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

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Báo cáo lâm nghiệp: "Variation of hydraulic conductance of under natural conditions some adult conifers" adult conifersVariation of hydraulic conductance of someunder natural conditions Granier, N. Breda, J.P. Claustres F. ColinA.INRA, Laboratoire de Bioclimatologie et Ecophysiologie Forestiares, Centre de Nancy, Champe- BP 35, 54280 Seichamps, Francenoux conductance; and 2) the influence of soilIntroduction drying on this parameter.Along the soil-tree-atmosphere system,liquid flow of water can be modelled satis- Materials and Methodsfactorily under many conditions by theOhm’s analog; in the liquid phase, the flowdepends upon water potential gradients Study sitesand hydraulic conductance from the soil to The experiments were carried out in the standsthe 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 withholdingwater relations in trees. incident rainfall with a plastic roof covering the We present here evaluations of some soil. of trees growinghydraulic conductanceunder natural conditions. Data were col- Sap flow measurementslected from different experiments involving A thermal device (Granier, 1985; 1987) allowedxylem 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 pathated through estimation of the sapwood area of potential (Pa); 1/ = predawn water potential 10the 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 hereare hourly averages.Xylem water potential ResultsThis parameter was measured with a pressurechamber on 1 yr old spruce and fir twigs and onpine needles. Data are averages of 2-6 mea- Fig. 1 shows an example of diurnal rela-surements made in sun-exposed and shadedparts of the crowns, on clear days at 2 h inter- tionships between sap flux density andvals; within-crown variability is not analyzed xylem water potential obtained on Abieshere, but the sampling procedure is very impor- from predawn to dusk. Loops are oftentant, 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) onEstimation L of hydraulic conductance gL spruce and larc:h. Nevertheless, Cohen etIn order to compare trees of different species, al. (1987) did not report this effect onage 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-rootwhere 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 ...