Assessing the role of tree growth patterns on the spatial variability of evapotranspiration on a subalpine transition zone in Kananaskis, Alberta
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Canada’s Rocky Mountains provide a large and essential supply of freshwater to downstream ecosystems and communities. Previous research has demonstrated that warmer temperatures, associated with climate change, are expected to increase the recruitment of trees towards alpine zones, by way of tree islands and krummholz. Tree islands and krummholz are coniferous trees that grow in isolated patches. Tree islands are stunted and deformed, yet their stems grow above the shrub layer, leaving them exposed to winter snowdrifts, unlike krummholz, which grow stunted or in matts, below the snowpack. These trees are unique, relative to conifers below the treeline limit, as they have growth mechanisms which allow them to persist in areas that are otherwise too harsh for full treeline expansion. This thesis addresses the complex relationships between the spatial variability of evapotranspiration (ET) in tree islands and krummholz on a subalpine ridge slope in Kananaskis, Alberta. As well, relationships between these canopies and controls on ET, such as snowcover, meteorological fluxes and vegetation characteristics are assessed. By addressing these objectives, this study will reduce existing knowledge gaps on how forest transition zones in mountain ecosystems may contribute to ecosystem water loss, should these tree patches continue to prosper at higher elevations. Atmometers, which measure the rate of ET from heterogenous landcover to the atmosphere, were used at FRS to determine the rate of potential crop evapotranspiration (ETC) from krummholz and tree islands. ETC was then converted to actual evapotranspiration (ETA) using patch-specific correction coefficients (KC) in order to address the influence of canopy dynamics and water availability on ET. ETA during the growing season was greater in the krummholz (190 mm) than the tree islands (131 mm). Krummholz were observed to be moisture rich tree patches that were shorter in height and more exposed than tree islands. Because of this, krummholz ET was controlled by the advection of sensible heat transported from drier areas downward over the krummholz resulting in oasis-effect ET (QE > Q*). Horizontal advection of sensible heat from the taller tree islands to the shorter krummholz increases clothesline-effect ET at FRS. In addition, the exposure of the krummholz to the effects of solar radiation to the their subsurface increases the rate of early growing season ETA by increasing soil water evaporation. Tree islands, which extend above the annual snowpack were capable of sheltering windblown snow, increasing the amount of water available to the tree islands and krummholz for the growing season. Water balances for the tree islands and krummholz indicated that SWE was the primary source of water to the patches and did not suggest water deficits during the observed growing season. Tree island ET rates were controlled by the evaporation of intercepted precipitation (2 - 58%), and growth characteristic such as increased canopy density, which increased subsurface sheltering, reducing soil water evaporation, while maintaining inner-canopy VPD (increases transpiration). The results of this study improve our knowledge of how tree islands and krummholz will influence ecosystem water storage, especially in terms of ET, and determined what dominant controls exist on ET in subalpine systems. As climate change is expected to decrease annual snowpack levels and increase seasonal air temperatures, ET from tree island and krummholz may contribute to water deficits in subalpine ecosystems.
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Jessica Williamson (2020). Assessing the role of tree growth patterns on the spatial variability of evapotranspiration on a subalpine transition zone in Kananaskis, Alberta. UWSpace. http://hdl.handle.net/10012/15941