Abstract
Several Andean countries have planned to restore forest cover in degraded and deforested land to enhance the provision of multiple ecosystem services in response to international commitments such as the Bonn Challenge. Hydrological services, e.g. water supply, hydrological regulation and erosion mitigation, are particularly important to sustain the life of forty million people. While rapid and important forest cover changes have occurred during recent decades, critical information on the impact of tree plantations or natural regeneration in previously deforested land on hydrological services has not yet been synthesized in the context of Andean ecosystems. To help improve decisionmaking on reforestation in the Andes, we reviewed the available literature concerning the impacts of tree plantation or natural regeneration of forest on water supply, hydrological regulation and mitigation of erosion and landslides. We also examined available data on the most relevant hydrological processes such as infiltration, evapotranspiration and runoff in forest stands. Hydrological services from native forests were also included as a reference state for comparing processes and services provided by reforestation. Following systematic review protocols, we searched peer-reviewed papers and unpublished theses and reports. We synthesized studies using different methods, including meta-analyses and meta-regressions. Results show that reforestation has had clear impacts on degraded soils, through reducing water erosion of soils and risk of moderate floods, increasing soil water infiltration and topsoil organic matter (SOM). We found that 20 years of tree plantation was sufficient to recover surface runoff and sediment yield close to the levels of native forests whereas SOM (used as a proxy of soil water storage) of native forests could not be recovered by reforestation in the time scales examined. The benefits in terms of hydrological regulation are at the expense of a reduction in total water supply since forest cover in the Andes was associated with higher water use (except for certain cloud montane forests). Nevertheless, we identified an important knowledge gap on forest transpiration that should be addressed to better manage water at local and regional scales. The impact of reforestation on landslides has also been largely overlooked in the Andes. At high elevations, Andean grasslands (e.g. paramo and puna) showed an excellent capacity for hydrological regulation and erosion mitigation but also lower water use than forests. People engaged in forest restoration initiative should be aware that hydrological services may take some time for society and the environment to show clear benefits after reforestation.