Evapotranspiration and Water Use Efficiency of Canadian Forests and Wetlands
Abstract
Plant growth and potential photosynthetic carbon uptake are strongly controlled by water availability and climatic conditions. The study of evapotranspiration (E) among a variety of terrestrial ecosystems is therefore fundamental to understand its role within the water cycle and linkage to carbon fluxes. In this study, we analyze tower-based eddy-covariance data of nine mature forests, two peatland and a grassland site across an east- west continental-scale transect in Canada. The seasonal pattern in E was closely linked to growing season length and rainfall distribution. Although annual precipitation (P) was highly variable (400-1450 mm) among sites, annual E was limited to 400-500 mm for annual P > 700 mm. Site-specific interannual variability in E could be explained by either changes in total P or changes in the amount of downwelling solar radiation due to a varying length of sunny or cloudy periods. A highly positive linear correlation was found between monthly mean values of E and net radiation at a coastal Douglas-fir site (BC-DF49), whereas a hysteretic relationship at all other investigated sites indicated that E lagged behind the typical seasonal progression of meteorological parameters. Furthermore, daytime average dry-foliage Priestley-Taylor alpha was relatively constant (0.55) at BC-DF49 throughout each of the years. However, significant seasonal variations with maxima occurring in the growing season were found among all other sites. Annual means of alpha mostly ranging between 0.5-0.7 implied stomatal limitation to transpiration. Growing season means of alpha were only slightly higher than annual means except for a deciduous (aspen) boreal site (1.23 at SK OA), indicating no or only marginal water supply limitation. At all sites, a high linear correlation between monthly mean values of gross primary production and E resulted in water use efficiency (WUE) being relatively constant. While at most sites, WUE was in a range of 2.6 to 3.6 g C kg-1 H2O, highest WUE of the 12 sites with values around 6.0 g C kg-1 H2O was found at BC-DF49. Significant differences were observed between an eastern ombrotrophic bog (ON-EPL, WUE 1.8 g C kg-1 H2O) and a western treed fen (AB-WPL, WUE 3.0 g C kg-1 H2O), which could be a result of site-specific water cycles and nutrient availability. Results of this study provide important insight into the coupling of water and carbon cycles at the ecosystem level and are very valuable for the validation of regional process-based models used to investigate the influence of climate change on plant functional types.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSM.B72A..04B
- Keywords:
-
- 1878 Water/energy interactions (0495);
- 4227 Diurnal;
- seasonal;
- and annual cycles (0438);
- 4806 Carbon cycling (0428);
- 4815 Ecosystems;
- structure;
- dynamics;
- and modeling (0439)