It is not often that a study completely upends a prevailing view, and, in doing so, offers hope of improving the lives of hundreds of millions of people.
But that is exactly what research recently published in Scientific Reports has done for the understanding of trees and water in dry regions.
In arid places where water is scarce, the planting of trees is often discouraged out of the belief that trees always reduce the availability of much-needed water.
Yet scientists working in Burkina Faso found that when a certain number of trees are present, the amount of groundwater recharge is actually maximized.
The study is a “game changer”, according to one of the study’s authors, Douglas Sheil, professor at the Norwegian University of Life Sciences and a senior research associate with the Center for International Forestry Research (CIFOR).
“We don’t get so many scientific studies in our lives where we see such a potential shift in how we do something,” Sheil said.
“It is very dramatic in the sense that it totally overturns the way we had looked at trees and water availability.”
Previously, few studies had examined tree cover in the tropics or what effect scattered, or intermediate, tree cover might have on water yields.
Drawing on the idea that trees can improve the movement of water in soil, the scientists worked with an ‘optimum tree cover theory’ that would provide for the maximum amount of groundwater recharge.
The research bridges two contrasting views on forests and water: the ‘trade-off theory’ and the ‘sponge theory’, explained Aida Bargués Tobella of the Swedish University of Agricultural Sciences, one of the study’s lead authors.
The ‘sponge theory’ holds that forests soak up water during the rainy season and slowly release it during the dry season, thereby sustaining stream flow during dry periods, whereas the ‘trade-off theory’—which has become the dominant paradigm—holds that more trees equals less water.
“Both perceptions are true to some extent, but what we show is that the net effect of trees on groundwater recharge depends on the degree of tree cover,” Bargués Tobella said.
“So trees can improve groundwater recharge to a point.”
By testing groundwater levels both near and far from trees in a typical semi-arid landscape over several years, the researchers found that an intermediate amount of tree cover created conditions in which more water was available than if there were no trees or a large number of them.
“Without trees, these sensitive tropical soils lose their large pores, which are responsible for leading water down into the ground quickly,” said Ulrik Ilstedt of the Swedish University of Agricultural Sciences (SLU), the study’s other lead author.
“Without these pores, the water flows away on the soil surface or is trapped in the compact soil surface and evaporates.”
“With that said, if there are too many trees, they will still consume more water than what is gained by their soil improvement.”
Other factors that also affect water availability include tree species, soil quality and type, and climate.
This study was done on a type of soil that is widespread in the tropics, but there are other types of less sensitive soils that would not have produced the same positive effects, according to Ilstedt.
However, some 70 percent of the semi-arid tropics have soils similar to those used for this research, he added.
“The most important point of our study is to show that a trade-off between water and tree cover doesn’t always exist, and that more trees can actually improve groundwater recharge,” Bargués Tobella said.
“This means that people could benefit from the many goods and services that trees provide while also seeing improved water availability.”
And with 340 million people in Africa lacking access to adequate and hygienic sources of water, that’s a lot of benefits.
The benefits of trees in the seasonally dry tropics for people in their daily lives are myriad and varied.
In particular, in the study area of Saponé in Burkina Faso, Shea trees dominate: the more Shea trees there are, the more nuts local people can sell.
Trees also support erosion control and climate change mitigation.
“With greater tree cover, there are also benefits like biodiversity, carbon and wood fuel that were being denied before,” Sheil said.
“Large areas of the arid tropics actually have no tree cover, and having more trees would be advantageous, as it would give people more access to fuelwood, fruit and many other benefits.”
The findings from the study enable people to control and manage such conditions by planting more trees, Sheil added, noting that it presented an opportunity for donor organizations to start working to support land management that facilitated the planting of trees in water-deprived areas.
For instance, this research is extremely relevant for ongoing tree-based restoration efforts in the Sahel region, such as the Great Green Wall Initiative or the African Forest Landscape Restoration Initiative (AFR100).
Goal Number Six of the recently agreed upon Sustainable Development Goals (SDGs) is to increase access to clean water, with the recognition that water is a basic human requirement.
Landscapes such as those studied in Burkina Faso house some of the world’s poorest people, where, as the study notes, limited water not only constrains food production, nutrition and health, but also reduces opportunities for education, work and improved livelihoods.
The finding that increased tree cover in tropical dry regions could increase people’s access to water could therefore have a major impact on their lives, the researchers believe.
“The study needs to repeated in other sites as the optimal tree cover will vary with conditions, and with the species involved, but there is no good reason not to expect similar results in other parts of the tropics,” Sheil said. “I think this will have global significance.”