Over the last decades, the Andean highlands of Ecuador have been characterised by intense afforestation efforts, in order to increase the economic return of less viable agricultural areas, reduce erosion and, more recently, to sequestrate atmospheric carbon. Afforestation with Pinus species is widespread in the high altitudinal grasslands known as páramos. The impact of Pinus patula afforestation on the water yield is studied and compared to the more common practice of intensive grazing and potato cultivation in four microcatchments in the Paute river basin in south Ecuador. Two catchments are covered with natural grassland vegetation, one is converted to pine forest, and one is drained, partly intensively grazed, and partly cultivated with potatoes. The results indicate that afforestation with P. patula reduces the water yield by about 50%, or an average of 242 mm year-1. The water yield of the cultivated catchment is very similar to that of the natural catchments, but analysis of the flow duration curves suggests a faster response and a loss of base flow. These effects may have important implications for a sustainable management of the páramo ecosystem, given that the páramo is the major water supplier for the Andean highlands. © 2007 Elsevier B.V. All rights reserved.