Autumn paints a birch forest as the sun sets at the Rauen Hills in Rauen, eastern Germany. (Patrick Pleul/DPA via AP)

Forests play a critical role in the global carbon cycle, helping to mitigate climate change by storing carbon that would otherwise end up in the atmosphere. So maintaining sustainable forest management practices that maximize the trees’ ability to act as a carbon sink — for example, planting more trees than we harvest — has been recognized by activists as a key strategy in the fight against climate change.

But forestry has not always had the desired effect, according to recent research. A new study out in the journal Science Thursday makes the provocative claim that more than 250 years of forest management in Europe have actually contributed to climate change, rather than helped to stop it.

The study reconstructs the land-use history of Europe from 1750 through the present day, taking into account both changes in land cover — that is, deforestation and afforestation — and management changes, including changes in the types of trees planted and the amount of wood harvested. The researchers then used models to examine the effects of these changes on the climate over time.

Through their reconstruction, the researchers made some key observations about the ways forests have changed in Europe in the past 250 years. First, they found that while deforestation removed nearly 200,000 square kilometers (or around 75,000 square miles) of forest cover in Europe between 1750 and 1850, subsequent reforestation efforts not only made up for the losses, but actually resulted in a net gain in Europe’s overall forest cover during the study period.

They also noted that these reforestation efforts tended to favor conifer trees, which produce a more commercially valuable type of wood, rather than the broad-leaved forests that dominated the landscape before. Broad-leaved  trees are usually deciduous trees and have flat leaves, like oaks. Overall, conifer cover increased by 633,000 square kilometers, while broadleaf cover decreased by 436,000 square kilometers.

Finally, they observed that in the past 250 years, approximately 85 percent of Europe’s forests came under human management and have been subjected to wood extraction practices, such as tree thinning and litter raking.

These changes have had a number of effects on the climate since 1750, the models suggested. While it may seem as though a net gain in tree cover should have been good for the climate, the researchers found that Europe’s forests have actually accumulated a carbon debt, releasing 3.1 billion metric tons of carbon into the atmosphere overall since 1750.

“European forests have failed to realize a net [carbon dioxide] removal from the atmosphere, and this is due to the fact that humans extracted wood from unmanaged forests by bringing these forests under management,” said the study’s lead author Kim Naudts, a researcher with the University of Versailles’ Climate and Environmental Sciences Laboratory at the time the work was conducted.

“Even a well-managed forest today stores less carbon than its natural counterparts in 1750,” Naudts said.

Additionally, the researchers found that replacing broadleaved forests with conifers had another unintended warming effect. Conifer leaves tend to be darker than those of broadleaved trees, Naudts pointed out, so they allow more sunlight to be absorbed.

And, Naudts added, “the other effect is that they are more conservative with water, which leads to less evapotranspiration, and to drier air.” Evapotranspiration is the process by which water evaporates out of a tree’s leaves into the atmosphere. The resulting drier air also contributed to a warming effect.

Overall, the researchers found that there was an increase in air temperature over the forests of about 0.12 degrees Celsius during the study period. This finding represents an especially interesting aspect of the paper, according to Rupert Seidl, a professor in the Institute of Silviculture at the University of Natural Resources and Life Sciences in Vienna, in an email to The Washington Post. (Seidl was not involved with the study). And extending the paper’s climate analysis beyond simply the amount of carbon released by the forests into the atmosphere is “important to get a better and more comprehensive understanding of the role forests play in the climate system,” he said.

Altogether, the study concludes that forest management in Europe over the past 250 years has not been the climate boon that some might have hoped for. In the paper’s abstract, the researchers suggest that “the political imperative to mitigate climate change through afforestation and forest management…risks failure, unless it is recognized that not all forestry contributes to climate change mitigation.”

However, not all experts agree with the authors’ harsh conclusions. Seidl, for instance, pointed out that using forests as a climate change mitigation tool is a new approach, one that did not exist for the majority of this paper’s study period.

“The goal of past management was primarily to ensure local energy supply and maintain food security, and not regulating global climate,” he said in his email. “So blaming past management for not cooling the climate is like blaming it for something it never set out to do in the first place.”

And other experts argued that the way the study was designed — examining the blanket effects of one long block of time from 1750 on — obscures the fact that more recent forest management techniques have had better results. According to Marcus Lindner, head of the Sustainability and Climate Change Research Programme at the European Forest Institute, forest management techniques started to have a positive mitigation effect in Europe in the 1950s.

“Obviously forest loss and forest degradation strongly contributed to greenhouse gas emissions until the 1950s,” said Lindner, who was not involved with the study, in an email to The Post. “I find it not credible to combine 200 years of suspected negative contribution to climate change mitigation with 60 years of positive climate change mitigation through management and then claim that (based on combined 260 years) forest management does not work.”

Asked about this, Naudts said that the researchers started with 1750 because that’s the year used as a reference by the U.N.’s Intergovernmental Panel on Climate Change, and “from a climate perspective, that’s a very logical reference.”

So it may be that forest management in Europe has had greater success in climate mitigation when a more recent history is considered. Regardless, the study’s authors suggested that paying more attention to the specific effects of certain land-use and management changes on the climate — as revealed by their historical effects — will benefit future forestry efforts.

Naudts, for instance, suggested that foresters should not make management decisions based solely on the amount of carbon that will be released or stored under certain strategies. “If we manage them only for carbon we ignore the other unintended changes, like changes in solar radiation, absorption and evapotranspiration, and our study shows that those are actually quite important,” Naudts said.

And she added, “In Europe, it could be wise to avoid the use of conifers when you do afforestation or when you are managing your forests.”

A further key point to consider, according to Seidl, is that even current forest management “not only aims to cool the climate, but also is providing a wide range of other ecosystem services to a growing human population, from fiber and fuel to recreation and the provisioning of clean water.”

This is an issue considered by the authors of the study in their conclusions as well. The question they raise at the end of their paper is whether it’s possible to design management strategies that cool the climate while simultaneously preserving the other ecosystem services that forests provide to humans, such as wood production.

“I agree with Naudts et al. when they conclude that the key challenge for forest management is to develop strategies that fulfill a wide range of these functions and services – including climate change mitigation – simultaneously,” Seidl said.