Sentinel-2 and environmental data as predictors of the spatiotemporal dynamics of bark beetle (Coleoptera: Scolytinae) - induced tree mortality in natural spruce forests

Biotic and abiotic disturbances affect forest ecosystems. Unlike sudden disturbances, bark beetle outbreaks are a gradual process. Bark beetle outbreak triggered by windstorm in Norway spruce (Picea abies) forests was analysed over a 6-year period (2016-2021) in mountainous conditions (890-2,100 m a.s.l.) with deep valleys and high ridges. The disturbance extent and dynamics were assessed via remote sensing (Sentinel-2 imagery) using a supervised maximum likelihood classification. Topographical variables, bark beetle-related spatial metrics and spectral indices were assessed for predictor importance for their influence on bark beetle-caused disturbance dynamics by boosted regression trees. The overall accuracy of the classification ranged from 87-92% (Kappa 0.84-0.89). Bark beetle spots were initiated mainly at relatively high altitudes and preferentially on exposed terrain. Spectral indices, such as the red-edge normalized difference vegetation index (RENDVI), played a consistent role across various years in predicting spot initiation. With respect to the spread of bark beetle spots, distance emerged as the most influential predictor across all years. Additionally, elevation and RENDVI had notable impacts on spot spreading. In the case of bark beetle spot initialization, the importance of different factors varied among years whereas for spot spreading distance was the most influential for each year.