Understanding the interplay between environmental conditions and physiological traits is essential to explain tree longevity in resource-limited environments. We studied old (up to 466 years) Thuja occidentalis trees growing on contrasting rock and gravel substrates to assess how substrate type and age influence hydraulic function, resource use, and survival strategies. Using dendroanatomical and physiological measurements, we compared key functional traits across 25 trees of varying ages and site conditions in Duparquet, Quebec, Canada. Our results show that trees on drier rock substrates exhibit smaller xylem tracheids, reduced theoretical hydraulic conductivity, and lower foliage density, enabling them to resist drought stress at the cost of limited growth. These conservative traits become more pronounced as the trees age and are associated with greater longevity, with the age of trees growing on rock substrates exceeding 300 years, whereas those on gravel substrates reaching up to 200 years. In contrast, trees on gravel demonstrate greater hydraulic efficiency and radial growth potential as a result of fewer resource constraints. These findings align with the hydraulic limitation hypothesis and provide valuable insights into how substrate conditions and age-related functional acclimation shape survival strategies in long-lived trees. Moreover, they highlight the importance of local site conditions in tree resilience and underscore the need for further research into the effects of environmental variability on tree physiology and morphology to better predict tree responses to future environmental changes.

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.