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

The high-Andes biome, which includes grasslands, peat-forming wetlands known as 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 resulting from conversion of native grasslands 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 the Cusco region, 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 and regional communities, we predict a 10-11 % reduction in water yield from the Upper Ramuschaka Watershed (2.12 km2) if the afforested Polylepis grows to 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.高安第斯生物群区（high-Andes biome）涵盖草原、可形成泥炭的博夫达莱斯湿地（bofedales）以及原生森林，为下游数百万民众提供水源涵养调节等至关重要的生态系统服务。尽管已有研究关注了原生草原与博夫达莱斯湿地的水文调节功能，并量化了外来物种造林导致的产水量（water yield）下降，但针对原生草原转换为波利利斯属森林（Polylepis）所引发的水资源变化，目前仍知之甚少。本研究在秘鲁库斯科地区季节性干旱普纳生物群区（puna biome）中，针对雅拉瓦伊丘草原（Jarava Ichu）、2016-2017年间完成0.568平方千米造林的幼龄波利利斯林，以及成熟波利利斯林，开展了从根区（root zone）到集水区尺度（catchment scale）的水文通量（hydrologic fluxes）观测。相较于草原，波利利斯林的根区土壤更为干燥，在湿润期达到田间持水量（field capacity）的时间更晚，且每年向下伏含水层（underlying aquifer）补给的水量平均减少41%。幼龄与成熟波利利斯林的年平均蒸散发（evapotranspiration）量，分别是雅拉瓦伊丘草原的1.9倍与3.2倍。尽管原生森林为当地及区域社区提供了多样的生态系统服务，但本研究预测，若造林形成的波利利斯林完全成熟，拉穆查卡上游集水区（Upper Ramuschaka Watershed）（2.12平方千米）的产水量将下降10%-11%。本研究结果可为该区域的水文模拟提供参考，并为社区主导的讨论提供依据——这类讨论旨在评估普纳生物群区内不同土地覆被所提供生态系统服务的相对重要性。