In a world facing climate change and increased pressure on land and natural resources, water availability is one of the major challenges for a sustainable future. More than 700 billion people on the planet still lack access to safe drinking water and nearly half of them live in Sub-Saharan Africa. Water scarcity also poses challenges to agricultural production, and being the main source of income for the worlds rural poor, it pinpoints water availability as key for realizing livelihood opportunities.

The target of increasing groundwater recharge and what kind of land use that best promotes water availability is one that has heckled scientists for years. The dominant paradigm in forest hydrology today is that increased tree cover in fact leads to reduced water yields, mainly due to water losses to the atmosphere in the form of higher evapotranspiration from trees compared to crops or grass. Meanwhile, large-scale tree-based restoration projects are being promoted in Africa and causes concern that these might jeopardize precious water resources.

Aida Bargués Tobella, researcher at the Swedish University for Agricultural Sciences (SLU) and member of the Focali Research Network, challenged this dominant paradigm with the optimum tree cover theory in her recently defended doctoral thesis The Importance of tree cover for water resources in semiarid Africa. The optimum tree cover theory suggests that, in the seasonally dry tropics, groundwater recharge can actually be maximized at an intermediate, non-zero, tree cover. Trees have both negative and positive influences on groundwater recharge, such as increased evapotranspiration where water is lost to the atmosphere, and enhanced infiltration capacity where the water stays in the ground. The balance between these negative and positive influences will change under different levels of tree cover. “More trees can result in more groundwater recharge if the trees’ positive influences exceed the negative ones” said Aida when presenting the theory during the event ‘Rainfall recycling’ as a landscape function at the Global Landscape Forum in Bonn 2017. 

Landscape of Sahel, Africa. Photo by Daniel Tiveau for CIFOR via Flickr (CC BY-NC-ND 2.0)

Aida means that the scientific evidence for the dominant paradigm has many limitations. Firstly, the research conducted is strongly biased towards humid temperate areas with scarce contributions of research in the tropics or semi-arid areas. Secondly, most studies have compared extremes – open land versus closed forest – and thereby neglected areas with intermediate levels of tree cover. She argues that sound conclusions concerning the net impact of tree cover on water yields cannot be drawn from current evidence. “In the tropics in general, the knowledge on the hydrological impacts of trees and how these relate to tree cover, tree species, tree spatial configurations, etc. is very thin” Aida explains. “We need to understand the specific conditions under which more trees can lead to improved water yields in tropical African drylands, because if land use policies are based on the current paradigm that more trees give less water there is a risk that tree-based restoration is discouraged.” she continues, when explaining why investigating the optimum tree cover has such a large importance.

Forests and trees provide, regulate and support a wide range of ecosystem services, and tree-based restoration has therefore been promoted as a tool to enhance the capacity of landscapes to deliver such services. More than half of the land in Africa suffers from degradation and smallholder farmers and households are largely affected as their activities depend on stable weather patterns, healthy soils, tree cover, and water. “If tree-based restoration is discouraged, this might have negative consequences for the environment and people’s livelihood in these regions” Aida further explains.

Irrigation project in Mozambique. Photo by Marcos Villalta for DFID via Flickr (CC BY-NC-ND 2.0)

Through the Formas mobility starting grant and the Swedish Research Council project grant for development research, Aida and her colleague and previous supervisor Ulrik Ilstedt (SLU, Focali researcher) have received about 7 million SEK to continue investigating the optimum tree cover theory, now focusing on the tropical drylands in Sub-Saharan Africa. The project attempts to fill some of the knowledge gaps on the hydrological impacts of trees and will thus be highly relevant for successful implementation of tree-based restoration initiatives in water scarce African drylands.

So far, there is evidence for the optimum tree cover theory from a site in Burkina Faso, but Aida and Ulrik believe that this will be widespread over many areas in the seasonally dry tropics and be of great value for dealing with water scarcity in these areas. Their research could thus contribute to improving the well-being of billions of people affected by water stress in these regions. “Understanding the net impact of changes in tree cover on water yields is key to formulate solid strategies and policies aimed at improving water availability. This can in turn improve the well-being of people in water-limited regions that are so dependent on water availability for their livelihoods.” Aida concludes.

Aida’s Formas mobility grant is for a period of four years. During the two and a half first years she will be based at the World Agroforestry Centre (ICRAF) headquarters in Nairobi. During the last part of her project, Aida will be based at SLU Umeå, where she will work within Associate Professor Ulrik Ilstedt’s research group on Tropical Forestry and Land Use Management at the Department of Forest Ecology of Management.