In Europe and North America, increased density has significantly raised carbon storage despite little or no expansion of forest area, according to the study, conducted by scientists at Rockefeller University, US, Connecticut's Agricultural Experiment Station, US, and the University of Helsinki, Finland. The findings were published in PLoS One.

Even in the South American nations studied, more density helped maintain regional carbon levels in the face of deforestation.

The researchers analysed information from 68 nations, which together account for 72% of the world's forested land and 68% of reported carbon mass. They conclude that managing forests for timber growth and density offers a way to increase stored carbon, even with little or no expansion of forest area.

Co-author Paul Waggoner, a forestry expert with Connecticut's Agricultural Experiment Station, says that remote sensing by satellites of the world's forest area brings access to remote places and a uniform method. "However, to speak of carbon, we must look beyond measurements of area and apply forestry methods traditionally used to measure timber volumes."

The authors say that most regions and almost all temperate nations have stopped losing forest and the study's findings constitute a new signal of what they call the Great Reversal under way in global forests after centuries of loss and decline.

To examine how changing forest area and density affect timber volume and carbon, the study team first focused on the US, where the US Forest Service has conducted a continuing inventory of forest area, timberland area and growing stock since 1953. They found that while US timberland area grew only 1% between 1953 and 2007, the combined national volume of growing stock increased by an impressive 51%. National forest density increased substantially. For an international perspective, the research team examined the 2010 Global Forest Resources Assessment compiled by the UN Food and Agriculture Organization (FAO), which provides consistent figures for the years 1990 to 2010.

The data reveal uncorrelated changes of forest area and density. Countries in Africa and South America, which lost about 10% of their forest area over the two decades, lost somewhat less carbon, indicating a small rise in forest density. In Asia during the second decade of the study period, density rose in 10 of the region's 21 countries. Indonesia's major loss of density and sequestered carbon, however, offset any gain in carbon storage in other Asian nations. Europe, like the US, demonstrated substantial density gains, adding carbon well in excess of the estimated carbon absorbed by the larger forested area.

Overall, of 68 nations studied, forest area is expanding in 45 and density is also increasing in 45. Changing area and density combined had a positive impact on the carbon stock in 51 countries.