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Climate Change and Tree Growth: Increased Rainfall Can Actually Reduce Tree Growth In the African Savanna, Researchers Say

Oct 21, 2015 02:06 PM EDT
Acacia Tree
Princeton University researchers found that the ability of grasses to more efficiently absorb and process water gives them an advantage over trees such as the acacia, one of which is pictured here.
(Photo : Kev Moses)

Given heavier rainfalls regularly sweeping through the African savanna, you'd expect to see observe thriving tree populations. But these areas are actually home to significantly fewer trees. Why? Researchers from Princeton University recently set out to answer this question and found that it all has to do with putting down roots.

Trees equipped with tougher, deeper roots are better able to survive droughts but that leaves them ill equipped to drink in water during sudden and frequent intense rainfalls. That's not the case with nearby grasses which can absorb water quickly and take advantage of their slower sipping arboreal neighbors, according to a news release.

"A simple way to view this is to think of rainfall as annual income," Xiangtao Xu, first author of the study and a graduate student at Princeton University, said in the release. "Trees and grasses are competing over the amount of money the savanna gets every year and it matters how they use their funds."

Essentially, large trees are able to store more water over a long period of time and have grown accustomed to working for and spending their income more slowly and over time. This could become a disadvantage for savannas' trees, though, since climate change models estimate more intense rainfals for such tropical areas.

"Because the savanna takes up a large area, which is home to an abundance of both wild animals and livestock, this will influence many people who live in those areas," Xu added. "It's important to understand how the biome would change under global climate change."

To better estimate how climate change may impact savannas, researchers modeled rates of photosynthesis for both the grasses and trees, and the rate at which the two plants absorb water or steal water from each other. Then, they integrated random rainfall amounts based on field observations across the savanna. This allowed them to see how the plants would respond to changing climate conditions.

"Past analyses of the savanna have only considered annual or monthly rainfall, but understanding how rainfall is distributed in different areas on a daily scale is critical in the savanna," Xu explained.  "We put realistic rainfall schemes into the model, then generated corresponding grass or tree abundance, and compared the numerical results with real-world observations."

Their findings suggest that grasses ultimately win out during periods of intense rainfall. This contradicts the previous belief that the trees would be able to suck up more water with their deeper, heavier roots.

"This hypothesis ignores the fact that grasses and trees have different abilities for absorbing and utilizing water," Xu said in a statement. "And that's one of the most important parts of what we found. Grasses are more efficient at absorbing water, so in a big rainfall event, grasses win."

This study will help researchers better understand how the savannas may be affected by climate change in the future and how a change in plant abundance may impact other local organisms that rely on either the grasses or trees for food or shelter. Their findings were recently published in the Proceedings of the National Academy of Sciences.


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