The impacts of partial cuts on carbon stocks remain uncertain due to elevated post-harvest windthrow mortality that may counteract the positive effects of thinning on stand growth. This study evaluated the effects of partial cuts and tree mortality on aboveground carbon stocks eight years after harvest in jack pine forests. The experimental design included 24 sites with varying harvest intensities: unharvested control, moderate (30–35 %), and heavy (40–50 %) thinnings. Dendrochronological analyses and allometric equations were used to quantify carbon in live and dead biomass (snags, broken, and uprooted trees). Increases in individual tree carbon stocks between the pre-harvest year and the eighth year post-harvest were lower in controls and similar among partial cut treatments. Eight years after harvest, live carbon stocks per hectare were 41 % lower in the heavy thinning than in unharvested stands but remained similar between moderate thinning and control treatments. Larger trees and those growing closer to skidding trails showed greater increases in carbon stock, with the latter exhibiting a 33 % higher increase on average. Mean total deadwood carbon stocks were 55 % higher in the control than in the partial cuts due to reduced snag carbon in harvested stands. Uprooting was influenced by harvest treatment and proximity to skidding trails, while stem breakage was driven by tree characteristics (height, age, and competition) and distance to trails. The proportion of snags was explained by mean annual wind speed. These findings suggest that moderate thinning could sustain forest carbon stocks with prescriptions aiming at maintaining snag carbon stocks.

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.