Ecohydrological feedbacks in subarctic and arctic ecosystems: deep soil water buffers ecosystems from climate variability

Ecohydrological feedbacks in arctic and subarctic landscapes are being altered through vegetation shifts and permafrost degradation, both of which are expected to affect evapotranspiration dynamics. Low precipitation and high vapor pressure deficits define much of the growing season period in arctic and subarctic areas in interior Alaska. However, low plant water stress and high vegetation cover relative to other semiarid ecosystems suggests that these ecosystems may rely on water sources other than seasonal precipitation. We hypothesized that (1) water originating at the seasonal thawing front in the soil is a consistent, deep water source that supports the water requirements of deeply rooted plants, (2) this 'deep' source may be permafrost derived in systems with permafrost degradation, and (3) summer rainfall supports more shallowly rooted plants that use water from the near-surface soil. To test these hypotheses, we utilized stable isotopes and water flux measurements to evaluate plant water sources across a gradient in permafrost temperatures and different ecosystem types (i.e., black spruce to shrub tundra). The data were analyzed in a hierarchical Bayesian framework that coupled isotope mixing models with water flux models. The analyses revealed that in a dry year, plants consistently use deep water produced by a thawing active layer, and in some cases this was derived from degrading permafrost. In a wet year, however, plants use a mixture of rain water and thawing active layer water. Our findings suggest that ecohydrological feedbacks in arctic and subarctic ecosystems are linked to the consistent water sources deep in the active layer for plant use, resulting in a climatically buffered ecosystem. Moreover, most studies evaluating plant water sources and ecohydrological feedbacks in low-precipitation systems tend to focus on warm/hot arid or semiarid systems. Thus, this study provides a unique contribution by exploring such feedbacks in low-precipitation, cold ecosystems.