Bark traits and their influence on thermal resistance to wildfires: an experimental study across six tree species common in Central Europe

Abstract Climate change is increasing the probability of wildfires in the non-fire-prone forests of Central Europe. Systematic knowledge on tree resistance to fire would help to improve the future management of forests to ensure their long-term adaptation to climate change impacts. However, a systematic classification of the fire resistance of tree species occurring in Central Europe remains largely unexplored. While observations from surface-fire-dominated ecosystems show that bark constitutes an important interface between fire and tree physiology, the influence of bark traits remains an open question, due to conflicting study results. To address this gap, we conducted heating experiments on stem discs with intact bark, assessing bark thermal conductivities of six tree species common in Europe (the native species Abies alba, Fagus sylvatica, Larix decidua, Picea abies, Pinus sylvestris, and the non-native Pseudotsuga menziesii ). By applying generalised additive models, we further investigated the influence of bark traits (thickness, moisture content, density), relative height along the tree stem, and tree species identity on insulation capability. Our results indicate that bark thickness may be the most robust predictor for assessing bark-related resistance to surface fires (i.e. insulation capability). A decrease in thermal resistance was observed with increasing relative height of the sample along the tree stem. Further, we found evidence for decreasing thermal resistance with increasing bark moisture content, but no significant effect of bark density on bark insulation capability. We identified significant interspecific differences in bark insulation capability between the investigated tree species, particularly in combination with species-specific patterns of bark investment. Accordingly, our results suggest higher resistance to fire for L. decidua , P. menziesii , and P. sylvestris , while A. alba , P. abies , and especially F. sylvatica may be associated with lower fire resistance. Our results underscore the need to incorporate diverse tree species' characteristics into assessments of disturbance resistance in the face of climate change.