How climate and vegetation type influence evapotranspiration and water use efficiency in Canadian forest, peatland and grassland ecosystems

Abstract The effects of climatic factors and vegetation type on evapotranspiration ( E ) and water use efficiency (WUE) were analyzed using tower-based eddy-covariance (EC) data for nine mature forest sites, two peatland sites and one grassland site across an east–west continental-scale transect in Canada during the period 2003–2006. The seasonal pattern of E was closely linked to growing-season length and rainfall distribution. Although annual precipitation ( P ) during the observation period was highly variable among sites (250−1450 mm), minimum annual E was not less than 200 mm and was limited to 400−500 mm where annual P exceeded 700 mm. Site-specific interannual variability in E could be explained by either changes in total P or variations in solar irradiance. A highly positive linear correlation was found between monthly mean values of E and net radiation ( R n ) at the grassland site (AB-GRL), the two peatland sites (AB-WPL and ON-EPL), and only one of the forest sites (coastal Douglas-fir, BC-DF49) whereas a hysteretic relationship at the other forest sites indicated that E lagged behind the typical seasonal progression of R n . Results of a cross-correlation analysis between daily (24-h) E and R n revealed that site-specific lag times were between 10 and 40 days depending on the lag of vapour pressure deficit ( D ) behind R n and the decoupling coefficient, Ω . There was significant seasonal variation in daytime mean dry-foliage Priestley–Taylor α with maxima occurring in the growing season at all sites except BC-DF49 where it was relatively constant (∼0.55) throughout all years. Annual means of daytime dry-foliage α mostly ranging between 0.5 and 0.7 implied stomatal limitation to transpiration. Increasing D significantly decreased canopy conductance ( g c ) at the forest sites but had little effect at the peatland and grassland sites, while variation in soil water content caused only minor changes in g c . At all sites, a strong linear correlation between monthly mean values of gross primary production (GPP) and E resulted in water use efficiency being relatively constant. While at most sites, WUE was in the range of 2.6–3.6 g C kg −1 H 2 O, the BC-DF49 site had the highest WUE of the twelve sites with values near 6.0 g C kg −1 H 2 O. Of the two peatland sites, AB-WPL, a western treed fen, had a significantly higher WUE (∼3.0 g C kg −1 H 2 O) than ON-EPL, an eastern ombrotrophic bog (∼1.8 g C kg −1 H 2 O), which was related to peatland productivity and plant functional type.