Measurement and modeling of the surface conductance of boreal aspen and black spruce stands

English Abstract:

Boreal forest accounts for 30% of the Canadian landscape and plays an important role in the global water and carbon cycles. How these processes will change with respect to climatic variability is poorly understood. Our goal is to improve our understanding of the integrated system response of the southern boreal forest to variation in climate. Continuous measurements have been made since 1996 at Old Aspen (OA) and 1999 at both Old Black Spruce (OBS) as part of the Boreal Ecosystem Research and Monitoring Sites (BERMS) program and Fluxnet Canada/Canadian Carbon Program. In addition to climate variables, we have high-quality year-round eddy-covariance measurements of evapotranspiration (E), and related micrometeorological fluxes of momentum, sensible heat, and CO₂. From 1996 to 2013, E at OA and OBS has ranged from 264 mm y^-1 to 450 mm y^-1 and 271 mm y^-1 to 331 mm y^-1, respectively. The lower average E at OBS has resulted in a significantly higher contribution to groundwater (i.e., precipitation (P) - E), especially over the 10 years after the drought that occurred in 2001-2003. During the drought years the difference between the sites was much greater, with mostly negative P - E values at OA. As part of research to improve models of forest E, data analysis is under way to compare two surface conductance models. Variations in the conductance are evaluated through comparisons of the models’ controlling variables on the conductance, such as photosynthetically active radiation, vapour pressure deficit, CO₂ concentration, soil water matric potential and leaf are index. The relationship between the controlling variables and conductance is being monitored for changes over the long term, including drought and wet periods. This research contributes to improving the parameterization of hydrologic processes in land surface models such as CTEM and CLASS.