Exotic Trees and Ecosystem Impacts in Sub-Saharan Africa: The Case of Sao Hill, Tanzania

• Forests are the major sources of water and economic sustainability. • Mature eucalyptus can exhaust groundwater of ∼30–50 m³/year. • More excessive depletion of groundwater is expected in the future • There should be balance between economic development and conservation of forests. • Forest, climate, and water need to be integrated into the development policies and practices. Most forests in sub-Saharan Africa are vital ecosystems that support the livelihoods of many people. However, the dominant tree species in the forest are alleged to cause significant water loss in the forest ecosystem and thus, reducing the recharge of the nearby catchments. Thus, there is a need to determine the actual amount of water depleted from the soil and its subsequent impact on groundwater recharge and ecosystem services in the neighbouring ecosystems. Using a Boolean search, this systematic review compiles 20 years of research from the Web of Science and Scopus on the synergies of these exotic trees and water loss in the area, and their subsequent impacts on groundwater recharge. Most findings indicate that, mature eucalyptus can exhaust groundwater of ∼30–50 m³/year. This amount reduces the capacity of groundwater recharge and pose several social-ecological impacts. As interventions, the following are research directions: (1) estimating the current and future amount of water depletion caused by the exotic tree species; (2) mapping the current and future level of peoples’ dependency on the river catchments and forest ecosystems; (3) developing suitable and sustainable measures that balance economic and ecological needs); (4) integrating new knowledge into wider forest, climate, water and development policies and practice to promote climate-resilient structural transformation and advance equity and inclusion. Therefore, this article summarizes and evaluates the ecosystem, economic and hydrological state in the Sao Hills Forest and identifies priorities for future research, particularly in the context of climate change and increasing population pressures.