California Trees Not Keeping Up with CO2

Forests aren’t absorbing as much carbon dioxide as in the past. Fire suppression might be to blame

Fire suppression in forest encourages the growth of smaller trees and, as a result, significantly reduces a forest's overall ability to store carbon, according to a new study by scientists at the University of California at Irvine. The researchers, studying forests in California, found that while the number of trees per acre increased in the sixty year period between 1930 and 1990, carbon storage actually declined about 26 percent.

This change in the nature of the forests, with greater numbers of smaller trees at the expense of large trees, seems to have been caused by the assiduous suppression of fires by human intervention, the researchers said.

Using detailed records, the scientists, compare forests as they were in the 1930s with forests in the 1990s and found that the "stem density" of the forests had increased, which would seem to enhance a forest's ability to store carbon. In fact, the smaller-tree factor outweighs the denser-forest factor because large trees retain a disproportionate amount of carbon, the researchers concluded.

Climate change, or at least the vast increase in carbon dioxide launched into the atmosphere by the combustion of fossil fuels over during the industrial era, has focused scientific attention on the ability of plants, especially trees, to take up and store the added CO 2. Trees are not the only carbon sinks (the oceans store vast amounts of CO 2), but they are often cited as a key indicator in the fight to stabilize the buildup of greenhouse gases in the atmosphere.

This study, published last month in the journal Geophysical Research Letters, pertains to California only, but Aaron Fellows, one of the study's authors, believes it will apply to other dry conifer (evergreen) forests in the U.S. western region.

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