Tropical vegetation benefits less from elevated atmospheric CO2 than expected

Carbon dioxide is known to have a fertilizing effect on plant growth, and the gas is often added to greenhouse crops to help boost yields.

Climatologists have suggested that this same CO2 fertilization effect – acting on a planetary scale rather than a greenhouse scale – could help offset global warming by promoting the growth of carbon-storing trees and shrubs. released by burning fossil fuels.

It has also been suggested that this effect is particularly pronounced in tropical ecosystems, where high levels of carbon dioxide would promote the spread of woody cover (trees and shrubs) over grasslands that store less carbon.

But this highly influential concept of “tropical greening” due to elevated anthropogenic carbon dioxide levels is difficult to test, and the idea has recently been challenged by results from long-term observational and experimental field studies.

Now, a study of the evolution of vegetation in tropical West Africa over the past 500,000 years reveals that atmospheric carbon dioxide levels have had a surprisingly small effect on the extent of tree cover.

Availability of moisture was found to be the most important factor in tree cover, and the frequency of forest fires was also an important factor, according to the study, which is scheduled to be published online May 5 in the Science magazine.

“Put simply, we’ve shown that the amount of CO2 in the atmosphere doesn’t matter if there’s not enough water, or if there are fires every year, or if all the seedlings are eaten by the animals,” said the study’s lead author, William Gosling. the Institute for Biodiversity and Ecosystem Dynamics of the University of Amsterdam.

University of Michigan climatologist Jonathan Overpeck is co-author.

“The benefits of increased CO2 are much smaller than originally thought, but many climate-vegetation interaction models still overestimate the influences of carbon dioxide and therefore underestimate the impacts that continued climate change will have on vegetation,” said Overpeck, dean of the UM School for Environment and Sustainability.

“Our paper is a nail in the coffin of the CO2-dominant hypothesis and will hopefully lead to more realistic models,” he said. “The idea that you can just plant trees to sequester carbon, and that those trees will be immune to a hotter, drier climate thanks to high levels of atmospheric CO2, that’s not a sure bet. .”

In their study, researchers from the Netherlands, the United States and the United Kingdom explored the relationship between tropical tree cover, atmospheric carbon dioxide concentrations, and five other variables known to influence tree cover: fire activity, density of herbivorous mammals, availability of moisture, temperature and temperature. seasonality of temperature at Lake Bosumtwi in Ghana over the past 500,000 years.

They used a combination of observed and simulated data obtained from lake sediment cores, ice core records and a climate model.

Scanning electron microscope image of numerous grass pollen grains (Poaceae Aristida sp.) from a sediment core from Lake Bosumtwi (Ghana). Image credit: Caixia Wei

Lake Bosumtwi was created by a meteorite impact around 1 million years ago, and the crater lake has been filling with sediment since then. Lake sediments provided pollen grains, charcoal fragments, fungal spores, and nitrogen isotope information that helped researchers piece together a record of past land cover and the factors that influenced it. have influenced.

Atmospheric carbon dioxide concentrations were obtained from ice core recordings, and the Earth system model GENIE-1 (climate-carbon cycle) provided information on temperature and temperature seasonality.

“We combined evidence of past environmental change extracted from sediments recovered from Lake Bosumtwi in Ghana with simulated past climate conditions and the record of global atmospheric CO2 change obtained from ice cores over the past 500,000 years” , Gosling said.

The model allowed researchers to explore the relative importance of the six drivers of vegetation change. It showed that moisture availability and fire activity were the most important factors in determining tree cover change in rainforest-savannah transitions, while the effect of carbon dioxide was weak.

“What’s new in our data is that when you go to the landscape scale, the resources available and the processes that happen in that landscape are more important than CO2 fertilization,” Gosling said.

The authors say their findings have important implications for global vegetation models and Earth system models that currently include a CO2 fertilization effect on vegetation but often do not include factors such as fire or herbivory. . More research needs to be done to further improve these models, they said.

“Efforts that promote carbon sequestration in tropical vegetation must carefully consider the roles of moisture, fire and herbivory if they are to be successful,” Gosling said.

He also noted that the research team used only one study site, and “it is possible that the relationship between CO2 and vegetation elsewhere in the tropics is different.”

Besides Gosling and Overpeck, the authors of the study are Charlotte Miller, Timothy Shanahan, Philip Holden and Frank van Langevelde.

Past data on Lake Bosumtwi’s environmental changes used in the study was funded by the NERC/The Open University Charter Scholarship and the NERC New Investigator Award. The authors thank the Lake Bosumtwi Drilling Project team, the International Continental Drilling Program, and the US National Science Foundation for supporting the acquisition of the Bosumtwi sediment cores and age model data.

Amber Heijboer of the University of Amsterdam contributed to this report.

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