Replacement of grasslands with Polylepis forests reduces aquifer recharge in the seasonally dry Andes

The high-Andes biome, which includes grasslands, bofedales, and native forest, provides vital ecosystem services, such as water regulation, to millions of people living downstream. Whereas recent studies have highlighted the hydrologic regulation of native grasslands and bofedales, and quantified reductions in water yield resulting from afforestation with exotic species, little is known about changes to water resources when native grasslands are converted to Polylepis forests. We measured hydrologic fluxes from the root zone to catchment scale in Jarava Ichu grasslands, young Polylepis (afforested over 0.568 km2 in 2016-2017), and a mature Polylepis forest in the seasonally dry puna biome of Cusco, Perú. Compared to grasslands, the Polylepis root zone experienced more drying, reached field capacity later in the wet-up, and passed an average of 41 % less water annually to the underlying aquifer. Average annual evapotranspiration in young and mature Polylepis was 1.9 and 3.2 times larger than in J. ichu grasslands, respectively. Despite the varied ecosystem services provided by native forests to the local community and region, we predict a 10-11 % reduction in water yield from the Upper Ramuschaka Watershed (2.12 km2) if the afforested Polylepis reaches maturity. The results presented here will guide hydrological modeling in the region and inform community-led discussions assessing the relative importance of the ecosystem services provided by the varied land covers of the puna biome.