Global warming and more frequent droughts have intensified the scourge of the spruce bark beetle (Ips typographus) in Europe. Forests are key to mitigating extreme temperatures and macroclimatic warming, thereby supporting biodiversity and resilience forests to climate change. The sap-eater, an ectothermic insect, benefits from longer and warmer growing seasons, which accelerates its development and allows for more generations per year. At the same time, spruce trees weakened by drought are more vulnerable to its attacks.

The bark beetle attacks trees by disrupting the vascular system and transmitting a fungus (Ophiostoma spp.) that blocks the flow of water and nutrients. These attacks lead to tree death and drastic changes in the forest ecosystem, affecting light, moisture and temperature. The bark beetles are thus ecosystem engineersOur study documents that climate-related disturbances accelerate the warming of forest microclimates.

Research revealed significant changes in thermal regimes in the understory and in the tree crowns after bark beetle attacks. In the understory, maximum daily summer temperatures increased by up to 2 °C with an increasing proportion of spruces being attacked. This increase is primarily a consequence of increased canopy openness and a significant decrease in tree transpiration, which reduces evaporative cooling. In addition, dead trees were warmer than alive, on average by 2.6 °C on a sunny day and by 0.7 °C on a cloudy day. Surprisingly, however, the calamities did not affect minimum night temperatures, suggesting that even dead trees can effectively dampen radiative heat loss at night.

Key finding: increasing the proportion of deciduous trees can mitigate microclimatic warming caused by the bark beetle. Deciduous trees maintain microclimate damping even in dying spruce stands. Diversifying forests with an emphasis on deciduous species not only reduces the risk of pests, but also ensures continued microclimate damping. Management decisions about logging after disasters should weigh the benefits of maintaining microclimate damping and biodiversity-supporting elements. Dead trees continue to damp the understory and provide valuable habitat for saproxylic insects and fungi.

The study contributes to the understanding of atmosphere-biosphere feedbacks, where the ecological impacts of climate change lead to accelerated local climate changes that may further exacerbate the calamities of the lycopod. Characterizing and taking into account these new microclimatic regimes in disturbed forests is crucial for informed management and promoting resilience. Our results support calls for diversification of Scandinavian forests to increase their resilience and ecosystem functions, such as microclimate moderation. JRi

The study is published in the journal Agricultural and Forest Meteorology .