Do more trees always mean less water? This used to be the predominant scientific view, but new research paints a more nuanced picture. It appears that intermediate tree cover can boost groundwater recharge in African tropical drylands, which is great news for people living there.
Everybody loves trees for the fruit, fuel, construction material, and soil restoration they provide. Not to mention their role when it comes to carbon storage. Yet, development organizations have often advised against tree planting in the African drylands since they thought it would deprive the soil of much-needed water. “The predominant scientific view used to be that each tree would withdraw more water than it contributed, leading many to believe tree planting should be avoided in arid areas. In South Africa, schoolchildren were even sent out to uproot trees to improve the water situation,” says Ulrik Ilstedt, Associate Professor of Forest Ecology and Management at the Swedish University of Agricultural Sciences (SLU).
But in 2016, the long-held beliefs about trees and groundwater were challenged. In the article Intermediate tree cover can maximize groundwater recharge in the seasonally dry tropics, published in Nature Scientific Reports, the researchers demonstrated that, under certain conditions, trees could contribute to groundwater recharge in arid regions. Ulrik Ilstedt was one of the authors, together with colleagues from the Swedish University of Agricultural Sciences as well as from the Center for International Forestry Research (CIFOR), the CGIAR Research Program on Forests, Trees and Agroforestry, the Congolese Institute for Agronomy Research (INERA) and World Agroforestry (ICRAF).
“It will be possible to manage landscapes more wisely to protect both soils and groundwater recharge”Ulrik Ilstedt, Associate Professor, Swedish University of Agricultural Sciences (SLU)
The article was a game-changer since the results indicated that many water-scarce tropical regions could, in fact, benefit from having more trees. Research carried out in Burkina Faso shows that when a certain number of trees are present, the amount of groundwater recharge is maximized. Trees turned out to be crucial for maintaining the large pores that characterize the soil in the area and which lead water into the ground. If there were no trees, the water would just run off or evaporate. But if the trees were too many, the water they consume would override the benefits they bring to soil improvement. “It is important to recognize that this issue is very complex. We are now continuing studies to understand different types of conditions – for example, the impact of different kinds of soils, trees, and climate management systems. With that knowledge, it will be possible to manage landscapes more wisely to protect both soils and groundwater recharge,” Ulrik Ilstedt says.
The new findings do not mean that the old way of looking at the relationship between trees and groundwater is necessarily wrong, only that it is not relevant everywhere. So far, most tree-water research has been carried out in northern, temperate regions where conditions are very different to the arid tropics. It is also more common to study forests than trees that are scattered in a landscape, which for example is the case when agroforestry is practiced. The “optimal tree cover theory”, suggested by Ulrik Ilstedt and others, argues that there is an ideal level of tree coverage in the landscape in tropical drylands that maximizes groundwater recharge. If that optimal tree cover can be identified in specific contexts and improved through better management, this could mean an important breakthrough in sustainable landscape management, according to Ulrik Ilstedt: “Better landscape management can be life-changing for hundreds of millions of people in water-scarce areas. With improved access to water, women and children especially will lead healthier lives and have more time to earn money or get an education. And people everywhere can then enjoy the many benefits of trees,” he says.
This article originally appeared in Waterfront in 2020. Read full issue