The authors have dubbed their finding The Great Reversal – while the area of planet Earth covered  by forests might still be slowly decreasing, the density of forests is increasing. The net result say a group of United States and European researchers is that the amount of carbon being sequestered in trees is going up.

Aapo Rautiainen, Iddo Wernick, Paul E. Waggoner, Jesse H. Ausube and Pekka E. Kauppi1 in their research article A National and International Analysis of Changing Forest Density point out that, like cities, forests grow by spreading out or by growing denser.

Both inventories taken steadily by a single nation and other inventories gathered recently from many nations by the United Nations confirm the asynchronous effects of changing area and of density or volume per hectare. United States forests spread little after 1953, while growing density per hectare increased national volume and thus sequestered carbon. The 2010 United Nations appraisal of global forests during the briefer span of two decades after 1990 reveals a similar pattern: A slowing decline of area with growing volume means growing density in 68 nations encompassing 72% of reported global forest land and 68% of reported global carbon mass. To summarize, the nations were placed in 5 regions named for continents. During 1990–2010 national density grew unevenly, but nevertheless grew in all regions. Growing density was responsible for substantially increasing sequestered carbon in the European and North American regions, despite smaller changes in area. Density nudged upward in the African and South American regions as area loss outstripped the loss of carbon. For the Asian region, density grew in the first decade and fell slightly in the second as forest area expanded. The different courses of area and density disqualify area as a proxy for volume and carbon. Applying forestry methods traditionally used to measure timber volumes still offers a necessary route to measuring carbon stocks. With little expansion of forest area, managing for timber growth and density offered a way to increase carbon stocks.

The group of scientists analyzed international data compiled by the United Nations Food and Agriculture Organization (UNFAO) in the 2010 Global Forest Resources Assessment with 68 countries included in an analysis by region.

Countries in the South American and African regions lost close to 10% of their forest area during the two decades. Asian and European forest area expanded several percent while the area in North America changed little.

The changing carbon mass combined with the area reflects changing density. During the second of the two decades, carbon mass in the Asian region changed little, while area expanded, indicating falling carbon density. For analyzed countries in the Africa and South America regions, carbon declined slightly less than area, reflecting small density increases. North America and Europe gained carbon well in excess of any area additions.

During 1990–2000 carbon density grew in all regions. While area changed little in the Asian and North American regions, rising density increased sequestered carbon. In the European region, increasing area plus carbon density together grew the mass of carbon. The data for the African and South American regions indicate shrinking carbon volume but slight gains in carbon density during the 1990s.

During the second period, 2000–2010, the Asian region displayed the greatly altered pattern. The great loss of density and sequestered carbon in Indonesia obscured the rising density in ten of the twenty-one nations included in the region. As during the preceding period, rising density contributed most to increased sequestered carbon in the European and North American regions. African and South American losses of sequestered carbon mass were again tempered by slightly rising carbon density.

In all regions apart from Asia, the sign of change in forest area and the stock of sequestered carbon was the same. However, their magnitude differed significantly, especially in Europe and North America where most of the change was attributable to increasing carbon density

The researchers reach the f0llowing conclusion:

For forests in a world with a growing population, with growing needs, but a fixed expanse of land, faster increases of volume or carbon than decreases of area during recent decades are encouraging. Technological improvements have been shown to improve greatly the efficiency of producing commercial timber products using less land [17], [18]. The major stresses on forest land are thus attributable to humanity’s appetite for forest land, not forest products.

To stop the loss of forest land in countries still experiencing deforestation, lifting crop yields as well as stabilizing population and switching fuels can reduce pressure on forest land [19], [20]. Forest management to increase carbon density by encouraging growth of young forests and improving degraded forests offer effective levers for higher global forest carbon sequestration.

Inventories taken steadily by a single nation, the United States, and other inventories gathered recently from many nations by the United Nations confirm the asynchronous effects of changing area and density. While assessing the environmental impacts from forest management has become integral to national environmental policies and the global debate over climate change, current global forest statistics continue to suffer from deficiencies in the availability, accuracy, and precision of measurements of key attributes. The need to address these issues with scientific rigor calls for increased attention to forest density measurement to produce a consistent global data set that acknowledges the importance of forest density, in addition to area, as a decisive factor.

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Peter Fray
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