Báo cáo lâm nghiệp: "Hourly and daily variations of xylem sapflow in sweet chestnut coppices using a thermal measurement method"

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Hourly and daily variations of xylem sapflow in sweet chestnut coppices using a thermal measurement method V. B. Saugier Bobay Laboratoire d’Ecologie v6g6tale, bit. 362, Universite Paris-XI, 91405 France Orsay, Introduction time scales (Bobay and Granier, 1987): F = 42.84 x SA x !231; where F sapflow = (cm3!h-!); SA = sapwood area at heated probe level (cm K flow index (without dimen- ); 2 A new method for measuring xylem sap- == sions). K !7’M - !T’f’u!T’!: where dTM = = flow (Granier, 1985) has been tested to temperature difference without any sapflow calculate forest transpiration. Two cop- (°C); dT(u) temperature difference with sap- = pices of sweet chestnut (Castanea sativa flow u (°C). Mill.) in the south of Ile-de-France were Psychrometer and net-radiometer were in- studied. One of them was thinned to a stalled on a tower in the thinned coppice. Potential evapotranspiration Ep was calculated density of 3500 stems/ha, the other (6000 with using Priestley-Taylor’s equation (1972), a stems/ha) was used as a control. hourly time step: This paper presents preliminary results Cp=1.26x(zt/(zl+!)x(f?n/A) of hourly and daily variations of xylem where Rn net radiation (W-m- A change ); 2 = = sapflow in the thinned coppice. of saturation vapor pressure per °C (kPa- ); 1 C- O psychrometer constant (kPa- A ); 1 C- o y = = latent heat of vaporization of water (J ). 1 g- ’ Use of this equation is not usually recom- Materials and Methods mended for forests but was found useful in chestnut coppices that have large leaves and a relatively large boundary layer resistance. A xylem sapflow measurement sensor com- prises two cylindrical probes inserted into the conducting xylem of the trunk. One of the probes is heated at constant power, the other is Results and Discussion unheated and used as a reference. The tem- perature difference between both probes is a measure of sap velocity around the heated Hourly variations of xylem sapflow were probe. It is recorded using differential thermo- couples connected to a data logger. This recorded and compared during days with method allows investigations of the variability of different irradiances. the sap velocity within a stem or between On 5 day, maximum July 19!37, stems. Sapflow is calculated with the following a sunny formula and may be averaged on hourly or daily and minimum ) max (T ) min (T temperatures
respectively 28.4°C and 16.5°C, with were global radiation Rg of 2637 J.1 -d- 2 - CM a and a sapflow of 2.05 mm (Fig. 1). By 1 d- ’ contrast, on 31 July 1987, a cloudy day, 7-! 20.6°C, T 12.2°C, Rg 744 n i m = = = 1 -d- 2 J-cm- and sapflow 0.8 mm (Fig. 1 d- ’ = 2). Both Figs. 1 and 2 show an influence of net radiation on sapflow variations. With a daily time step, transpiration T is about equal to sapflow, as was shown by mea- surements of weight loss and water up- take by cut stems. Using sensors in 7 stems, the sapflow of the whole coppice was computed and plotted against the vapor pressure deficit (VPD) (Fig. 3). The relationship is quite good and reveals apparent limit to an Conclusion forest transpiration. Daily variations of sapflow F and Ep are shown in Fig. 4. Because of a particularly The method used here enabled continu- wet season, no water stress was found ous measurements of forest transpiration during 1987. The maximum transpiration throughout the entire growing season. was 2.58 mm-d- This is quite low in com- . 1 These results will be compared to a soil- parison with Ep values that reach 6 water balance approach using a neutron mm.d-I . This may be due to a low leaf probe and rainfall measurements. The data will be further analyzed in order to area index (2.8) caused by thinning and also by poor weather. derive a forest transpiration model.
Granier A. (198!) Une nouvelle m6thode pour References la mesure du flux de sbve brute dans le tronc des arbres. Ann. Sci. For. 42, 81-88 Bobay V. & Granier A. (1987) Etude de la trans- piration d’un taillis de chataignier !I’aide d’une Priestley C.H. & Taylor R.J. (1972) On the nouvelle m6thode thermique de mesure du flux assessment of surface heat flux and evapora- de s6ve. Bull. Group. Fr. Humidimetrie Neutro- tion using large scale parameters. Mon. Wea- nique 21, 33-44 ther Rev. 100, 81-92