Interannual variation of evapotranspiration from forest and grassland ecosystems in western canada in relation to drought

Climate models predict rising temperatures and more frequent and prolonged droughts, particularly in the northern hemisphere and in the Canadian Prairies. However, few studies have examined the interannual variation in evapotranspiration (E) of northern terrestrial ecosystems in relation to drought. This study analyses multi-year (1998-2006), eddy-covariance measurements to quantify the seasonal and interannual variability in E from grassland and mature aspen, black spruce, and jack pine forest ecosystems in the dry interior plains of western Canada. It also investigates the response of E to the historic 2001-2003 drought in this region. Leaf area index (LAI) was a primary factor controlling the difference in E among different ecosystems in the same ecozone. Annual E was higher and more variable for the aspen forest (405 ± 84 mm, mean ± s.d.) and grassland (395 ± 90 mm) than the black spruce (374 ± 34 mm) and jack pine (300 ± 20 mm) forests. Interannual variation of E was controlled by early spring soil temperature at all four sites, with warm springs enhancing annual E. Only the aspen forest and grassland showed a significant suppression of E by summer drought, related to reduced surface conductance at the aspen site, and to reduced surface conductance and early leaf senescence at the grassland. We conclude that the potential for drought impacts on annual E of northern ecosystems is greatest for grasslands, moderate for deciduous broadleaf aspen forests, and smallest for coniferous black spruce and jack pine forests. The forests of the Boreal Plains, adjacent to the prairie region, may ameliorate the onset of drought through the recycling of moisture to the atmosphere, whereas the prairie grasslands have only limited capacity to counteract drought through moisture recycling.

Research highlights▶ Climate models predict rising temperatures and more frequent and prolonged droughts, particularly in the northern hemisphere and in the Canadian Prairies. However, few studies have examined the interannual variation in evapotranspiration (E) of northern terrestrial ecosystems in relation to drought. This study analyses multi-year (1998-2006), eddy-covariance measurements to quantify the seasonal and interannual variability in E from grassland and mature aspen, black spruce, and jack pine forest ecosystems in the dry interior plains of western Canada. It also investigates the response of E to the historic 2001-2003 drought in this region. Leaf area index (LAI) was a primary factor controlling the difference in E among different ecosystems in the same ecozone. Annual E was higher and more variable for the aspen forest (405 ± 84 mm, mean ± s.d.) and grassland (395 ± 90 mm) than the black spruce (374 ± 34 mm) and jack pine (300 ± 20 mm) forests. Interannual variation of E was controlled by early spring soil temperature at all four sites, with warm springs enhancing annual E. Only the aspen forest and grassland showed a significant suppression of E by summer drought, related to reduced surface conductance at the aspen site, and to reduced surface conductance and early leaf senescence at the grassland. We conclude that the potential for drought impacts on annual E of northern ecosystems is greatest for grasslands, moderate for deciduous broadleaf aspen forests, and smallest for coniferous black spruce and jack pine forests. The forests of the Boreal Plains, adjacent to the prairie region, may ameliorate the onset of drought through the recycling of moisture to the atmosphere, whereas the prairie grasslands have only limited capacity to counteract drought through moisture recycling.