Evapotranspiration dynamics and partitioning across an assemblage of vegetation types in a subarctic, alpine catchment, Yukon Territory, Canada

Abstract:

As a result of altitude and latitude amplified impacts of climate change, widespread changes in vegetation composition, density and distribution have been observed across northern, alpine catchments. Quantification of evapotranspiration (ET) across a range of vegetation units is critical to predicting water yield from alpine catchments, yet challenging due to complex environmental and phenological controls on transpiration (T) and energy partitioning. Feedbacks of vegetation change on northern hydrology is currently unknown as these regions face increased temperatures and altered precipitation (P) regimes. Here, we assess the influence of vegetation type and structure, meteorological, phenological and soil controls on ET dynamics and partitioning within and among three sites in a subarctic, alpine catchment near Whitehorse, Yukon for 5 years. These sites span a gradient of thermal and vegetation regimes, providing a space-for-time comparison as ecosystems shift in the future: 1) a low-elevation boreal white spruce forest (~20 m), 2) a mid-elevation subalpine taiga comprised of tall willow (Salix) and birch (Betula) shrubs (~1-3 m) and 3) a high-elevation subalpine taiga with shorter shrub cover (< 0.75 m) and moss, lichen, and bare rock. Eddy covariance and sap flow sensors ran year-round at the forest and during the growing season at the mid-elevation site on both willow and birch shrubs for two years. Results show growing season ET decreased and interannual variability increased with elevation, as May to September ET totaled 348 mm (+-4mm) at Forest, 244mm (+-14 mm) at the tall shrub site, and 247 mm (+-34mm) at the short shrub site. Comparatively, AET:P ratios were the highest and most variable at the forest (2.4 +-0.3) and similar at the tall and short shrub (1.2+-0.1). From July to August, T comprised 89% of ET at the shrub sites and 46% at the forest. Controls on ET and T varied at each site depending on time of year and following P events of varying intensity. ET at the forest and short shrub site was primarily driven net radiation while at the tall shrub site, ET was controlled by surface Results suggest that predicted changes in vegetation type and structure in northern regions will have a considerable impact on water partitioning, and will also vary in a complex way in response to changing temperature and P regimes.