High Biomass Forests are more Susceptible to Bark Beetle Disturbance in Europe

European forests have seen a rise in both harvest and natural disturbances (i.e., bark beetle and windthrow disturbances) in the last decades, with consequences for Europe’s Forest carbon sink, which is already declining in some countries. Among natural disturbances, drought-driven bark beetle outbreaks accounted for a third of unplanned canopy openings between 2015 and 2020. Bark beetles are theorised to preferentially target high-biomass forests, due to favourable breeding material, suggesting that the major outbreaks in 2018–2020 were inevitable in high-biomass spruce forests. However, direct comparison of forest biomass in pre-disturbance and undisturbed forests using remote sensing data remain unexplored despite their critical implications for future forest management.

We hypothesise that forests subject to upcoming bark beetle disturbances have a higher biomass than forests that have remained undisturbed throughout the satellite era. To test this, we combine 30m spatial scale forest disturbance data (1984 to 2023) with a 30m PlanetScope-based aboveground biomass (AGB) map for 2019 and a 10m forest genus map based on Sentinel-1/2. This approach allows us to examine forest AGB in 2019 before disturbances between 2021 and 2023 occurred, which we term “forests with upcoming disturbances.” We compared this with AGB in nearby (within 10km) forests that remained unaffected by disturbances throughout the entire period, termed “undisturbed forests”. Additionally, we included nearby forests that experienced disturbances between 1984 and 2019, referred to as “disturbed forests”.

Preliminary results show that needleleaf forests subject to upcoming unplanned disturbances have significantly higher AGB compared to nearby undisturbed forests, particularly in spruce forests, where biomass values are, on average, 30Mg/ha higher than undisturbed spruce forests. In contrast, no statistical difference was found between the biomass of spruce forests subject to upcoming harvest and undisturbed forests.

Enhancing the carbon sink in European forests is a crucial climate mitigation strategy and for achieving the European Green Deal goals. Prioritising the restoration of spatially heterogeneous forests over merely high biomass forests is therefore a crucial consideration. This strategy could help mitigate the increasing risk of bark beetle outbreaks under global warming.