Water limits to curb climate change through large scale afforestation in the tropics and impacts on water availability for food production

Tree restoration plays a key role in curbing climate change by storing carbon. However, the impacts these strategies have on water resources is still unclear. While more and more areas are afforested every year, the extent to which these trees can grab rainwater and displace it from other uses while inducing water scarcity should be further investigated. Here, we examine the hydrological limits to tree restoration in tropics and the impacts of these strategies on water availability locally and downstream. We consider tropical biomes as case study since water is the main limiting factor to plant growth there. First, we use the spatially distributed agro-hydrological model WATNEEDS to evaluate the tree water requirement in suitable and available areas for tree restoration at pixel scale. Available areas for tree restoration are shrub areas not yet occupied by agricultural purposes or urbanization. Then, we compute the hydrological balance for different land use scenarios starting from present conditions (i.e., current land use) to tree restoration with and without yield gap closure (i.e., expansion of irrigation on currently rainfed areas).

We find that more than one third of suitable and available areas for tree restoration are in zones at high water stress for the plant, where rain alone can meet just up to the 40% of plant water requirement. Here, tree restoration causes substantial increase in water scarcity conditions, especially in the Horn of Africa and in Oceania. When accounting for both tree restoration and agricultural yield gap closure, water scarcity on these areas is exacerbated. Here, more than half of suitable and available areas for tree restoration will experience water scarcity conditions for at least six months per year.

It appears clear how competition for water resources reduces the benefits of tree restoration practices as plant growth is overall constrained by the lack of water.