U. researchers find correlation between Savanna rainfall, trees

More rainfall on the African savanna could lead to fewer trees, according to a recent study conducted by University researchers published in the Proceeding of the National Academy of Sciences.

Authors for the study were David Medvigy, an assistant professor of geosciences, and Xiangtao Xu GS, one of his doctoral candidates. Ignacio Rodríguez-Iturbe, a professor in the Civil and Environmental Engineering department at the University, co-authored the study.

This strange phenomenon of a negative correlation between rainfall and trees in the region can be partially explained by grass’s relationship to rain, Xu said. In an environment with heavy rainfall, grasses are better able to absorb the moisture and use it to support photosynthesis.

Trees, though, are better suited for drier environments, where roots and leaves aren’t susceptible to heavy rains, he said. When these trees are exposed to long, heavy periods of rainfall, they have difficulty taking advantage of the newly-supplied water — especially if there isn’t plentiful grass to help in the absorption.

Xu explained that the idea came after a series of brainstorming sessions regarding topics on plants’ water use. Due to the scarcity of rain that it receives, the African savanna seemed to be the most promising environment from which to gather data on rainfall and how plants use it, he said.

He explained that he, Medvigy and Rodríguez-Iturbe used a numerical model to illustrate the differences in water-use strategies between grasses and trees. This model, and a rainfall simulator, allowed the researchers to ensure their theories were accurate in reflecting the reality of the African savanna.

“It’s just because [grasses and trees] are in competition,” Xu said. “It’s just because if we just plant trees you can have a very good prediction, if we just plant grasses we can have a very good prediction — but if you put them together, they’re competing with each other.”

Xu said that fewer trees on the savanna is not necessarily a problem for the ecosystem, but that it could affect the composition of the savanna over time. He added that this change is analogous to how the industries and cultures in a city can change over time.

Although grasses may benefit from more rainfall in this prediction, it is hard to determine whether or not this is a good scenario, since trees would likely not reap similar benefits, Xu said.

He noted that while the study is not directly connected with climate change, climate change is one of the reasons why researchers conduct these kinds of studies.

“We know that in the future, rainfall is going to become more extreme,” Xu explained. “If it did become more extreme, what would happen?”

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Gabriel Katul, a professor of hydrology and micrometeorology at Duke University, said casino online that the study is relevant because rainfall may be increasing due to climate change. He added that the study focused on how climate could alter the vegetation composition of the savanna, particularly the composition between grasses and trees, rather than how the savanna may be a reflection of overall climate change.

Xu said that the study cannot determine how the lives of those in the region may be affected, but can be a reference point for experts to make public policy decisions.

“This is how different disciplines should operate,” he added.

Xu noted that, while the study provided one sufficient explanation for the phenomenon of fewer trees on the savanna, there could be many more. He said that it would be interesting to analyze how any overlooked physiological differences among trees and grasses might influence their competition, and that it would be intriguing to create a more complex model that included factors like temperature and radiation changes, in addition to rainfall variations.

Titled “Relation between rainfall intensity and savanna tree abundance explained by water use strategies,” the paper was published Oct. 5 and was supported by the Princeton Environmental Institute and the Andlinger Center for Energy and the Environment.

Xu noted that both institutions supported the research financially and helped engineers communicate with researchers.

The Princeton Environmental Institute and the Andlinger Center for Energy and the Environment declined to comment.