Effects of bark beetle disturbance and fuel types on fire radiative power and burn severity in the Bohemian-Saxon Switzerland

Abstract

Until recently, forest fires were considered a rare phenomenon in the temperate forests of Central Europe because of the moderate summer temperatures and the humid climate. However, many of those forests (monocultures of Picea abies, Norway Spruce) were affected by bark beetle infestations in the past years and recent fires such as that in the Bohemian-Saxon Switzerland in 2022 raised widespread debates about the effects of forest mortality on fuel accumulation and hence fire occurrence and severity. Here, we aim to investigate the association of fire radiative power (FRP) and burn severity with fuel types during the fire in Bohemian-Saxon Switzerland in 2022. We enhanced a European fuel type classification with a class for dead and dying spruce and mapped pre-fire fuel types using data on habitats, tree species, bark beetle disturbances and tree coverage. FRP was derived from observations of the Visible and Infrared Radiometer Suite (VIIRS) sensors. Burn severity was quantified by the differenced normalized burn ratio (dNBR) through pre- and postfire Landsat 8, 9, and Sentinel-2 images and was assessed in the field using the Composite Burn Index (CBI). We found the highest FRP at sites with dead spruce forests. Burn severity was moderate with high variability across all fuel types but highest severities occurred in dead spruce stands. The dNBR correlated positively with char height and the presence of torched trees (R = 0.48, P < .05), especially in dead spruce stands, likely due to the large amount of dry fine woody debris and the presence of initial natural regeneration. Our results demonstrate that surface fuel accumulation from past bark beetle disturbances resulted in more intense fires (higher FRP, char height and torching) and higher burn severity. The results highlight the need for improved cross-border fire risk management in Central European temperate forests that were previously not considered as fire-prone.