A review of the literature on the relationships between trees, land use, and hydrological processes in the Andes

Abstract

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 ecological functions of watersheds, depending on diverse factors. In this study, we compiled and synthetized the literature on the relationships between woody plants, land use and hydrologic processes in the Andes. 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 very little 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 cloud 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 higher 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 lower water provision than other land uses. However, their effect on water regulation defies simple comparisons with other uses: they have lower base flow and higher peak flow than other uses, and lower infiltration and water storage than forests and paramos, but perform better in these respects than natural pastures. 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. Even though there remain important knowledge gaps on the role of forests and trees in the hydrology of Andean watersheds, especially regarding the combined impacts of land use and land cover change, our review results can help inform degraded land restoration practices and policies in the Andean region