Most Forests are Growing Faster

On a global scale, forests increased their net primary production by 6 percent between 1982 and 1999, but on a smaller scale there were notable declines in tree growth. Forest productivity has been influenced by changes in the last 55 years to temperatures, precipitation, sunlight and carbon dioxide, among other factors.

Temperate and boreal forests of Canada, Europe and northern Asia are where vegetation growth rates have increased the most, according to satellite images and field studies. Even there, exceptions exist at regional levels. For instance, aspen stands along the northern edge of Canada's prairies slowed their growth by 50 percent several times between 1951 and 2000 in response to climate change and insect attacks. In northern Europe, Norway spruce trees were suppressed by industrial pollution. Elsewhere in Europe, in areas where water limits growth, tree productivity declined as temperatures increased.

Where climate change has most slowed forest productivity, though, is in tropical forests. When the El Niño/Southern Oscillation (ENSO) has driven temperatures up, wide-spread spikes in tropical tree deaths have occurred. Extensive mortality of large trees was documented in Central America and Southeast Asia in conjunction with the ENSO events of 1982-83 and 1997-98.

Large-scale circulation patterns have also had a positive effect on tree growth. For instance, when the Pacific Decadal Oscillation (PDO) brings warm winters and little snow to the Pacific Northwest, growth improves for trees at higher elevations. In the same region, though, lower-elevation trees in water-stressed locations fair poorly in response to PDO-generated warm spells.