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

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 (250−1450mm), minimum annual E was not less than 200mm and was limited to 400−500mm where annual P exceeded 700mm. 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 (Rn) 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 Rn. Results of a cross-correlation analysis between daily (24-h) E and Rn revealed that site-specific lag times were between 10 and 40 days depending on the lag of vapour pressure deficit (D) behind Rn 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 (gc) 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 gc. 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 2.6–3.6gCkg−1 H2O, the BC-DF49 site had the highest WUE of the twelve sites with values near 6.0gCkg−1 H2O. Of the two peatland sites, AB-WPL, a western treed fen, had a significantly higher WUE (∼3.0gCkg−1 H2O) than ON-EPL, an eastern ombrotrophic bog (∼1.8gCkg−1 H2O), which was related to peatland productivity and plant functional type.