Bark beetle attacks increase daytime forest temperatures

Hot summers have made bark beetles a familiar menace in northern Europe, but a new twist is now clear. When these insects kill spruce, they also change the air within the forest, raising daytime temperatures close to the ground by as much as 2 degrees Celsius (about 3.6 degrees Fahrenheit).

That shift matters because many organisms depend on steady conditions near the forest floor. It also affects how new trees take root after a disturbance, and how people feel when they walk these trails on a scorching day.

Increasing bark beetle outbreaks

The research was led by Caroline Greiser of Stockholm University and the Bolin Centre for Climate Research.

The team measured temperatures both beneath and above the trees at 31 spruce-dominated sites in southern Sweden between July and September 2021.

Sensors mounted at 6.5 feet recorded air in the understory, the shaded space below the canopy. A drone flight added a thermal map of treetops over one reserve to compare living crowns with dead ones.

The focus was the European spruce bark beetle, Ips typographus. “We’ve known for a long time that hot, dry summers increase bark beetle outbreaks,” said Greiser.

The goal of the study was to see how attack severity plays out in the micro-scale temperatures that plants and animals actually experience.

Temperatures of dead trees

Daytime maximums beneath attacked stands ran up to 2 degrees Celsius warmer along a gradient from healthy to heavily damaged.

Nights did not drop below the temperatures seen in healthy stands, which suggests a surprising kind of thermal stability after mortality.

Dead trees were not neutral at the top of the forest either. On a sunny day their crowns averaged 2.6 degrees Celsius warmer than living crowns, and on an overcast day the gap shrank to 0.7 degrees.

Why deciduous trees matter

Where birch and aspen were mixed in with spruce, the warming was smaller. Broad leaves restore shade, keep moisture cycling, and help hold down the daily highs under the trees after an outbreak.

“Our study adds a new reason to diversify forests,” said Greiser. Mixed stands also spread risk across species, so a single pest cannot topple everything at once. 

Restoring deciduous trees to conifer heavy stands has been a policy discussion in Scandinavia for years. These results add temperature control to the list of reasons to bring them back.

Temperature effects of bark beetles

Forests tend to be cooler by day and warmer by night than open areas because of shading, wind reduction, and moisture fluxes. That buffering is documented in a global review that links canopy structure to steadier conditions below.

Living trees also move water from soils to air through evapotranspiration, which absorbs heat on hot afternoons. When many trees die, that cooling pathway weakens, so the daytime highs under the canopy can tick upward.

Despite the warmer days, attacked stands did not show colder nights in this work. Crowns and stems, even when dead, still slow the loss of heat to the sky, and that can keep minimums steady.

Earlier field study in shelterwood systems found higher night minimums under partial cover than in clear cuts. That helps explain why leaving some structure standing can still protect frost sensitive seedlings.

Why dead trees run hotter

The thermal drone maps showed dead crowns run warmer than living crowns, especially on clear days. That pattern lines up with reduced water flow from dead wood and the loss of evaporative cooling at the top of the stand.

Independent remote sensing work in Poland reported roughly a 2.5 degrees Celsius (about 4.5 degrees Fahrenheit) difference between dead and healthy tree canopies on a hot day. Weather conditions and tree species can change the exact number, but the direction is consistent.

Clear skies strengthen these differences because sunlight drives heating while living leaves are busy cooling. Clouds narrow the gap by cutting incoming energy.

Preparing for bark beetles

Once again, diversity proves valuable. Adding birch and aspen to spruce stands softens the heat surge that follows beetle die-offs and reduces the risk of a single insect wiping out the entire stand.

Decisions about salvage logging are not one size fits all. Where standing dead trees help hold night minimums steady and support insects and fungi that need dead wood, managers can leave structure to maintain that buffering.

Fire risk is often raised after beetle outbreaks, yet large scale analyses show area burned in the western United States did not increase directly because of beetles, with fire weather playing a larger role.

That does not remove local hazards, but it does suggest careful, site specific planning rather than blanket assumptions.

Future bark beetle outbreaks

Better maps of fine scale temperature will help link microclimate to seedling survival and wildlife activity.

Repeated drone flights across seasons and weather types could sharpen guidance on when and where structure matters most.

Microclimate is part of the climate feedback too. As bark beetles change how heat moves through forests, they shape the very environment that affects future outbreaks and regeneration.

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

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