University of Texas at Austin: Tropical Forests Benefit Less from Elevated Atmospheric CO2 Than Thought

The amount of carbon dioxide (CO2) in the Earth’s atmosphere has only a small influence on changes in tropical ecosystems despite evidence of enhanced plant growth under elevated CO2 scenarios in greenhouse experiments, according to a new international study.

That means it’s unlikely that tropical forests will expand in response to rising greenhouse gas levels, an outcome that some had hoped might lead to increased CO2 storage in tropical ecosystems as carbon-rich woody plants replace grasslands, which are less adept at storing CO2.

The study was published May 5 in Science. The research team was made up of scientists from the Netherlands, United Kingdom and United States, including scientists at The University of Texas at Austin.


A drilling rig taking sediment samples from Ghana’s Lake Bosumtwi. Photo credit: T Shanahan/UT Jackson School of Geosciences.
The researchers analyzed 500,000 years of tropical vegetation change in West Africa and found that CO2 had less of an influence on tropical forest growth than water, wildfires and animal grazing did.

“When you scale up to the landscape scale, the resources available and processes occurring within that landscape are more important than the CO2 fertilization,” said the study’s lead author, William Gosling of the Institute for Biodiversity and Ecosystem Dynamics at the University of Amsterdam.

The research examined data extracted from the sediments of Ghana’s Lake Bosumtwi, a meteorite impact crater lake that formed more than 1 million years ago. By examining pollen and biological and geochemical records preserved in sediment cores, the researchers were able to reconstruct ecosystem changes in the region and compare them with published records of atmospheric carbon dioxide from ice cores and new climate model simulations of past temperature and precipitation changes.

Together, the data helped the researchers evaluate the relative importance of the six main drivers of vegetation change over the past half-million years: CO2, wildfires, plant-eating mammals, water, temperature and seasonality.

The results showed that water availability and wildfires were the most important factors when it came to increasing the coverage of woody plants and set the threshold for transitioning savanna into tropical forest. The effect of CO2 was small. Even when different methods incorporating uncertainties into the reconstructions were analyzed, the results were the same, said co-author Timothy Shanahan, a professor at the UT Jackson School of Geosciences.

“Whatever model you use, basically the dominant control on vegetation turns out to be precipitation and disturbance,” he said. “What our study shows is that CO2 plays, at least in the geologic record, a very small role.”

Most climate models used today include a CO2 fertilization effect that causes plant growth to increase with rising CO2. However, the models do not account for other key factors that control woody plant expansion into grassy savanna, such as wildfires or animal grazing.

Co-author Jonathan Overpeck, a professor at the University of Michigan, said that the study shows the importance of changing models so they take these factors into account.

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

The lake sediment record at the center of the study was extracted more than a decade ago during a National Science Foundation-funded mission led by Shanahan and Overpeck. The international team included collaborators from Syracuse University, the University of Akron, The University of Rhode Island, the University of Vienna and the University of Toronto.

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