Relationship between trees, land cover and use and hydrological services in the Tropical Andes: a synthesis of knowledge.
In the Andean region, interest is growing in the use of trees to restore degraded areas through afforestation, reforestation or agroforestry. In addition to the economic benefits that these interventions can generate, particularly in the case of commercial plantations with exotic trees, one of the main arguments used to support these actions has been their purportedly positive effects on the recovery of the water provision and regulation functions of ecosystems. However, the relationship between tree cover and water is complex and may have positive or negative effects on the ecosystem functions of watersheds, depending on diverse factors. This study seeks to compile and systematize existing scientific and local knowledge on the relationship between woody plants, land cover/use, and hydrological services in the tropical Andes, in order to provide recommendations and inform restoration practices and policies. To this end, we carried out an exhaustive bibliographic search of publications from 1990 onwards in the countries belonging to the tropical Andes (Bolivia, Colombia, Ecuador, Peru and Venezuela) as well as of relevant global references. We evaluated, systematized and synthesized these publications according to four categories of land cover and use that included forests, paramos (Andean moorlands), agroforestry and forested areas with exotic species, and their relationship with different types of hydrological services. We gave priority to the Andean forest and non-forest ecosystems, defined as those above 500 m.a.s.l. The results showed that there is very limited and fragmented knowledge, with significant gaps in specific areas. While most studies focus on native forests and on agricultural land uses, there is comparatively less research on paramos (Andean moorlands) or on agroforestry, the latter limited to coffee agroforestry. Furthermore, the results from different studies are often not comparable and are sometimes contradictory because of varying research design and methods. Our qualitative review highlighted the importance of montane forests in water provision, in particular through the interception of horizontal precipitation. In addition, native forests play an important role in water regulation in comparison with crop and pasture lands, as they reduce runoff and store more water. Our results also showed that paramos provide better water regulation than forested areas with exotic species and other land covers by maintaining base flows. Plantations with exotic taxa, such as pines and eucalypts, provide overall less water than other land uses. They have lower base flow and higher peak flow than other uses and lower infiltration and water storage than forests and paramos. The reviewed studies indicated that water provisioning from coffee agroforestry systems is lower than in coffee systems without shade, but that these levels vary according to the tree species. The tree species, and in particular the management practices, influence the water regulatory role of coffee agroforestry systems with shade as compared to systems without shade. While run-off is higher in systems with exotic taxa (e.g. pine, eucalypts), the level of infiltration varies according to management and leaf characteristics. Among the main recommendations, we highlight the need to conserve cloud forests and paramos, given their importance in the interception of horizontal precipitation, water regulation, among others. In areas with direct intervention, it is key to adopt sustainable forest management practices that do not affect hydrological processes, medium densities of trees in plantations and agroforestry systems that reduce evaporation but at the same time increase the fall flow and infiltration. In cultivated areas, best practices need to be promoted, which may include agroforestry activities to avoid soil and water contamination by chemical inputs, rotation of grazing to avoid soil compaction and reduce evaporation and runoff. Also, in certain cases, plantations with exotic species can be considered for the restoration of highly degraded areas given their positive effect on infiltration and water quality. We also recommend increasing research on the knowledge that local farmers have about the hydrological functions of woody species and their relationship with land use, as this can contribute to the development of more appropriate management practices. In this regard, it is key to involve local farmers in the design of interventions. Finally, we urge the implementation of long-term monitoring systems, in order to have accurate, relevant and useful hydrometeorological information; as well as the production of systematic comparative studies that take into account the land use and land cover change history in a given place.