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<em>Science</em>: Centuries of European Forest Management Have Not Cooled Climate

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Coniferous (dark green) and broadleaved(light green) trees in the summertime in Alsace, France. | Ernst-Detlef Schulze

Despite an overall increase in forested areas in Europe over the past couple hundred years, forest management practices of humans have caused these green carbon sinks to contribute to climate warming, rather than mitigate it. A major reason is the strong preference of foresters to re-plant certain tree species, which has altered environmental processes on a large scale.

The results, published in the 5 February issue of Science, demonstrate that simply re-planting trees after harvesting them is not sufficient to offset climate change, and more carefully planned forest management is required.

Between 1750 and 2010, Europe had to sustain a population that grew from 140 million to 580 million people — and sustaining such a population requires plenty of resources, including wood. Therefore, by the mid-1800s, Europeans were in fact planting more trees than they were harvesting, causing a net increase in forested area by 2010.

However, not all types of trees were re-planted equally. Foresters strongly favored planting more commercially valuable coniferous trees — such as Scots pines, Norway spruce and beech — resulting in the reforestation of 633,000 square kilometers of conifers at the expense of broadleaved forests, which decreased by 436,000 square kilometers since 1850.

Kim Naudts of the Max Planck Institute for Meteorology explained, "In areas where both trees grow well, foresters have favored coniferous species over broadleaved species because they are fast-growing species. The wood from conifers has different properties — lighter, less lignin [less fibrous] and often straighter — which are valued by industrial users."

To gain more insights into the impact of this favoritism on a large scale, Naudts and colleagues reconstructed 250 years of European forest history using modelling that included forest management factors, such as changes in tree species. Their analysis reveals that the conversion of broadleaved forests to coniferous forests caused significant changes in evapotranspiration, the evaporation of water through leaves, and albedo, the amount of solar energy reflected from the Earth back into space. Both can affect warming.

"In general coniferous species absorb more solar radiation, resulting in a warming. Also, coniferous species are generally more conservative in their water use, which could lead to warming through a decrease in [evapotranspiration]," said Naudts.

Humans also are altering the ability of these forests to mitigate climate warming by removing their wood and biomass litter like leaves and branches.

"By extracting wood from unmanaged forest and bringing these forests under production, humans released carbon into the atmosphere that would otherwise be stored in the biomass, litter, dead wood, and soil of the forest," Naudts said. "So, even well-managed present-day forests store much less carbon than their natural counterparts in 1750, which explains the [net] lack of carbon dioxide removal from the atmosphere."

Naudts said that, since such biophysical effects of forest management are not always accounted for, "we thought it was worth it to do the calculations."

Although the scope of this study is restricted to Europe, similar effects may apply to other regions where similar underlying processes are or have been occurring, the authors note. For example, people have pursued large-scale replanting in natural forests and created entirely new forests in China, the U.S., and the Russian Federation, among other places.

Land-use changes over the past 250 years in Europe have been huge, yet, they only caused a relatively small temperature increase, equal to roughly 6% of the warming produced by global fossil fuel burning, Naudts noted. "Nevertheless, we should manage European forest to safeguard other functions, like biodiversity conservation, wood production, flood control, prevention of soil erosion. If, additionally, such management could result in climate mitigation, then this is an extra benefit."

Author

Michelle Hampson

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