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

Wednesday, 14 May 2014: 1:45 PM
Bellmont A (Crowne Plaza Portland Downtown Convention Center Hotel)
Hughie Jones, University of British Columbia, Vancouver, BC, Canada; and T. A. Black, R. S. Jassal, Z. Nesic, J. S. Bhatti, and D. Sidders

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 2nd year (2010) and became a C sink of 80, 153 and 173 g C m-2 y-1 during the 3rd, 4th and 5th years after planting, respectively. HP11 was a C source of 140 and 22 g C m-2 y-1 in the 1st (2011) and 2nd year, respectively, and became a C sink of 105 g C m-2 y-1 during 3rd year after planting.

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

At HP11 measured and modelled growing season (May1-September 31) E values during the 2nd year (2012) were 208 and 223 mm, respectively, where modelled values of ES and ET were 140 and 83 mm, respectively. During the 3rd year of growth, measured and modelled growing season E at HP11 were 235 and 240 mm, respectively, where modelled values of ES and ET 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 2nd and 3rd 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 3rd, 4th and 5th 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 ES 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.

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