Trees function as water harvesters in African drylands
Tree cover works well for water retention in African drylands – but the right tree density is needed for best results, and there are pitfalls in extrapolating findings from temperate regions to tropical ones.
Author: Daisy Ouya. Source: Agroforestry World
Native trees that dot African dryland areas bring a welcome respite from the tropical sun. In addition, and contrary to old assumptions, they “… may function as water harvesters, contributing to deeper drainage and recharge.” They might thereby help recharge groundwater bodies.
These findings by researchers from the Swedish University of Agricultural Sciences (SLU) and World Agroforestry Centre (ICRAF), recently published in the journal Water Resources Research, refute the commonly held notion that trees in drylands worsen water scarcity.
With funding support from the Swedish International Development Cooperation Agency (Sida), Aida Bargués Tobella of SLU’s Department of Forest Ecology and Management worked with colleagues from ICRAF and farmers in Burkina Faso, to study the effect of sparse trees on the flow of water through the soil. The study was done in Saponé, 30 km south of Ouagadougou, in an agroforestry parkland—a land use system in which scattered multipurpose trees are deliberately preserved on land also used for crops and/or grazing.
The researchers used water sprinklers to simulate a 45mm/hour rainfall in three types of experimental sites: under Vitellaria paradoxa (shea) trees (6 sites), under shea trees with a termite mound (6 sites), and in open areas among trees (6 sites); the open areas ranged in size, in order to study the effect of tree density. They followed the journey of the water from the point at which it entered the soil to a 50-cm depth.
By capturing surface runoff from the simulated rainfall, the researchers were able to work out how much of the ‘rainwater’ entered the ground, and thus assess soil infiltrability. Soil infiltrability measures the rate at which water may enter the ground, and makes it possible to determine the proportion of water that infiltrates the soil vs. that which runs off on the surface.
They then poured a non-toxic dye solution into the wet ground; after this had seeped through they photographed soil sections, to investigate preferential flow. Preferential flow indicates where the water goes once it enters the ground; specifically, how much of it seeps into the soil (dye-stained areas) and how much is fast-tracked through preferential channels—similar to VIP fast-track lanes on queues—to eventually reach the groundwater table. High values for both parameters mean better possibility for groundwater recharge.The results showed that soil infiltrability was nearly five times higher under single shea trees (39 mm/h) than in open areas (8 mm/h). This means that in areas around trees, more of the rainwater infiltrated the ground than was lost to surface runoff. Infiltrability for soils with a tree and a termite mound was midway between the two, at 10 mm/h.
In soils with a tree and a termite mound, the degree of preferential flow was 77%, double that for soils with a tree alone (38%) and nearly triple that for open areas (27%).
These results, say the researchers, point to the important role of trees as rainwater harvesters.
Trees are thought to improve preferential flow by forming large open areas in the soil (macropores) through their roots and associated soil fauna, which serve as spaces for water to pass. Under trees associated with termite mounds, preferential flow is helped further by below-ground termite galleries, which fast-track water into the deep layers of the ground.Besides their role as water harvesters, the researchers point out that trees in drylands reduce evaporation losses and soil erosion. As more water seeps into the ground around trees and it does it faster, less water is available on the surface to be lost through evaporation; and by reducing the amount of surface water runoff, the trees reduce soil erosion.
The researchers say their findings clearly show the pitfalls of extrapolating results from temperate areas to arid tropical ones: the long-held assumption that trees in African drylands would have a drying effect on groundwater have been based on findings from temperate regions, comparing the effect on watersheds of open land versus dense plantations of fast-growing species. Based on these results, people have expected that trees in drylands would behave in the same way as temperate trees. This has led to mixed policy direction regarding trees in drylands: In some cases tree planting has been discouraged outright, while in others, farmer-managed natural regeneration of trees has been actively encouraged, for the many benefits trees bring to people.
Jules Bayala, a scientist at ICRAF’s Sahel node and collaborator in the research, says trees bring myriad products and services to people in drylands. For instance, Shea trees, around which the research in Burkina Faso was carried out, are “a lifeline for people and the environment throughout the Sahel.” Beyond their environmental attributes such as erosion control and soil improvement, shea trees give the world-famous shea butter, extracted from the nuts and widely used as an affordable vegetable fat throughout the Sahel. Shea butter is an important export commodity, too, bringing in cash income to people and countries. The tree provides numerous other products, from fruit, nectar, and pollen for bees, fodder, firewood, timber, and natural dyes.
“Our findings that shea trees can have beneficial impacts on water availability provides additional good evidence for policy-making on land use in drylands,” says Bargués Tobella.
Water constrains the livelihoods of millions of people in arid, semiarid and dry subhumid regions, which cover around 35% or the global land area and are home to more than a third of the world’s population, pointing to the importance of evidence-based policies for drylands.
Bargués Tobella warns, however, that the overall effect of tree planting on water resources in drylands will be strongly dependent on tree density.
“Tree density and the spatial distribution of trees are key variables that affect soil hydraulic properties,” she says. “These warrant further studies to get the right balance.”
She explains: “Trees transpire water which is then lost to the atmosphere. So at the end what we have is a balance between positive (higher infiltrability and preferential flow) and negative (higher transpiration) effects of trees on groundwater recharge.”
Bargués Tobella, A.,H. Reese, A. Almaw, J. Bayala, A. Malmer, H. Laudon, and U. Ilstedt (2014), The effect of trees on preferential flow and soil infiltrability in an agroforestry parkland in semiarid Burkina Faso, Water Resour. Res., 50, 3342–3354, doi:10.1002/2013WR015197