Soil drying induces widespread productivity loss but unequal climate vulnerability among ecotypes of a foundational Arctic sedge

1. As temperatures increase in the Arctic, hydrological change may lead to local soil drying through altered snowpack, evapotranspiration, and drainage due to permafrost thaw. These changes threaten to alter soil moisture regimes that control plant productivity and ecosystem carbon cycling. Eriophorum vaginatum, a foundational sedge that accounts for up to 30% of carbon uptake in moist tundra ecosystems, exhibits substantial local adaptation across its range, yet its capacity to maintain productivity under changing soil moisture conditions remains unknown. We conducted a common garden experiment using tussocks from three populations along a latitudinal gradient in northern Alaska, subjecting them to treatments simulating both surface soil drying and deeper drainage from permafrost thaw. Through measurements of plant water status, photosynthetic capacity, and seasonal growth patterns, we found that soil drying substantially reduced productivity across all populations through both decreased photosynthesis and reduced leaf area. Plants responded to moisture stress primarily by reducing canopy size and accelerating senescence rather than altering leaf-level physiology, with southern populations showing greater vulnerability to drought stress. Our findings highlight regional differences in drought susceptibility and suggest that shifts in soil moisture could influence Arctic plant productivity and carbon cycling under future climate.