Tang, J., P.V. Bolstad, B.E. Ewers, E.V. Carey, A.R. Desai, and K.J. Davis, in preparation. Canopy transpiration, stomatal conductance and water use efficiency in an old-growth northern forest, Journal of Geophysical Research - Biogeosciences.

Combining sap flux measurements of transpiration and eddy covariance measurements of carbon dioxide provide a means to study plant stomatal behavior and the relationship between transpiration and photosynthesis from tree to stand levels. We measured sap flux using Granier-type sensors in a hardwood-dominated old-growth northern forest in the upper peninsula of Michigan, USA in 2002 and 2003. We also measured fluxes of carbon dioxide and water vapor with the eddy covariance method and derived daytime gross primary production (GPP). We upscaled sap flux measurements into canopy transpiration based on sapwood area and calculated canopy conductance from transpiration and vapor pressure deficit (D). The diurnal patterns of sap flux and canopy transpiration were mainly controlled by D and photosynthetically active radiation (PAR). Sugar maple and yellow birch had higher sap flux than hemlock. Daily sums of sap flux and canopy transpiration had exponential relationships to D and linear relationships to PAR. Average canopy transpiration per leaf area over the growing season were 327.4, 228.9, 317.7 g m-2 day-1 for sugar maple, yellow birch, and hemlock, respectively, resulting in total canopy transpiration of 2.30 mm day-1, compared with the average evapotranspiration of 2.42 mm day-1. The diurnal pattern of canopy conductance was mainly controlled by PAR, but the day-to-day variation of canopy conductance was controlled by D. Water use efficiency (WUE = GPP/transpiration) had a strong relationship with D with an exponential decay function. WUE dramatically increased when D <0.3 kPa, and approached a minimum of 3.0 mg g-1 when D was high. WUE provides an alternative potential to estimate GPP from measurements of sap flux.