Tree and stand-level patterns and predictors of Norway spruce mortality caused by bark beetle infestation in the Tatra Mountains

• We analyzed long-term plot data taken from a spruce forest affected by bark beetles.
• Norway spruce mortality significantly increased as the outbreak progressed.
• Larger trees were killed more frequently and stand complexity decreased.
• Stand structure characteristics, particularly diameter, best predicted mortality.
• Fine-scale outbreak severity is mostly governed by outbreak stage and stand structure.

This study investigates temporal shifts in Norway spruce (Picea abies) mortality, stand structure characteristics, and stand complexity facilitated by a bark beetle (Ips typographus) outbreak that affected an unmanaged subalpine forest region in Tatra National Park, Poland in the late 2000s. Changes in survivorship and stand structure characteristics (diameter (DBH), basal area, height, age, and crown length ratio) of nearly 2500 spatially-referenced trees located in 64 long-term survey plots were compared over four time periods that spanned the duration of the outbreak disturbance event. Stand structure characteristics, topographic factors (slope, elevation, and aspect), and solar equinox radiation were tested as predictors of mortality for multiple stages in the outbreak using boosted regression tree modeling. Our findings showed that: (1) spatial synchrony was not reflective of mortality severity; (2) mortality rates increased significantly as the outbreak progressed; (3) the stand’s structure was altered significantly by the outbreak (larger trees were killed most frequently); (4) stand structure characteristics were the best predictors of mortality in all stages of the outbreak, though topographic factors and solar equinox radiation also exhibited moderate to strong predictive power in some stages; and (5) stand complexity decreased significantly as the outbreak progressed. This illustrates the inherently complex nature of bark beetle outbreaks on fine spatial scales and suggests that the extent and severity of spruce mortality during an outbreak event is largely dependent on the relative stage of the outbreak and the structure of the stand.