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

摘要

Using 20 mm-long Granier-type sensors, the effects were quantified of tree size, azimuth and radial position in the xylem on the spatial variability in xylem sap flux in 64-year-old T. distichum trees growing in a flooded forest in Durham, North Carolina, USA. This information was used to scale flux to the stand level to investigate variations in half-hourly and daily (24-h) sums of sap flow, transpiration per unit of leaf area, and stand transpiration in relation to vapour pressure deficit (D) andphotosynthetically active radiation (Qo). Measurements of xylem sap flux density (Js) indicated that Js 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 andJs was not significant. Js in the inner xylem (20-40 mm from the cambium) was 0.40 of that in the outer 20 mm xylem. Js on the north side of trees was 0.64 of that in directions 120degree C from the north. Daily transpiration was linearly related to daily daytime mean D, and reached a 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 < 0.6 kPa during both night and day,increasing to saturation during daytime at higher values of D. The positive effect of Qo on Js was significant but relatively minor. Thus, a second-order polynomial with D explained 94% of the variation in Js and transpiration. An approximately 40% reduction in LAI by a hurricane resulted in decreases of about 18% in Js and stand transpiration, indicating partial stomatal compensation.