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Global Ideas

Why more rain means fewer trees in the savanna

In the arid African savanna, it stands to reason that trees would thrive with increased rainfall, yet the opposite is true. Researchers have now uncovered the reason.

Water is often scarce in Africa's savannas, so it would seem logical to conclude that more rainfall would allow the few trees in the expansive grassland ecosystems to thrive. But a 2011 study revealed the opposite to be true - the more it rained in a savanna, the fewer trees grew there. Now #link:http://www.pnas.org/content/early/2015/09/29/1517382112:researchers# at Princeton University may have figured out why.

In essence, it boils down to competition. The trees want the water, but so does the grass. Conventional wisdom held that trees - with their extensive root systems that penetrate deep into the ground - would have an edge. And they do when it comes to extensive periods of drought, but not when it rains a lot.

"This hypothesis ignores the fact that grasses and trees have different abilities for absorbing and utilizing water," said lead study author Xiangtao Xu. "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."

It is therefore important to look not only at the total amount of rainfall in an area but also its patterns and intensity in order to predict how the vegetation in the ecosystem will evolve, according to the study. As a result of climate change, periods of intense rainfall are expected to increase throughout the world, so it is likely that we will see even fewer trees in the savanna in the future.

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