How plants and vegetation assemblages use water in a rapidly changing subarctic, alpine environment

Background/Question/Methods

As high latitude ecosystems warm, precipitation regimes change, and vegetation shifts, there is limited knowledge of how changing species composition and density will affect critical zone water fluxes across vegetation types underlain with seasonally frozen soils and permafrost. Here, we assess the role of soil moisture, frozen ground status, precipitation dynamics and plant species on the timing, magnitude, and sources of plant water uptake at three sites across a subarctic, alpine catchment in Yukon, Canada. The sites represent a chronosequence and include a low-elevation boreal forest, a mid-elevation subalpine taiga comprised of tall, dense willow and birch shrubs and a high-elevation subalpine taiga with shorter, sparse shrub cover. We sampled soil and xylem water every 3 weeks from pre-leaf out to post-senescence over 2 hydrologically distinct years. Isotopic data was supplemented with sap flow and eddy covariance measurements of ecosystem evapotranspiration (ET). We answer the questions: 1) What sources of water, and where in the profile, do subarctic plants access water within and among seasons? 2) How does soil moisture and the seasonal nature of frozen ground influence plant sources? and 3) What are the primary abiotic and biotic controls and limits on T and ET across vegetation covers?

Results/Conclusions

Results show that T at the forest was more sensitive to changes in soil moisture than the shrub sites and early season xylem water varied in response to snowmelt timing and frozen ground. Plant water uptake was more reliant on snow water at the forest site than both shrub sites. Near-surface bulk soil water had more negative lc-excess at the forest throughout the season and with depth, highlighting increased contributions from soil evaporation. Of particular note, willow and birch shrubs had different lc-excess values, and indicate shallower, more evaporatively enriched sources for birch. Mixing analyses reveals that these subarctic plants were opportunistic, using both meltwater and rain dependent upon season. This study combines direct measurements of sap flux, ET, and critical zone isotopes to provide new details on multi-year plant-soil-water dynamics and water cycling in seasonally frozen soils, which have not previously been reported in cold regions. Results imply that the current rapid changes in vegetation will have considerable impacts on future blue-green water flux partitioning.