Carbon Sequestration and Water Use of Hybrid Poplar Plantations in Western Canada

Hybrid poplar (HP) has the potential to supply a significant amount of wood for bioenergy, pulp and other uses, as well as long-term carbon (C) sequestration in woody biomass and soil organic matter in the form of tradeable C credits. However, fast-growing HP are known to have high water and nutrient requirements, and growth and sustainability of HP may be compromised due to limited precipitation with possible adverse effects on the regional water balance, especially in Western Canada. Thus the key to ecologically, environmentally and economically sustainable HP is the availability of optimum water and nutrients to meet increased growth and evapotranspiration rate. The latter is expected to increase with increasing vapor pressure deficits as a result of increasing temperature as forecasted by climate models. To understand the effects of different soil and climatic conditions on the growth and water use efficiency, and to develop models for forecasting sustainability of rainfed HP in Western Canada, we have begun making measurements of ecosystem-level fluxes of CO2 and water vapor using the eddy-covariance (EC) technique in representative HP plantations in northern Alberta and southern Manitoba. EC measurements are supplemented with measurements of above- and below-ground C stocks, soil respiration including its heterotrophic and autotrophic components, and root zone water storage and drainage. EC measurements showed that the annual C balance of an HP plantation on Class 1 land in St Albert, Alberta (mean annual precipitation 450 mm) shifted from a C source of 0.60 Mg C ha-1 y-1 in the 1st year to a C sink of 0.80 Mg C ha-1 y-1 in the 2nd year. Compared to this, an HP plantation grown on a marginal agricultural land (Class 3) in Ashmont, Alberta (mean annual precipitation 350 mm) took five years to shift from a strong C source of 3.3 Mg C ha-1 y-1 in the 1st year to C neutrality. In Manitoba, a recently (July 2011) planted HP plantation on Class 1 land at Cooks Creek near Winnipeg (mean annual precipitation 510 mm) lost 0.9 Mg C ha-1 during Jun-Dec 2011. The results suggest that both soil type and precipitation influenced the net ecosystem productivity of HP plantations. Growing season water use at the three sites increased with growing season precipitation but exceeded the latter by as much as 50% thereby indicating appreciable use of stored soil water, whereas annual water use was estimated to vary from 10% lower to 10% higher than annual precipitation. These results emphasize the need to study the long-term sustainability of HP plantations in relation to annual total precipitation and its distribution, especially when large contiguous areas would likely be brought under HP to supply biomass feedstock for expanding pulp and bioenergy industries in Western Canada.