Interpreting Long-Term Measurements of Surface Conductance at a Boreal Aspen and a Black Spruce Stand

Boreal forest accounts for 30% of the Canadian landscape and plays an important role in the global water and carbon cycles. How they 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 Old Black Spruce (OBS) as part of the Boreal Ecosystem Research and Monitoring Sites (BERMS) program in the Fluxnet-Canada Research Network and now the Changing Cold Regions Network. 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 2015, climate stationarity was maintained with an interannual variability of ±2°C in growing season mean air temperature and ±0.25 kPa in vapor pressure deficit (D), while CO₂ mixing ratio increased ~ 2.0 µmol mol^-1 y^-1. E at OA and OBS ranged from 264 mm y^-1 to 450 mm y^-1 and 271 mm y^-1 to 331 mm y^-1, respectively, with the minimum values occurring in the 2001 to 2003 drought. As part of research to improve models of forest E, variations in the surface conductance are evaluated using different relationships with controlling variables, such as photosynthetically active radiation, D, atmospheric CO₂ mixing ratio, soil water matric potential and leaf area index. Over the observation period, the conductance has ranged from 5.0 to 11.6 mm s^-1 at OA and 4.7 to 8.2 mm s^-1 at OBS. To decipher long-term changes from inter-annual variability, a five-year moving window is used in determining the relationships of the controls on growing-season conductance. This research contributes to improving the parameterization of hydrologic processes in land surface models such as CTEM and CLASS.