If you thought planting trees would take care of global warming, think again. The results of a new study, which looked at how increased carbon dioxide concentrations influence forest growth, are not as promising as some had expected. In the past, some people have argued that the increase in carbon dioxide (CO2) in the air would be partially offset by an increase in plant growth, caused by that additional (CO2): increased CO2 concentrations in the atmosphere should work like extra fertilizer and lead to increased plant growth. This growth in turn should bind to much of the CO2. In other words, the plant growth should act like a sink, absorbing the gas released into the air by burning fossil fuel.

But the new analysis, published in last week's issue of Science, found that although there has been an increase in biomass, most of it must be attributed to land use history. The authors, a team of scientists from Princeton University and the U.S.D.A. Forest Service, uncovered plant growth rates of only 2 to 4.4 percent. These numbers stand in sharp contrast to some earlier studies, which suggested that rising CO2 concentrations would bring a 25 to 75 percent growth increase. The researchers used data from the Forest Inventory and Analysis (FIA) database, taking samples from more than 20,000 acre-size plots in Minnesota, Michigan, Virginia, North Carolina and Florida. To examine historical changes in growth and mortality rates of the vegetation there, the scientists looked at forest biomass, the cumulative result of past growth. "The U.S. has a fairly unique history in that a hundred years ago, a large fraction of the landscape was deforested," explains John Caspersen, who led the study. "Subsequently, there has been a reforestation of much of the eastern U.S."

It is this reforestation to which Caspersen and his colleagues attribute most of the increase in biomass--a finding with far-reaching implications: "If the reason that forests are taking up CO2 right now is due to land use history, then that sink will not last indefinitely," Caspersen says. Trees only absorb large quantities of carbon during their growth, so once they are fully grown--in other words, when the reforestation is complete--they will not be able to serve as CO2 sinks. "It's important to realize that when the forests have recovered," Caspersen adds, "we have only gotten back to ground zero in terms of the carbon balance in the atmosphere."