Published April 1, 2000

Forests cover 27 percent of the total land area of the Earth, and they contain 60 percent of the carbon stored in the terrestrial biosphere. For this reason, they are the subject of much study among those scientists interested in the effect of increased atmospheric carbon dioxide on the planet.

Greening Up! often features studies showing how the forests of the Earth benefit from increased atmospheric carbon dioxide (CO2). Most often, these studies are of a short duration–six weeks to one year. Those who claim that the effects of elevated CO2 could be harmful often point this out, questioning whether this atmospheric change could possibly be good in the long run. But scientific evidence provides the answer, and that answer is a resounding yes.

For example, Barton and Jarvis note that most experiments designed to investigate the response of trees to elevated atmospheric CO2 have frequently been performed on seedlings or juvenile trees in controlled environments or open-top chambers.

To more fully understand the impacts of elevated CO2 on forests, they say, experiments must be conducted on mature trees under actual field conditions. Accordingly, they selected a mature 16-year-old stand of Sitka spruce near Edinburgh, Scotland, for their research. They enclosed selected branches in bags with either natural or doubled atmospheric CO2 in an experiment that ran from May 1991 to November 1994.

Elevated CO2 doubled the rate of photosynthesis for the current-year shoots and nearly doubled the photosynthetic rate for the older shoots within the bags. Obviously, the measurements from the small bags cannot be extrapolated to the entire tree, let alone to an entire forest, but the positive biological response from the bags is certainly encouraging for the future of Sitka spruce.

In another rare long-term study, Soule and Knapp studied changes in the density and cover of western juniper trees in central Oregon from 1951 to 1994. At all sites examined, they found that the density of mature western juniper was increasing, but they could find no evidence that domestic grazing practices, fire regimes, or climate changes contributed to the increase. Soule and Knapp suggested the rise in atmospheric CO2 may have enhanced the growth of the juniper and increased its water-use efficiency, thereby contributing to the observed increased density of mature juniper.

Robert C. Balling Jr., Ph.D. is director of the Laboratory of Climatology at Arizona State University and coauthor of The Satanic Gases.


Barton, C.V.M., and P.G. Jarvis, 1999. Growth response of branches of Picea sitchensis to four years exposure to elevated atmospheric carbon dioxide concentration. New Phytologist, 144, 233-243.

Soule, P.T., and P.A. Knapp, 1999. Western juniper expansion on adjacent disturbed and near-relict sites. Journal of Range Management, 52, 525-533.