Bark beetles as microclimate engineers – thermal characteristics of infested spruce trees at the canopy surface and below the canopy

• Bark beetle attacks increase maximum daytime understory temperatures by up to 2°C. • Minimum night-time temperatures remain unaffected despite canopy loss. • Dead trees are warmer than living trees, with a 2.6°C difference on sunny days. • Deciduous trees help moderate beetle-induced microclimate warming. • Results inform forest management on post-disturbance regeneration and conservation. Over recent decades, Spruce bark beetle outbreaks have expanded and intensified across Europe, driven by a warming climate and more frequent drought events. While research has largely focused on early detection and vulnerability prediction, little is known about the consequences of beetle infestations on forest microclimates. Bark beetle attacks are expected to alter forest microclimates due to changes in canopy cover, albedo, wind patterns, and evapotranspiration. We explored the effect of bark beetle attacks on summer forest microclimate using two approaches. Firstly, we measured understory microclimate at 2-m height in 31 Swedish forest stands using small temperature loggers. Along a gradient of attack severity, represented by increasing proportions of attacked spruces, maximum summer day-time temperatures increased by up to 2°C, with this warming effect being moderated by the presence of deciduous broadleaf trees. Surprisingly, night-time minimum temperatures were not affected by bark beetle attacks. Secondly, we mapped canopy surface temperature over one part of the study area using multispectral and thermal drone imaging, contrasting canopy temperatures of living and dead trees. We observed that dead trees were generally warmer than living trees, by an average of 2.6°C on a sunny day and 0.7°C on a cloudy day. Our study documented changes in thermal regimes in both the understory and overstory after bark beetle attacks, indicating that climate-change related disturbances are fuelling rapid increases in microclimate warming. However, even dead forest stands may function as thermal buffers for understory vegetation, as we found minimum temperatures being unaffected by bark beetles. Finally, our results suggest that increasing the proportion of deciduous trees can decrease the risk of bark-beetle induced microclimate warming. The insights gained can guide forest succession and regeneration management after disturbances, contributing to critical decisions on conservation areas and salvage logging strategies.