A COMPARISON OF SAP FLOW AND EDDY FLUXES OF WATER-VAPOR FROM A BOREALDECIDUOUS 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 chang es in transpiration were monitored using two sap flow techniques and w ere compared against the difference between eddy correlation measureme nts of water vapor flux made above and below the aspen canopy. The thr ee methods showed similar diurnal and seasonal trends in water flux, a lthough sap flow lagged the eddy correlation measurements by about 1 h our diurnally due to changes in water storage within the trees. During the growing season, all methods showed a linear increase in midday tr anspiration with above-canopy vapor pressure deficit (VPD) up to simil ar to 1 kPa, beyond which transpiration was relatively constant (VPD 1 -2.5 kPa). A similar relationship was obtained when total daily transp iration was plotted against mean daytime VPD. The results are consiste nt with other observations that stomatal conductance of the aspen cano py decreases at high VPD. The complementary benefits of simultaneous m onitoring of canopy transpiration by both eddy correlation and sap flo w measurements are discussed.