Fast-growing trees such as hybrid poplar (Populus spp.) are characterized by rapid growth and high biomass production. However, their tree productivity can vary depending on tree spacing, site conditions and genotype selection. In this study, we evaluated the impact of site × spacing × clone interaction on tree productivity. We selected plantations of four hybrid poplar clones (747215; Populus trichocarpa Torrey & A. Gray × P. balsamifera L., 915004, 915005; P. balsamifera × P. maximowiczii Henry and 915319; P. maximowiczii × P. balsamifera) established at three spacings (1 × 4 m, 2 × 4 m and 3 × 4 m) across three sites of contrasting productivity in northwestern Quebec, Canada. Increasing spacing from 1 × 4 m to 3 × 4 m led to larger stem and higher mean annual increment (MAI) and above-ground biomass on a per tree basis at the most productive site. In contrast, tree size remained smaller and unchanged across spacings at the least productive site, and for the least productive clone. The proportion of biomass allocated to the stem decreased when spacing increased from 1 × 4 m to 3 × 4 m to the benefit of branches. Hybrid poplar plantations’ productivity was promising, as MAI per hectare reached 20.10 m3 ha−1 year−1 at narrower spacing (1 × 4 m) at the most productive site for the most productive clone (915319). These findings highlight that site conditions modulate spacing effects and that high-yielding clones with optimal densities can maximize hybrid poplar productivity.

IntroductionClimate change is exerting profound impacts on ecosystems worldwide. In Quebec, Canada, mean annual temperatures are projected to rise by 2°C over the coming decades. In addition, the frequency and intensity of climate extremes are expected to increase, posing significant risks to forest ecosystems and affecting the performance and survival of tree species.MethodsTo assess the potential impacts of climate change on the local adaptation of white spruce, six geographically distant seed sources were evaluated seven years after planting at two test sites with contrasting growing conditions and latitude in Quebec, eastern Canada. Fall phenology was monitored from mid-July to mid-August 2021. For frost tolerance assessment, representative samples were collected in early September. Additionally, two sampling dates at the end of the growing season were used to characterize the dynamics of non-structural carbohydrates (NSC).Results and DiscussionBud set phenology, autumn frost tolerance, and late-season NSC concentrations (fructose, glucose, sucrose, and pinitol) were significantly influenced by test site, seed source, and their interactions. The average bud set initiation occurred one week later at the southern site compared to the northern site, whereas the final stage of bud set was similar between sites. Significant differences among seed sources were observed for certain phenological stages. By early October, all seed sources growing at the southern site showed reduced frost tolerance compared to those at the northern site, with visible needle tissue damage occurring at temperatures below –12°C, highlighting the increased risk of early autumn frost damage at the southern site. Fructose and glucose exhibited contrasting site-specific temporal trends, increasing from September to October at the southern site while decreasing at the northern site. In contrast, sucrose showed the opposite pattern, with the southwestern seed source exhibiting the lowest content, emphasizing the contrasting roles of NSC in sustaining late-season growth under warmer conditions and in regulating dormancy induction and cold hardiness under colder conditions. Overall, our study highlights the crucial role of NSC dynamics in cold tolerance and local adaptation to early autumn frost events. The implications of these findings for assisted migration strategies are discussed.