Báo cáo lâm nghiệp: "Variation of hydraulic conductance of under natural conditions some adult conifers"

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adult conifers Variation of hydraulic conductance of some under natural conditions Granier, N. Breda, J.P. Claustres F. Colin A. INRA, Laboratoire de Bioclimatologie et Ecophysiologie Forestiares, Centre de Nancy, Champe- BP 35, 54280 Seichamps, France noux conductance; and 2) the influence of soil Introduction drying on this parameter. Along the soil-tree-atmosphere system, liquid flow of water can be modelled satis- Materials and Methods factorily under many conditions by the Ohm’s analog; in the liquid phase, the flow depends upon water potential gradients Study sites and hydraulic conductance from the soil to The experiments were carried out in the stands the mesophyll, which thus has an impor- described in Table I. tant significance for the description of In the A. bornmulleriana stand, 4 trees were subjected to an imposed drought by withholding water relations in trees. incident rainfall with a plastic roof covering the We present here evaluations of some soil. of trees growing hydraulic conductance under natural conditions. Data were col- Sap flow measurements lected from different experiments involving A thermal device (Granier, 1985; 1987) allowed xylem water potential and sap flow mea- continuous measurement of sap flux density, surements and we will analyze: 1) be- which was integrated along a radial axis into the sapwood of the trunk. Total sap flow was evalu- tween-tree variability of liquid path
ated through estimation of the sapwood area of potential (Pa); 1/ = predawn water potential 10 the tree at flowmeter level. Measurements were (Pa). Each summation was calculated over a complete daylight: period at 2 h intervals. made at 10 s intervals and data presented here are hourly averages. Xylem water potential Results This parameter was measured with a pressure chamber on 1 yr old spruce and fir twigs and on pine needles. Data are averages of 2-6 mea- Fig. 1 shows an example of diurnal rela- surements made in sun-exposed and shaded parts of the crowns, on clear days at 2 h inter- tionships between sap flux density and vals; within-crown variability is not analyzed xylem water potential obtained on Abies here, but the sampling procedure is very impor- from predawn to dusk. Loops are often tant, especially in dense stand conditions, noticed on clear days; they indicate non- where tree water potential shows a large varia- bility. steady state water transfer with a time lag between water potential and sap flow, as reported also by Schulze et al., (1985) on Estimation L of hydraulic conductance gL spruce and larc:h. Nevertheless, Cohen et In order to compare trees of different species, al. (1987) did not report this effect on age and stand conditions, we calculated the citrus trees. The slopes of these relation- specific hydraulic conductivity as follows: ship represent the inverse of gL, e.g., gL=EflE(W!-4%! (mol ) Pa- 1 s 2 m- ’ (1) hydraulic resistance from the soil-root where f= sap flux density (e.g., per unit of sap- interface to the twig xylem. wood area mol I water ) 1 s; 2 m- ’ ff, = xylem
conductivity, assuming that in a closed 1) was calculated for the 3 gL (see eqn. stand there is a linear relationship be- coniferous species under different water tween leaf area and sapwood cross-sec- stress conditions, as estimated by pre- tional area. Thus, the same values of gL dawn water potential (y/0) (Table II). obtained for trees ranging from interme- diate to dominant crown classes led us to Table 11 shows that: 1) for the same spe- think that mean vapor flux density across cies (see P. abies), between-tree variabili- the leaf surfaces was rather constant be- ty depended upon crown class: gL was tween these trees. It was much lower for rather constant for the dominant, co- rain-deprived trees. dominant, intermediate and isolate trees. It was much lower for rain-deprived trees; Water stress produced a decrease in 2) with increasing water stress (de- 3 between -0.18 and -1.14 4 gL by a factor creasing V/0 gL decreased continuously, ), MPa under our experimental conditions. as previously reported by Cohen et al. Two hypotheses can be proposed: 1) (1987); 3) maximum values of gL for the 3 drought increases the hydraulic resistance studied species were comparable even between soil and root; 2) drought reduces under different stand and climate condi- the hydraulic conductivity of sapwood. tions: 0.34 x 10- mol for P. 5’ .Pa- 1 s 2 m- Measurements of water potential at an abies, 0.40 x 10- for P. pinaster and 0.37 5 intermediate level, especially at the lower x 10- for A. bommulleriana. 5 part of the living crown, may be made on twigs of branches enclosed in a plastic bag (Hellkvist et al., 1974); they allow esti- mates of soil to mid-crown and mid-crown Discussion to twig hydraulic conductivity. A few measurements seems to indicate, as that the gL of the mid- first results, The specific hydraulic conductivity can be crown to twig path does not decrease as interpreted in terms of leaf specific
Granier A. (1987) Evaluation of transpiration in 5 10- water stress increases 0.50 (gL x = Douglas fir stand by means of sap flow mea- a moi-m-2-s-i-Pa!), reinforcing the first surements. Tree P!ys