Delayed Fall Colors a Positive Development for Forests?

Scientists have been at a loss to account for why the traditional autumnal spectacle of disheveled trees and changing colors has gotten gradually pushed back over the last few years. Some have attributed the delayed autumnal senescence to increasing global temperatures; others have attributed it to the length of day.

David F. Karnosky, a professor at Michigan Technological University, believes rising atmospheric levels of carbon dioxide may be to blame — and, perhaps surprisingly, to thank. Karnosky explains that delaying senescence may in fact be good news for forestry industries since it prolongs the trees' growing season. The extra carbon dioxide taken up in the autumn, in addition to that taken up during the growing season, would also boost their productivity.

Karnosky and a team of international researchers analyzed two years' worth of data on autumnal senescence from forests near Rhinelander, Wisconsin, and Tuscania, Italy; the forests stayed greener longer as carbon dioxide levels rose, irrespective of temperature variations. Because of the brevity of this study, however, they were not able to assess what the longterm effects on mature forests would be or whether other factors, such as higher ozone levels in the troposphere, would cancel out the extra carbon's benefits.

Past studies have demonstrated that higher levels of carbon dioxide are causing tree growth to start earlier in the spring. In the future, more longer term research projects will need to be carried out in these and other forests to determine whether this phenomenon persists and become widespread - and if it significantly affects elevated carbon dioxide levels.

Michael Smith (Veshengro), February 2008