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We analyzed assumptions and measurement errors in estimating canopy transpiration ( E  L ) from sap flux ( J  S ) measured with Granier-type sensors, and in calculating canopy stomatal conductance ( G  S ) from  E  L  and vapor pressure deficit ( D ). The study was performed in 12-year-old  Pinus taeda  L. stands with a wide range in leaf area index ( L ) and growth rate. No systematic differences in  J  S  were found between the north and south sides of trees. However,  J  S  in xylem between 20 and 40 mm from the cambium was 50 and 39% of  J  S  in the outer 20-mm band of xylem in slow- and fast-growing trees, respectively. Sap flux measured in stems did not lag  J  S  measured in branches, and time and frequency domain analyses of time series indicated that variability in  J  S  in stems and branches is mostly explained by variation in  D . Therefore,  J  S  was used to estimate transpiration, after accounting for radial patterns. There was no difference between  D  and leaf-to-air vapor pressure gradient, and  D  did not have a vertical profile in stands of either low or high  L  suggesting a strong canopy–atmosphere coupling. Therefore,  D  estimated at one point in the canopy can be used to calculate  G  S  in such stands. Given the uncertainties in  J  S , relative humidity, and temperature measurements, to keep errors in  G  S  estimates to less than 10%, estimates of  G  S  should be limited to conditions in which  D  ≥ 0.6 kPa.

Analyses of assumptions and errors in the calculation of stomatal conductance from sap flux measurements