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Forests in many regions are becoming larger carbon sinks thanks to higher density, U.S. and European researchers say in a new report.
In Europe and North America, increased density significantly raised carbon storage despite little or no expansion of forest area, according to the study, conducted by Rockefeller University scientists with colleagues at Connecticut’s Agricultural Experiment Station and the University of Helsinki in Finland. The findings were published in the online, open-access journal PLoS One.
The authors say most regions and almost all temperate nations have stopped losing forest and the study’s findings constitute a new signal of what co-author Jesse Ausubel of Rockefeller calls the Great Reversal under way in global forests after centuries of loss and decline. “Opportunities to absorb carbon and restore the world’s forests can come through increasing density or area or both.”
To examine how changing forest area and density affect timber volume and carbon, the study team first focused on the United States, where the U.S. Forest Service has conducted a continuing inventory of forest area, timberland area and growing stock since 1953.
They found that while U.S. timberland area grew only 1 percent between 1953 and 2007, the combined national volume of growing stock increased by an impressive 51 percent. National forest density increased substantially.
CO2 is the source of the carbon that plants turn into organic compounds, and it is well established that higher CO2 levels can have a fertilising effect on many plants, boosting growth by as much as a third.
However, some plants already have mechanisms for concentrating CO2 in their tissues, known as C4 photosynthesis, so higher CO2 will not boost the growth of C4 plants.
Where water is a limiting factor, all plants could benefit. Plants lose water through the pores in leaves that let CO2 enter. Higher CO2 levels mean they do not need to open these pores as much, reducing water loss.