Climate, not CO₂, may drive make-up of plant communities

Rising carbon dioxide levels tied to global warming may not directly determine the composition of plant communities. Localized climate shifts appear to play a larger role, according to a Brown-led research team’s report in this week’s Science.

PROVIDENCE, R.I. — The types of plants that dominate a particular landscape may depend more on localized climate changes, particularly the availability of moisture, than on the concentration of carbon dioxide in the atmosphere, says a new study in Science.

Scientists have debated whether falling concentrations of carbon dioxide fueled the flourishing of tropical and subtropical grasslands worldwide that occurred about 7 to 8 million years ago, during the Miocene epoch and, with carbon dioxide concentration rising today, whether carbon dioxide will be the primary factor for potentially reshaping plant communities.

But a new study of carbon isotope ratios in compounds specific to land vegetation in Mexico and Central America since the height of the last glacial period 21,000 years ago shows that climate change, particularly drought, exerts the strongest control over the relative abundance of grasslands compared to tree-and-shrub communities.

Carbon dioxide concentration has always been an important factor affecting vegetation. But climate change, particularly drought, appears to be a key driver for vegetation changes, said lead researcher Yongsong Huang, assistant professor of geological sciences at Brown University.

Huang and colleagues say that in the absence of favorable moisture and temperature conditions, a low carbon dioxide concentration alone is insufficient to propel the expansion of certain grasses. These grasses are called C4 plants, named for the pathway that defines their process of photosynthesis. Compared to other plants, grasses and sedges use lower levels of atmospheric carbon dioxide more efficiently during photosynthesis.

“The study suggests that if the climate gets drier worldwide today, then we may see more C4 grasslands appear,” Huang said. This would occur even though carbon dioxide concentration is on the rise, because of the high water-use efficiency of C4 plants, he said.

Huang and colleagues tried to establish what might have caused the expansion of grasslands 21,000 years ago at the zenith of the last ice age, when carbon dioxide was relatively low. Similar falling concentrations of carbon dioxide levels occurred during the Miocene epoch.

The researchers studied sediments from two different low-altitude lakes that have experienced the same increasing levels of carbon dioxide but have had contrasting changes in precipitation since the last ice age, about 13,000 to 27,000 years ago. One lake is in northern Mexico and the other is in northern Guatemala.

Huang measured carbon isotope composition of leaf-wax hydrocarbons isolated from sediments. The hydrocarbons were derived from the leaves of grasses, trees and shrubs that grew around the lakes. Trees and shrubs are called C3 plants based on their photosynthetic mechanism. C3 and C4 plants used unique pathways to process carbon dioxide, resulting in different isotope ratios. Studying the isotope ratios allowed the researchers to describe the relative abundance of vegetation types around the lakes in the past.

The wet conditions around Lake Alta Babicora in Mexico harbored abundant trees and shrubs during the last ice age. When the climate dried after the ice age ended, grasses flourished.

In contrast, the abundance of trees and shrubs increased around Lake Quexil in Guatemala, from the last glacial period when the general climate was drier, to today, called the Holocene epoch, when climate has been wetter.

The results raise many questions about what may happen to flora and fauna from global warming, which is fueled primarily by increased atmospheric carbon dioxide, Huang said. Accurate forecasts of future vegetation changes must take into account predicted changes in temperature and precipitation patterns, as well as carbon dioxide concentrations, he said.

The researchers note that the evolution of C4 grasses capable of efficient use of lower concentrations of carbon dioxide probably resulted from the gas’s decline in the atmosphere during the last 100 million years. Atmospheric concentrations during the last 15 million years were “sufficiently low to create conditions that generally, but not decisively” favored C4 grasses.

Large-scale expansion of C4 grasslands, however, was triggered mostly by major changes in precipitation and temperature, they concluded.

The study’s other researchers are F.A. Street-Perrott, University of Wales Swansea; S.E. Metcalfe, University of Edinburgh; M. Brenner, University of Florida; and K.H. Freeman, Pennsylvania State University.

The research was funded by grants from the National Science Foundation.