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Rising global carbon dioxide levels tied to global warming may not be as crucial in determining the composition of plant communities as other, localized climate changes.

“Nobody really knows what the increases in carbon dioxide are going to entail in terms of future changes in vegetation types, ” said Mark Brenner, a University of Florida assistant professor of paleolimnology, the study of ancient lakes. “It looks like climate changes in different areas may be more important than carbon dioxide, at least carbon dioxide by itself, ” he said.

Brenner’ s research team based their conclusions on an analysis of sediment from two lake bottoms, one in northern Mexico and one in northern Guatemala. The researchers used new techniques that allowed them to analyze only the remains of land plants, specifically their leaf waxes. By measuring the composition of the leaf waxes, the researchers were able to distinguish two broad categories of plants living in these areas — so-called C3 and C4 plants, which have different photosynthetic processes. Many C4 plants are tropical grasses, while most tropical trees are C3 plants. The researchers analyzed sediments deposited over the last 27, 000 years, from the last ice age to the current geological period. Over this period, there was a worldwide, relatively uniform increase in atmospheric carbon dioxide concentrations.

Brenner said that if carbon dioxide played the major role in determining plant composition, one would assume that analysis of the sediments would reveal very similar changes in relative abundance of C3 and C4 plants in the two places over the study period. But, in fact, the researchers found that trends in the two types of plants were different at the two locations. The changes were related not with carbon dioxide levels, but with shifts in rainfall. “The result appears to be that climate factors, especially moisture availability, determine whether C4 or C3 plants dominate in an area, not carbon dioxide, ” Brenner said.

Many scientists believe global warming will cause major variation in local climates worldwide, with some wet areas becoming dry and dry areas becoming wet. If that happens, it could have more impact on relative C3 versus C4 plant distribution than the rising carbon dioxide levels.