In grasslands around the world, climate change is occurring in tandem with woody encroachment (the spread of woody vegetation in grass-dominated ecosystems). State transitions from grassland to shrub/woodland have been identified aboveground via changes in species cover and composition, but the hydrological impact of these transitions is not well understood. Shifts from grass- to woody-dominance have the potential to impact evapotranspiration, soil moisture dynamics, and groundwater recharge. Therefore, it is possible that the consequences of aboveground vegetation change may be observable in the hydrological system. We leveraged long-term hydrological records from a tallgrass prairie site in northeastern Kansas, USA, to examine how concurrent changes in climate and land-cover have altered hydrological dynamics over the last century. Stream discharge has declined in recent decades despite > 100 years of climate wetting. The relationship between incoming precipitation and streamflow has weakened over the last 40 years, suggesting that shifts in the physical landscape are altering patterns of hydrological connectivity. Long-term isotope records show a divergence in the isotopic composition of precipitation (no change in δ18O) and stream/groundwater (decreasing δ18O) over the last decade. These results suggest that woody encroachment is accelerating the hydrological cycle, potentially by decreasing groundwater recharge (via increased evapotranspiration) and/or increasing infiltration rates (via creation of macropores). Holistically, these changes illustrate the interdependence of above- and below-ground processes in the local hydrological cycle, and the cascading long-term consequences (decades to centuries) for critical zone function once woody encroachment has occurred.

中文翻译：

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高草草原对生态系统状态变化的意外水文响应


在世界各地的草原上，气候变化与木本植物侵占（木本植被在以草为主的生态系统中的传播）同时发生。通过物种覆盖和组成的变化，已经确定了从草地到灌木/林地的状态转变，但这些转变的水文影响尚不清楚。从草本为主转变为木本为主有可能影响蒸散、土壤水分动态和地下水补给。因此，在水文系统中可以观察到地上植被变化的后果。我们利用美国堪萨斯州东北部高草草原遗址的长期水文记录，研究了气候和土地覆盖的同步变化如何改变了上个世纪的水文动态。尽管 > 100 年的气候湿润，但近几十年来溪流流量有所下降。在过去 40 年中，流入的降水和径流之间的关系已经减弱，这表明物理景观的变化正在改变水文连通性的模式。长期同位素记录显示，在过去十年中，降水（δ18O 没有变化）和溪流/地下水（δ18O 减少）的同位素组成存在差异。这些结果表明，木本侵蚀正在加速水文循环，可能是通过减少地下水补给（通过增加蒸散）和/或增加渗透速率（通过产生大孔隙）。从整体上讲，这些变化说明了当地水文循环中地上和地下过程的相互依存关系，以及一旦发生木质侵占，关键区功能将产生级联的长期后果（几十年到几个世纪）。