How bark beetle–induced dieback and silvicultural harvesting alter carbon and nitrogen pools in mountain Norway spruce monocultures

Large-scale forest dieback and other disturbances caused by insect outbreaks, windstorms, or fires are integral components of forest ecosystem dynamics. In managed forests, regeneration occurs through natural or artificial silvicultural practices, including planting or natural regeneration. However, extensive forest dieback can substantially reduce ecosystem services, particularly carbon accumulation and sequestration, and may be accompanied by changes in soil properties and nutrient pools. This study evaluated the recovery of soil organic carbon (SOC), total nitrogen (Nt), and selected labile fractions—cold-water-soluble carbon (WSC), water-soluble nitrogen (WSN), hot-water-extractable carbon (HWC), and hot-water-extractable nitrogen (HWN)—in Norway spruce ( Picea abies (L.) H. Karst.) forest ecosystems under two disturbance regimes: (i) bark beetle outbreaks followed by sanitary logging, and (ii) routine silvicultural harvesting in areas without outbreaks or large-scale tree mortality. Soil properties were assessed along chronosequences representing 10–20, 20–40, and 40–60 years post-disturbance, with mature managed stands (> 80 years) serving as reference sites. In the youngest stands recovering from both bark beetle outbreaks and regular harvesting, SOC and Nt were depleted in the upper mineral soil (0–10 cm) and enriched in deeper horizons (10–40 cm) compared with the oldest stands. However, soils beneath regenerated stands exhibited less stable nitrogen pools following bark beetle outbreaks than after silvicultural harvesting. Differences in SOC and Nt between the youngest and oldest stands largely disappeared within 20–40 years following disturbance. These findings suggest that, in spruce monocultures, silvicultural harvesting and outbreak-induced stand replacement have comparable long-term effects on soil carbon storage. In contrast, nitrogen labile fractions respond differently to cyclic stand dieback driven by biotic agents, a disturbance type that is increasing in frequency under ongoing climate change and declining stand resistance.