ATHENS, Ga. -- The steady warming of the Earth's atmosphere, along with increased concentrations of carbon dioxide, could one day bring cataclysmic changes to the planet, some scientists believe. They have suggested global warming could cause anything from the widespread elimination of species to the melting of polar ice caps.
But new studies in USDA's Southern Global Change Program indicate there is at least one hidden advantage to increased CO2 concentrations: much better tree growth due to improved photosynthesis. Four separate groups of scientists in the South agree that managed timber stands will actually benefit from higher levels of CO2 in the atmosphere.
"What we've found so far is that pine trees are better adapted to increases in CO2 than most species, but others respond positively as well," said Dr. Bob Teskey, a professor in the University of Georgia's Warnell School of Forest Resources.
Teskey, along with colleagues in North Carolina, Alabama, Mississippi, and Oklahoma, studied the response of loblolly pines to increasing concentrations of CO2 and provided detailed and extensive measurements of photosynthesis, respiration and growth under a variety of experimental conditions.
The research will be published this spring in the book The Sustainability and Productivity of Southern Forest Ecosystems in a Changing Environment. Co-authors of the chapter outlining the research are Dr. Phillip Dougherty of the Westvaco Timber Co. of Sumter, S.C., and Dr. Robert Mickler of the U.S. Forest Service.
Teskey and his colleagues wanted to know if the benefits from elevated CO2 levels would last and if these beneficial effects might be modified by other stresses on the trees, such as increasing air temperatures. In both cases, the preliminary news is good. The researchers believe it will take hundreds of years for slowly increasing CO2 levels to reach the maximum benefits for growing pine trees. And the benefits of increased CO2 are apparently greater than harm caused by rising global temperatures.
Carbon dioxide is a natural part of the Earth's atmosphere, but it has been steadily increasing over the past 100 years, largely due to air pollution from burning fossil fuels. By measuring gases trapped in ice, researchers know that a century ago the air contained about 280 parts per million of CO2. Now, that level is up, on average, to more than 350 ppm and is climbing by about 1.5 ppm a year. While 80 percent of the increase is due to the use of fossil fuels, tropical deforestation also adds to the problem. (Oddly enough, Teskey said, cement production is responsible for about 3 percent of the atmospheric CO2 increase annually.)
Carbon dioxide is the engine that drives photosynthesis in plants, so in a sense the new findings are logical. But the researchers were surprised at the steady, increasing effect that more CO2 had on loblolly pine trees.
"There are many things we don't know about global warming," said Teskey, "such as whether or not elevated temperatures may cause shifts in precipitation patterns; however, if these patterns are the same, there is no doubt in my mind that we will see increasing production in both managed and natural stands of trees."
Teskey's own research has examined the interaction between levels of CO2 and increasing atmospheric temperatures.
While the studies focused on loblolly pines, a greenhouse experiment at the University of Georgia examined the sweetgum, and a Mississippi study measured responses in the flowering dogwood. For these species, the effect of elevated CO2 concentrations on net photosynthesis was almost always positive, but the magnitude of the response was quite variable, depending on species and growth conditions.
Other effects in pine trees included large increases in branch length, dry stem weight, total plant biomass and root growth. Leaf area was also increased, but the growth of individual needles was the same.
One surprise in the studies was the discovery that the effects of elevated CO2 on physiological processes primarily adds to, rather than interacts with, other processes.
"This is remarkable because it means tree growth in the field (on sites of high growth potential as well as sites of low growth potential) is likely to benefit from elevated CO2 concentrations in the atmosphere," said Teskey.
While the studies point toward benefits for managed stands of loblolly pines, concerns remain for native forests. Indeed, global warming could play havoc with forest ecology, leading to the loss of biological diversity among plants and trees and among the animals that live on and around them. Another worry is the attendant increase in ozone during global warming, since ozone does decrease productivity, according to Teskey. Levels of ozone in the Earth's atmosphere are more than twice what they were a century ago.
Still, the beneficial effects of higher CO2 on tree growth far outweigh the negative effects of increasing ozone, Teskey said.