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We used 20-mm-long, Granier-type sensors to quantify the effects of tree size, azimuth and radial position in the xylem on the spatial variability in xylem sap flux in 64-year-old trees of  Taxodium distichum  L. Rich. growing in a flooded forest. This information was used to scale flux to the stand level to investigate variations in half-hourly and daily (24-hour) sums of sap flow, transpiration per unit of leaf area, and stand transpiration in relation to vapor pressure deficit ( D ) and photosynthetically active radiation ( Q  o ). Measurements of xylem sap flux density ( J  s ) indicated that: (1)  J  s  in small diameter trees was 0.70 of that in medium and large diameter trees, but the relationship between stem diameter as a continuous variable and  J  s  was not significant; (2)  J  s  at 20–40 mm depth in the xylem was 0.40 of that at 0–20 mm depth; and (3)  J  s  on the north side of trees was 0.64 of that in directions 120 degrees from the north. Daily transpiration was linearly related to daily daytime mean  D , and reached a modest value of 1.3 mm day −1 , reflecting the low leaf area index (LAI = 2.2) of the stand. Because there was no soil water limitation, half-hourly water uptake was nearly linearly related to  D  at  D  &lt; 0.6 kPa during both night and day, increasing to saturation during daytime at higher values of  D . The positive effect of  Q  o  on  J  s  was significant, but relatively minor. Thus, a second-order polynomial with  D  explained 94% of the variation in  J  s  and transpiration. An approximately 40% reduction in LAI by a hurricane resulted in decreases of about 18% in  J  s  and stand transpiration, indicating partial stomatal compensation.

Sap-flux-scaled transpiration responses to light, vapor pressure deficit, and leaf area reduction in a flooded Taxodium distichum forest