A comparison of sap flow and eddy fluxes of water vapor from a boreal deciduous forest

Water flux to the atmosphere was measured from a mature stand of aspen (Populus tremuloides Michx.) in Saskatchewan, Canada, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Diurnal and seasonal changes in transpiration were monitored using two sap flow techniques and were compared against the difference between eddy correlation measurements of water vapor flux made above and below the aspen canopy. The three methods showed similar diurnal and seasonal trends in water flux, although sap flow lagged the eddy correlation measurements by about 1 hour diurnally due to changes in water storage within the trees. During the growing season, all methods showed a linear increase in midday transpiration with above-canopy vapor pressure deficit (VPD) up to similar to 1 kPa, beyond which transpiration was relatively constant (VPD 1-2.5 kPa). A similar relationship was obtained when total daily transpiration was plotted against mean daytime VPD. The results are consistent with other observations that stomatal conductance of the aspen canopy decreases at high VPD. The complementary benefits of simultaneous monitoring of canopy transpiration by both eddy correlation and sap flow measurements are discussed.