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A from Stockholm University reveals that spruce bark beetles, already infamous for killing millions of trees in Sweden, are also changing the forest microclimate. Using a combination of temperature sensors attached to trees and thermal drone imagery, researchers have found that beetle-attacked forests can heat up by as much as two degrees during summer days—but that deciduous trees helped to cool down attacked forest stands.

The research, published in Agricultural and Forest Meteorology and conducted in Södermanland, Sweden, measured how bark beetle damage affects the microclimate both below the canopy and at canopy level. The findings suggest that forest disturbances not only respond to climate change, but also feed back into it by creating novel temperature regimes.

"We've known for a long time that hot, dry summers increase bark beetle outbreaks," says Caroline Greiser, lead author of the study and researcher at the Department of Â鶹ÒùÔºical Geography and the Bolin Center for Climate Research. "But here we show the reverse: bark beetles themselves create new thermal environments in forests."

Forest microclimates are often a bit cooler during the day and warmer during the night compared to open surroundings, leading to a buffering of warm and cold temperature extremes. The team set up a network of temperature sensors at 31 forest sites in five nature reserves in Södermanland, south of Stockholm, to study the microclimate in attacked and healthy forest stands.

In one of the areas, Ekeby nature reserve, they also created temperature maps of the canopy surface with the help of a drone. They found that dead trees were on average over 2° C warmer than living trees on sunny days, and that these "skeleton forests" with attacked and killed trees allowed more warming solar radiation to reach the understory. This could have domino effects on forest biodiversity, tree seedling survival, and even human comfort during heat waves.

However, there was one silver lining.

"Where there were more deciduous trees like birch or aspen, the warming effect during the day was much smaller," says Greiser. "And, as opposed to our expectations, we didn't see colder nights in beetle-attacked forests. The canopy cover was still high enough to slow down heat radiation during the night—so these stands may still protect young trees and other species from frost."

The results have clear implications for forest management in a warming climate.

"Our study adds a new reason to diversify forests," Greiser says. "Mixed stands with more deciduous trees are not only less vulnerable to beetle outbreaks, but they also help to keep the forest cooler afterwards."

But what to do with the already damaged forests? From a microclimate perspective, the dead trees seem to provide a similar protection against night frost as shelterwood and therefore could be left standing instead of being salvage logged.

The researchers hope that their findings can guide decisions about post-disturbance regeneration, conservation area management, and salvage logging—all of which are critical in an era of more frequent extreme weather and pest outbreaks.