Balancing biodiversity, carbon, and profit with pathogen risk: Effects of felling age and thinning in spruce forests

Global forestry is balancing multiple societal needs, including carbon sequestration and biodiversity conservation, in addition to timber production. We aim to guide decision-making in managed forests by providing a test case that navigates the trade-offs between these objectives in clear-cut oriented boreal forests. We measured timber yield, biodiversity-critical habitats, aboveground tree carbon and fungal pathogen pressure in 66 thinned (from below) and unthinned Norway spruce dominated stands spanning 40 to >100 years in hemiboreal Estonia. At comparable stand age, the volume of spruce sawlogs - the main commercial timber assortment - in thinned stands was similar to unthinned forests, but total aboveground tree carbon was lower. Thinned stands experienced greater fungal pathogen damage. In these stands, the availability of most dead-wood habitats remained limited to 15-92 % of the levels in unthinned stands of comparable age; and the difference from unthinned forests increased with stand age. Aboveground tree carbon peaked in 81-100-years-old unthinned stands (i.e. 20-40 years beyond their normal final felling age), surpassing both younger and older stands. By that time, biodiversity-critical habitats in unthinned forests reached 61-86 % of old-growth forest levels. The heavy pathogen load and increasing divergence from unthinned forests over time suggest that thinning may restrict future options for multifunctional forest use, such as extending final felling age-an approach that would have been effective for both carbon and biodiversity goals in unthinned spruce dominated managed forests. Consequently, some management decisions made decades earlier may limit the flexibility to adapt forest management to evolving societal needs.