Key messages

• This brief summarizes the findings of a systematic review on the impacts of forestation on water and soils in the Andes (detailed in Bonnesoeur et al., 2018).
• Exotic tree plantations and, to a lesser extent, native forests consume water and therefore often reduce the total water supply to downstream users in most Andean regions.
• Only in areas immersed in clouds, such as in the eastern slope of the Andes, might native forests increase downstream water availability compared to other land covers.
• Decreased total water supply could be acceptable to many users if it confers other benefits, such as increased water availability during the dry season or a reduction in water turbidity.
• When trees, including exotic species, are planted on degraded soils (bare and/or compacted soils), they can improve soil infiltration, reduce peak flows and control erosion.
• Exotic tree plantations on well-conserved grasslands (páramos, jalcas, punas) have detrimental impacts on total water supply and hydrological regulation.
• Existing native forests provide excellent water regulation and erosion control, more than mature tree plantations.
• As restoring degraded native forests does not necessarily recover original hydrological services, the conservation of existing forests must be a priority for watershed management.
• The hydrological impacts of native species forestation, however, have largely been overlooked and require further research.
• Long-term hydrological monitoring and research are necessary to fill the multiple data and knowledge gaps identified in this review.