Episodic, patchy disturbances characterize an old-growth Picea abies dominated forest landscape in northeastern Europe

• We studied landscape-level disturbance in primeval forest dominated by Picea abies.
• Forest dynamics was driven by episodic intermediate scale disturbance.
• The study demonstrate the role of intermediate-scale disturbance in boreal forests.

The conventional theory of boreal forest dynamics recognizes two distinct disturbance regime types, small-scale gap dynamics and dynamics driven by large stand-replacing disturbances. We used satellite imagery and extensive field measurements to examine the landscape-level pattern and impact of an extensive disturbance episode that occurred in the early 2000s in primeval forest dominated by Picea abies in the Arkhangelsk region, Russia, due to drought and subsequent bark beetle (Ips typographus) outbreak. We also quantified forest age structures and deadwood characteristics at the landscape-level to study how such disturbances shape forest structures over larger scales. Satellite image analysis revealed that disturbance patches covered about 16% of the land area in the 12 km × 12 km landscape studied. The size of the disturbance patches was strongly skewed toward small ones (median size 0.12 ha) and they were distributed across the landscape with some tendency of aggregation. The landscape forest matrix was dominated by old-growth forest. The dominant trees in the forest were established prior to 1850, and approximately half of the forest had established prior to 1800. However, the patchy occurrence of younger forest suggests that the landscape previously was subject to patchy disturbance similar to the recent one. This conclusion also gained support from historical records. We conclude that the structure and dynamics of the studied primeval forest landscape was driven by the combined impact of small-scale “background” mortality (classical gap dynamics) and infrequent episodes of patchy intermediate severity and scale disturbances.