Dead forests release less C[O.sub.2] than expected.

• Position: Your Life

Billions of trees killed in the wake of mountain pine beetle infestations have not resulted in a large spike in carbon dioxide released into the atmosphere, contrary to predictions. Massive tree die-offs release less carbon into the atmosphere than previously thought, suggests research led by the University of Arizona, Tucson.

Across the world, trees are dying in increasing numbers, most likely in the wake of a climate changing toward drier and warmer conditions, scientists suspect. In western North America, outbreaks of mountain pine beetles (Dendroctonus ponderosae) have killed billions of trees from Mexico to Alaska over the last decade.

Given that large forested areas play crucial roles in taking carbon dioxide out of the atmosphere through photosynthesis and turning it into biomass, an important question is what happens to that stored carbon when large numbers of trees die.

"The general expectation we had was that when trees die on a large scale, it would lead to a big pulse of carbon into the atmosphere through microorganisms metabolizing all that dead wood," explains David Moore, assistant professor in the School of Natural Resources and the Environment.

Large amounts of dead trees, it turns out, hold onto their carbon for a long time and prevent it from being released quickly into the soil or the atmosphere. This might be due to several reasons: first, while trees take up carbon dioxide during the day during...