Africa Water, Sanitation & Hygiene

Ecology & Research More rain leads to fewer trees in the African savanna income,” said first author Xiangtao Xu, a graduate student in the laboratory of second author David Medvigy, a Princeton assistant professor of geosciences. “Trees and grasses are competing over the amount of money the savanna gets every year and it matters how they use their funds.” Xu explained that when the “bank” is full with rain, grasses, which build relatively cheap structures, thrive. When there is a deficit of rain, the trees suffer less than grasses and therefore win out. The problem is that several high-profile papers over the past decade have predicted that periods of intense rainfall will become more frequent around the globe, especially in tropical areas, Xu said. The Princeton research suggests that these global climate changes will eventually lead to a reduced abundance of trees on the savannas. Single camel-thorn acacia on the savannah in Etosha National Park. The Etosha Pan in in the background. Princeton University researchers might have finally provided a solution to the ecological riddle of why tree abundance on Africa’s grassy savannas diminishes in response to heavy rainfall despite scientists’ expectations to the contrary. The researchers found that the ability of grasses to more efficiently absorb and process water gives them an advantage over trees such as the acacia. Credit: © marietf / Fotolia In 2011, satellite images of the African savannas revealed a mystery: these rolling grasslands, with their heavy rainfalls and spells of drought, were home to significantly fewer trees than researchers had expected. Scientists supposed that the ecosystem’s high annual precipitation would result in greater tree growth. Yet a 2011 study found that the more instances of heavy rainfall a savanna received, the fewer trees it had. To this ecological riddle, Princeton University researchers might have finally provided a solution. In a study published in the Proceeding of the National Academy of Sciences, researchers use mathematical equations to show that physiological differences between trees and grasses are enough to explain the curious phenomenon. The researchers found that under very wet conditions, grasses have an advantage because they can quickly absorb water and support high rates of photosynthesis, the process by which plants convert sunlight into energy. Trees, with their tougher leaves and roots, are able to survive better in dry periods because of their ability to withstand water stress. But this amounts to a disadvantage for trees in periods of intense rainfall, as they are comparatively less effective at utilizing the newly abundant water. “A simple way to view this is to think of rainfall as annual “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 said. “It’s important to understand how the biome would change under global climate change.” The study highlights the importance of understanding the pattern and intensity of rainfall, not just the total annual precipitation, which is where most research in this area has focused, Xu said. In 50 years, a region may still experience the same overall amount of precipitation. If the intensity changes, however, that will affect the abundance of grasses and trees. This, in turn, will influence the herbivores that subsist on them, and other animals in the biome -- essentially, affecting the entire ecosystem. Xu, Medvigy and co-author Ignacio Rodriguez-Iturbe, Princeton’s James S. McDonnell Distinguished University Professor of Civil and Environmental Engineering, created a numerical model that mimicked the actual mechanistic functions of the trees and grasses. They put in equations for how both plants photosynthesize, absorb water and even steal water from one other. These equations were coupled with a random rainfall generator based on rainfall parameters derived from field observations across the savanna. This configuration allowed the team to observe how the plants would respond under different 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 said. Daily rainfall intensity determines who will win in a competition between grasses and trees for the finite resource of water. Africa Water, Sanitation & Hygiene • November - December 2015 25

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