Twenty years after thinning: Aboveground carbon balance, growth and deadwood dynamics in boreal black spruce forests

Boreal forests store a large fraction of global terrestrial carbon and play a key role in climate regulation. However, this carbon reservoir is increasingly influenced by natural disturbances and forest management. Understanding how silvicultural practices influence long-term carbon storage is critical for adapting boreal forest management to climate change. We assessed the effects of commercial thinning, a partial harvesting practice, on in-situ aboveground carbon stocks in managed boreal forests 20 years after treatment. Specifically, we examined whether post-release growth of residual trees offsets the biomass removal from density reduction without inducing sustained increases in tree mortality. We compared moderately thinned and unthinned even-aged black spruce stands in northeastern Québec, Canada. Aboveground carbon stocks were quantified using field inventories of live trees, understory vegetation, and deadwood, while tree-ring analyses reconstructed residual growth and mortality patterns. Tree and understory carbon stocks did not differ between treatments. Residual trees in thinned stands showed sustained growth release, with basal area increment rising by ∼77% over the post-treatment period. Thinned stands also contained higher deadwood carbon stocks, primarily from downed woody debris concentrated near skid trails. No consistent increase in tree mortality was associated to thinning. Despite higher necromass, total aboveground carbon stocks remained comparable between treatments, owing to carbon retention in long-lived deadwood and enhanced growth of residual trees. This reflects a redistribution of carbon among aboveground pools rather than a net change in total stocks. Overall, our results indicate that moderate commercial thinning can maintain in-situ aboveground carbon stocks two decades after treatment, supporting potential compatibility between timber production and climate mitigation objectives under similar ecological conditions. • Moderate thinning altered carbon pool dynamics while maintaining aboveground stocks in resource-limited boreal conditions. • Residual trees showed substantial and sustained growth release after thinning. • No significant long-term increase in mortality following thinning. • Deadwood inputs created long-term on-site carbon storage. • Operational layout shaped spatial patterns of deadwood accumulation.