In addition to global warming, carbon dioxide (CO2) has fueled heated debate among scientists over how rising levels of the gas will change the world's vegetation. Some experts argue that CO2 concentrations hold tremendous sway over which plants thrive and which ones fail. But new research in today's issue of Science suggests that local climate changeparticularly the occurrence of droughtmay have a far greater influence on the makeup of plant communities. "Nobody really knows what the increases in CO2 are going to entail in terms of future changes in vegetation types," co-author Mark Brenner of the University of Florida says. "But it looks like climate changes in different areas may be more important than CO2, at least CO2 by itself."

To reach this conclusion, the scientistsa team of six geologists and geographers led by Yongsong Huang of Brown Universityanalyzed sediments from the bottoms of two Central American lakes: Lake Alta Babicora in Northern Mexico and Lake Quexil in Northern Guatemala. They measured the amounts of carbon isotopes in plant-leaf wax found in the sediments and from that determined what types of plants had lived in the two areas during the past 27,000 years. In particular, they looked at so-called C3 plants, a group that includes most tropical shrubs and trees, and C4 plants, such as tropical grasses.

The group discovered that whereas abundant trees and shrubs had given way to grasses in Mexico, the reverse had occurred in Guatemala. Moreover, these trends reflected shifts in rainfall in the two areas and not CO2 concentrations. Over the millennia studied, Mexico had become drier, while Guatemala grew moister. "The result," Brenner says, "appears to be that climate factors, especially moisture availability, determine whether C3 or C4 plants dominate in an area, not CO2."