Forestry practices have negative climate effects, researchers say

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 study in the journal Science Thursday makes the provocative claim that more than 250 years of forest management in Europe has contributed to climate change, rather than helped stop it.

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

Through their reconstruction, the researchers made key observations about the ways forests have changed in Europe. First, they found that while deforestation removed about 75,000 square miles of forest cover in Europe between 1750 and 1850, subsequent reforestation efforts not only made up for the losses but resulted in a net gain in Europe's forest cover during the study period.

They 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, such as oaks. Conifer cover increased by 244,400 square miles, while broadleaf cover decreased by 168,300 square miles.

Finally, they observed that in the past 250 years, about 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, the models suggest. 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 accumulated a carbon debt, releasing 3.1 billion metric tons of carbon into the atmosphere since 1750.

“Even a well-managed forest today stores less carbon than its natural counterparts in 1750,” said the study's lead author Kim Naudts, a researcher with the University of Versailles' Climate and Environmental Sciences Laboratory.

The researchers found that replacing broad-leaved 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.” The resulting drier air contributed to a warming effect.