Extended growing seasons and decreases in hydrologic connectivity indicate increasing water stress in humid, temperate forests

Abstract

Forested headwater catchments are important sources of stable and abundant freshwater resources. Interactions between vegetation and topography influence lateral hydrologic connectivity by altering shallow subsurface flow paths. This in turn influences vegetation density along those paths, and subsequent hydrologic partitioning between localized water use and subsurface flows at catchment scales. Climate change impacts on forests, and the degree to which they reshape feedbacks between evapotranspiration (ET) and hydrologic connectivity, remain unclear. To clarify the extent and drivers of changing lateral hydrologic connectivity, we assessed relative changes in upslope to downslope vegetation density using the Normalized Difference Vegetation Index (NDVI) from 1984 – 2021 in 30,044 forested catchments across the Southern Appalachian Mountains. Increasing upslope NDVI relative to downslope NDVI was used as a proxy for decreasing lateral hydrologic connectivity. We then related changes in connectivity to climate and streamflow dynamics across 28 sub-regional reference watersheds. We found decreases in the ratio of downslope to upslope NDVI in almost half of the catchments (48.5%), primarily due to increasing upslope NDVI. This indicates increasing ET upslope and a decline in lateral hydrologic subsidy to downslope given precipitation. This was also supported by faster streamflow recession and increasing ET estimates relative to precipitation in over half of reference watersheds. The strongest predictor of decreasing connectivity was growing season minimum temperature (Tmin), which increased in 88% of catchments (Mean R2 = 0.27 +/- 0.13). While Tmin is not a dominant atmospheric driver of ET, this pattern has been closely linked to lengthened growing seasons. This suggests that alteration of lateral hydrologic connectivity is mainly driven by ecophysiological responses to changing climate rather than directly by atmospheric drivers. Our results emphasize the importance of vegetation dynamics shifting hydrologic partitioning and driving water limitations even in humid, temperate forests.