title: 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-1tree-1). 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 m2of 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-1that is equivalent to 112 mmol m-2s-1at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m-2s-1at the top and 150 mmol m-2s-1at 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.
authors.affiliation: Lehrstuhl Pflanzenökologie and Bayreuther Institut für Terrestrische Ökosystemforschung, P.O. Box 10 12 51, W-8580, Bayreuth, Federal Republic of Germany.
authors.forename: B M M
authors.initials: BM
authors.lastname: Köstner
authors.affiliation: Lehrstuhl Pflanzenökologie and Bayreuther Institut für Terrestrische Ökosystemforschung, P.O. Box 10 12 51, W-8580, Bayreuth, Federal Republic of Germany.
authors.forename: E -D
authors.initials: E-
authors.lastname: Schulze
authors.affiliation: Forest Research Institute, P.O. Box 31-011, Christchurch, New Zealand.
authors.forename: F M
authors.initials: FM
authors.lastname: Kelliher
authors.affiliation: Forest Research Institute, P.O. Box 31-011, Christchurch, New Zealand.
authors.forename: D Y
authors.initials: DY
authors.lastname: Hollinger
authors.affiliation: Forest Research Institute, P.O. Box 31-011, Christchurch, New Zealand.
authors.forename: J N
authors.initials: JN
authors.lastname: Byers
authors.affiliation: Forest Research Institute, P.O. Box 31-011, Christchurch, New Zealand.
authors.forename: J E
authors.initials: JE
authors.lastname: Hunt
authors.affiliation: Forest Research Institute, P.O. Box 31-011, Christchurch, New Zealand.
authors.forename: T M
authors.initials: TM
authors.lastname: McSeveny
authors.affiliation: Lehrstuhl Pflanzenökologie and Bayreuther Institut für Terrestrische Ökosystemforschung, P.O. Box 10 12 51, W-8580, Bayreuth, Federal Republic of Germany.
authors.forename: R
authors.initials: R
authors.lastname: Meserth
authors.affiliation: Forest Research Institute, P.O. Box 31-011, Christchurch, New Zealand.
authors.forename: P L
authors.initials: PL
authors.lastname: Weir
keywords: Canopy conductance; Canopy transpiration; Humidity response of stomatal; Nothofagus; Xylem sap flow