By Robert C. Balling Jr., Ph.D.
Arizona State University

Mature pine forests throughout the world represent a major sink of carbon, and their survival in the future will play an important role in the planet's carbon budget. How will mature pine trees fare as the climate becomes more CO₂-rich and temperatures rise?

In one recent experiment, a team of scientists from throughout the United States exposed a mature loblolly pine forest to elevated carbon dioxide (natural + 200 ppm) for four years; there were some with sweetgum and yellow poplar mixed in as well. Hamilton and colleagues report that the elevated CO₂ increased (a) the basal area of the trees by 9.2 percent, (b) the overall growth rate by approximately 25 percent, (c) net ecosystem production by 41 percent, and (d) plant biomass by 57 percent. Those results are even more amazing given that the forest is growing on soils described by the authors as having "low nitrogen and phosphorus availability."

A team of scientists from Denmark constructed a large greenhouse to enclose an old pine forest ecosystem that included trees up to 160 years old. Rasmussen and colleagues elevated the atmospheric CO₂ concentration by 200 ppm and increased the air and soil temperatures by 3°C to 5°C. They found that the higher soil and air temperatures and elevated CO₂ did not cause any significant changes in growth rates of the old trees. With respect to effects on annual shoots, they reported that "Even though the environmental conditions were pushed hard, increasing the concentration of CO₂ by more than 50 percent and increasing the air and soil temperature by 3-5°C, no effects from the treatments could be observed." The team did find that current-year needles increased their weight by 63 percent, while the one-year-old needles increased their weight by 27 percent due to elevated temperatures and CO₂. Consider: Here is a team that selected old trees that should be far more vulnerable to environmental change than the seedlings used in hundreds of other studies dealing with elevated CO₂. Yet—in the presence of elevated CO₂—the old trees did not suffer at all when soil and air temperatures were increased 3°C to 5°C.

With carbon dioxide to aid it in its mission, the noble pine tree will do its part to improve the future biosphere.

References:

Hamilton, J.G., et al., 2002. Forest carbon balance under elevated CO₂. Oecologia, early on-line release.

Rasmussen, L., C. Beier, and A. Bergstedt, 2002. Experimental manipulations of old pine forest ecosystems to predict the potential tree growth effects of increased CO₂ and temperature in a future climate. Forest Ecology and Management, 158, 179–188.