Local climate may be more important than carbon dioxide levels in determining what types of plants thrive and what types don't do so well, a team of scientists reports in this week's edition of the journal Science.
Their findings suggest that 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 shifts, such as droughts or temperature 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 UF assistant professor of geology and co-author of the paper, which appears in Friday's Science. "But it looks like climate changes in different areas may be more important than carbon dioxide, at least carbon dioxide by itself."
The team, composed of six geologists and geographers from around the world and led by Geologist Yongsong Huang of Brown University, based their conclusions on an analysis of sediment from two lake bottoms, one in northern Mexico and one in northern Guatemala.
The sediments came from core samples retrieved by driving a hollow tube into the lake bottom. Over time, these sediments -- which include terrestrial plant remains -- are deposited layer by layer, like a wedding cake, with the oldest layer on the bottom. Such cores provide an environmental archive that allows researchers to obtain a continuous record of changes in climate, vegetation and land use.
The cores were retrieved as long ago as 1980 by UF researchers and currently are kept in the core storage facility at UF's Land Use and Environmental Change Institute. The researchers used new techniques that allowed them to analyze only the remains of land plants, specifically their leaf waxes.
"The technology has come on line to allow us to do studies that we couldn't do at the time we collected these samples," Brenner said.
By measuring the isotope 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 roughly 27,000 years, from the last ice age to the current geological period, called the Holocene. During 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.
In fact, the researchers found that trends in C3 versus C4 plants were quite different at the two locations -- and that the changes were correlated not with carbon dioxide levels, but with shifts in rainfall.
Over the millennia of the study period, the climate shifted from wet to dry in Mexico and dry to wet in Guatemala, with corresponding shifts in the plant communities. At Lake Alta Babicora in Mexico, abundant trees and shrubs shifted to grasses. At Lake Quexil in Guatemala, the abundance of trees and shrubs increased.
"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 significant variation in local climates worldwide, with some wet areas becoming dry and vice versa. If that occurs, it could have more impact on relative C3 versus C4 plant distribution than the rising carbon dioxide levels, researchers said.
"The study suggests that if the climate gets drier worldwide today, then we may see more C4 grasslands appear," said Huang, an assistant professor of geological sciences at Brown.
The findings also contribute to an enduring mystery about why the range of C4 plants expanded 7 million to 8 million years ago. The results suggest that climate factors may have also been important in ancient Miocene C4 expansion, Brenner said.
The research was funded by a grant from the National Science Foundation. - By Aaron Hoover
[Contact: Mark Brenner, Aaron Hoover]