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 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.
“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.”
Continue reading at Princeton University.
African savanna image via Shutterstock.