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In a Pinus taeda L. (loblolly pine) plantation, we investigated whether the response to vapour pressure deficit ( D) of canopy average stomatal conductance ( GS) calculated from sap flux measured in upper and lower branches and main stems follows a hydraulically modelled response based on homeostasis of minimum leaf water potential ( ΨL). We tested our approach over a twofold range of leaf area index ( L; 2–4 m2 m−2) created by irrigation, fertilization, and a combination of irrigation and fertilization relative to untreated control. We found that GS scaled well from leaf-level porometery [porometry-based stomatal conductance ( gs)] to branch-estimated and main stem-estimated GS. The scaling from branch- to main stem-estimated GS required using a 45 min moving average window to extract the diurnal signal from the large high-frequency variation, and utilized a light attenuation model to weigh the contribution of upper and lower branch-estimated GS. Our analysis further indicated that, regardless of L, lower branch-estimated GS represented most of the main stem-estimated GS in this stand. We quantified the variability in both upper and lower branch-estimated GS by calculating the SD of the residuals from a moving average smoothed diurnal. A light model, which incorporated penumbral effects on vertical distribution of direct light, was employed to estimate the variability in light intensity at each canopy level in order to explain the increasing SD of both upper and lower branch-estimated GS with light. The results from the light model showed that the upper limit of the variability in individual branch-estimated GS could be attributed to incoming light, but not the variation below that upper limit. A porous medium model of water flow in trees produced a pattern of variation below the upper limit that was consistent with the observed variability in branch-estimated GS. Our results indicated that stems acted to buffer leaf- and branch-level variation and might transmit a less-variable water potential signal to the roots. [ABSTRACT FROM AUTHOR]

ABSTRACTResponses of stomatal conductance (gs) to increasing vapour pressure deficit (D) generally follow an exponential decrease described equally well by several empirical functions. However, the magnitude of the decrease – the stomatal sensitivity – varies considerably both within and between species. Here we analysed data from a variety of sources employing both porometric and sap flux estimates of gs to evaluate the hypothesis that stomatal sensitivity is proportional to the magnitude of gs at low D (≤ 1 kPa). To test this relationship we used the function gs = gsref – m · lnD where m is the stomatal sensitivity and gsref = gs at D = 1 kPa. Regardless of species or methodology, m was highly correlated with gsref (average r2 = 0·75) with a slope of approximately 0·6. We demonstrate that this empirical slope is consistent with the theoretical slope derived from a simple hydraulic model that assumes stomatal regulation of leaf water potential. The theoretical slope is robust to deviations from underlying assumptions and variation in model parameters. The relationships within and among species are close to theoretical predictions, regardless of whether the analysis is based on porometric measurements of gs in relation to leaf-surface D (Ds), or on sap flux-based stomatal conductance of whole trees (GSi), or stand-level stomatal conductance (GS) in relation to D. Thus, individuals, species, and stands with high stomatal conductance at low D show a greater sensitivity to D, as required by the role of stomata in regulating leaf water potential. [ABSTRACT FROM AUTHOR]

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 (Qo). Measurements of xylem sap flux density (Js) indicated that: (1) 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 and Js was not significant; (2) Js at 20–40 mm depth in the xylem was 0.40 of that at 0–20 mm depth; and (3) Js 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 < 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. [ABSTRACT FROM PUBLISHER]