In mountainous headwater catchments, downslope flow of subsurface water could mitigate the impact of dry periods in convergent topographic areas, buffering downslope forest communities from soil moisture stress during drought. Here we investigated changes in landscape-scale vegetation patterns at five forested headwater catchments in the Coweeta Hydrologic Laboratory in the southern Appalachians. We used a ca. 30-year Landsat Thematic Mapper (TM) image record of normalized difference vegetation index (NDVI), spanning a period of recorded warming since the mid-1970s, and then we related spatial and temporal canopy patterns to seasonal water balance, streamflow recession behavior, and low flow dynamics from the long-term hydrologic records.
Results/Conclusions
All hydrologic metrics indicated increased localized water use, decreased hydrologic connectivity, and reduced downslope subsidy from ridge to stream during low-flow periods. Contrary to expectations, upslope leaf area index (LAI) increased more than downslope LAI over time, coincident with warming. Downslope vegetation appeared to respond to more frequent drought stress driven by a set of interacting processes resulting in decreased hydrologic connectivity and downslope subsidy. These processes have led to a change in canopy density patterns in which hillslope-scale NDVI has been homogenized along hydrologic flow paths over time. Trends in the ratio of NDVI in upslope and downslope topographic positions were also supported by long-term tree basal area increment, litterfall, and sap flux data in one of the reference watersheds. This study suggests that the vegetation downslope may be experiencing lower growth than upslope vegetation, due to their strong dependency on upslope water subsidy.