Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements

Abstract

Tree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg day tree). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m of sapwood per kg of water transpired over one hour.Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s that is equivalent to 112 mmol m s at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m s at the top and 150 mmol m s at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was -27.76±0.27‰ (average ±s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.

Evapotranspiration of a declining Quercus robur (L.) stand from 1999 to 2001. I. Trees and forest floor daily transpiration

“…When integrated to stand scale, the control plot transpired more, as far as oak is concerned, mainly because of that species greater density. The inter-tree variability, estimated through the relative contribution of each tree to total daily SFD, confirmed that in thinned and/or declining stands SFD is more heterogeneous [6,25,26]. Our results in the thinned plot showed that from year to year, despite its SA or crown area, a tree SFD may be ranking upwards or downwards depending on biological events like caterpillar attacks and their consequences on LAI.…”

Crop coefficient approaches based on fixed estimates of leaf resistance are not appropriate for estimating water use of citrus

“…Although calibration focused on the wound width, it will also account for possible uncertainties in sapwood moisture content, sapwood density and the upscaling from individual trees to orchard scale. Similar sap flow calibrations have been performed by Williams et al (2004) in an olive orchard assuming evaporation from the soil is negligible and by using a combination (Köstner et al 1992). The advantage of this method of calibration is that it can be performed in field without the use of weighing lysimeters, which although ideal, are expensive to install and require a number of years for the tree planted in the lysimeter to reach an adequate size for measurement.…”

Sensitivity of mean canopy stomatal conductance to vapor pressure deficit in a flooded Taxodium distichum L. forest: hydraulic and non-hydraulic effects

“…Measured sap flux (J S , kg H 2 O m -2 sapwood s -1 ) in the xylem of trees is increasingly used to estimate G S (Köstner et al 1992;Granier and Loustau 1994;Phillips and Oren 1998;Oren et al 1998aOren et al , 1998b, based on the assumption that J S scaled by A S :A L is equal to transpiration rate per unit of leaf area (E L ). Thus, G S for conifers or other species with small leaves (Landsberg 1986;Phillips and Oren 1998;Ewers and Oren 2000) can be calculated using the function (modified from Monteith and Unsworth 1990): (2) where G S is the canopy stomatal conductance for water vapour (m s -1 ), γ is the psychrometric constant (kPa·K -1 ), λ is the latent heat of vaporization (J·kg -1 ), ρ is the density of air (kg·m -3 ), c p is the specific heat of air at constant pressure (J·kg -1 K -1 ), and D is the vapor pressure deficit (kPa).…”