°N). We exposed cuttings to varying amounts of chilling (0–10 weeks) and monitored subsequent GDD to bud flush at a constant forcing temperature. ResultsChilling duration strongly accelerated bud flush timing, with increasing CDD resulting in fewer GDD to flush. Genotypic variation for bud flush was significant and stratified by latitude, with southern genotypes requiring more GDD to flush than northern genotypes. The latitudinal cline was pronounced under minimal chilling, whereas genotypic variation in GDD to bud flush converged as CDD increased. ConclusionsWe demonstrate that increased chilling lessens GDD to bud flush in a genotype‐specific manner. Our results emphasize that latitudinal clines in bud flush reflect a critical genotype‐by‐environment interaction, whereby differences in bud flush between southern vs. northern genotypes depend on chilling. Our results suggest selection has shaped chilling requirements and depth of rest as an adaptive strategy to avoid precocious flush in climates with midwinter warming.