Evaporation, Transpiration and Water Use Efficiency in Two Young Hybrid Poplar Plantations in Canada's Aspen Parkland

Hybrid poplar (HP) plantations established on former agricultural land in the aspen parkland of Canada have the potential to provide biomass for high-quality fibre, bio-energy, and ecosystem services including carbon (C) sequestration. The low precipitation and large summertime vapor pressure deficits (VPD) in the aspen parkland raises questions about HP plantation water use and its effects on local and regional water supplies. In May 2010 and June 2011 we began using the eddy-covariance (EC) technique to measure CO₂, water vapor and sensible heat fluxes above two young HP plantations. These plantations (HP09 and HP11, respectively) were planted (2 m x 2m spacing) in June 2009 and May 2011 on Class 1 (high productivity) clay loam Chernozemic soil located near Edmonton, AB and Winnipeg, MB, respectively. Our measurements have shown that HP09 was a C source of 154 g C m^-2 y^-1 during the 2^nd year (2010) and became a C sink of 80, 153 and 173 g C m^-2 y^-1 during the 3^rd, 4^th and 5^th years after planting, respectively. HP11 was a C source of 140 and 22 g C m^-2 y^-1 in the 1^st (2011) and 2^nd year, respectively, and became a C sink of 105 g C m^-2 y^-1 during 3^rd year after planting.

EC-measured evapotranspiration (E) and climate data were used to calculate bulk surface conductance (g_s) using the inverted Penman-Monteith equation and were compared to g_s estimates derived from a biophysical model that permits the partitioning of E into transpiration (E_T) and evaporation from the soil (E_S). The dependence of leaf stomatal conductance (g_stom) on attenuated incoming short-wave radiation through the canopy as a function of leaf area index (LAI) was derived and parameterized using EC data to create a canopy conductance (g_c) model (Saugier and Katerji, 1991. Agric. For. Meteorol., 54: 263-277), and was further modified by incorporating g_stom dependence on VPD (Leuning et al., 2008. Water Resour. Res., 44, W10419). E_S was estimated using the equilibrium evaporation rate. The resulting g_c and calculated climatological conductance values were input into a bulk g_s model originally derived by Kelliher et al. (1995, Agric. For. Meteorol., 73: 1-16) and later modified using a multiplier (f_s) to account for soil moisture effects on E_S by Leuning et al. (2008). After applying f_s (derived from soil water content data), initial modelled half-hourly values of g_s showed excellent diurnal and seasonal agreement with EC-calculated g_s, but results indicated the need for a separate canopy soil water function (f_c) for g_c.

At HP11 measured and modelled growing season (May1-September 31) E values during the 2^nd year (2012) were 208 and 223 mm, respectively, where modelled values of E_S and E_T were 140 and 83 mm, respectively. During the 3^rd year of growth, measured and modelled growing season E at HP11 were 235 and 240 mm, respectively, where modelled values of E_S and E_T were 185 and 55 mm, respectively. Values of water use efficiency (WUE) at HP11 during the growing season, calculated as gross ecosystem photosynthesis (GEP) divided by E, were 1.53 and 2.54 g C kg^-1 H₂O in the 2^nd and 3^rd years, respectively. At HP09 growing season EC-measured E of 376, 401, 399 mm exceeded growing season precipitation of 312, 385, 225 mm during the 3^rd, 4^th and 5^th years, respectively, and the respective values of growing season WUE were 1.90, 2.31, 2.03 g C kg^-1 H₂O. Results suggest that E_S dominates plantation E during the first 3-4 years of HP development, and that growth of HP planted on highly productive former agricultural soils in Canada's aspen parkland can become water limited.