The Heat Is Online

Forest Growth Stunted by Increased Heat, CO2

Researchers studying the effects of warming on the northern Canadian and Alaskan forests -- where temperatures have warmed by 2 degrees C. over the last 90 years -- reported in a March, 1995, article in Science that, after an initial growth spurt, the trees' growth rate flattened.

The type of forest they studied -- boreal -- is the earth’s largest terrestrial ecosystem and comprises a third of all the world’s forests. The scientists had expected that a warmer climate with its diet of enriched CO2 would increase the trees’ growth rates and expand the northern edge of the Canadian forests further into the arctic. But researchers Gordon Jacoby and Roseanne D’Arrigo of the Lamont-Doherty Earth Laboratory, who studied growth rings in trees near the timberline in northern and central Alaska, reported that while the growth rate accelerated during the 1930s and 1940s with warming weather, it has subsequently plateaued.

"[T]he warming appears to be stressing the forests by speeding moisture loss and subjecting them to more frequent insect attacks," Said Jacoby. If there’s enough moisture, he added, the warmer temperatures help these trees to grow better. But if it’s too warm, the evaporation of tree water through its leaves and needles reverses that benign effect. Warm, sunny days draw the trees’ moisture out through their leaves and needles. If there is not enough rain or soil moisture to compensate, tree growth will slow.

The scientists’ strongest worry, however, is that climate warming in Alaska will spur a dramatic increase in the population of insects. "Alaskan forests have suffered from severe outbreaks of bark beetles, which have devastated several million acres of forest. Entomologists have pointed out that warmer temperatures can halve the reproductive cycle of the bark beetle from two years to one," according to the authors of the Science article.

Citation: "Is a Warmer Climate Wilting the Forests of the North?" Science, Vol. 267, March 15, 1995; See also