Báo cáo khoa học: "Evapotranspiration measurements in a Mediterranean forest stand by means of ecophysiological and microclimatic techniques"

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Evapotranspiration measurements in a Mediterranean forest stand by means of ecophysiological and microclimatic techniques M. Sabatti R. Valentini, G.E. Scarascia-Mugnozza Istituto Universiti della Tuscia, Via De Lellis 4, 01100 Viterbo, Italy Biologico Selvicolturale, Materials and Methods Introduction In the Mediterranean region, dry summers The forest stand is located in central Italy, consists mainly of oaks (Quercus cerris L.) and quite common and water availability is are is managed as coppice with standard trees ori- one of the most important factors deter- ginated by gamic reproduction. The stand av- mining forest growth and development. erages 2400 sprouts and 97 standards of 2 age Furthermore, the lack of water for public classes per ha (corresponding to the double use experienced in central and southern and single harvesting cycle, respectively). The leaf area index (LAI ) of the whole stand is 4.5 Italy during the summer period, makes the (Scarascia-Mugnozza et al., 1989). analysis of the water cycle in forest and its impact on watershed management impor- Energy balance measurements were carried tant. by means of an exchange mechanism car- out rying 2 ventilated ceramic wick psychrometers The techniques available today for (Gay, 1971). Net radiation was measured with a net radiometer (Middleton Inc.). All the micro- determining water consumption by forest meteorological data were collected every 10 stands range from large scale hydrological min by a data logger (HP3421 a) connected to a balances to transpiration measurements microcomputer (HP71Net radiation was as- of an individual leaf. Among these, the sumed to be mainly dissipated by the latent and the sensible heat fluxes. In order to compare heat pulse velocity technique (hpv) for the 2 methods, evaporation from the soil was measuring sap fluxes seems to be simple considered negligible. and reliable (Marshall, 1958; Swanson, 1962; Granier, 1985), but different ap- Heat pulse velocity was measured with a proaches have to be compared, in order to device constituted by 2 temperature probes (0.5 mm diam. J-type thermocouples) inserted determine its degree of accuracy. In this into the xylem and one heat source (1 mm paper we compare the energy balance/ diam.) placed 2 cm from the downstream sen- Bowen ratio and the hpv technique for sor and 1.6 cm from the upstream one. hpv determining water consumption of an oak data were collected every 10 min together with micrometeorological measurements. stand.
beginning of the season, the predawn The calibration procedure for the hpv system consisted of measuring in the field the water potential was about -0.5 MPa water uptake of sampled trees, which represented the for both sprouts and standards and it averages of the size classes. A plastic tank was reached minimum values (-2.6, -2.8 MPa attached around the stem and filled with water. for sprouts and standards, respectively) The bark and the first xylem rings were cut in the water in order to allow water uptake. At tha during September. same time, hpv data were collected. The sap- wood cross sectional area was determined by rates measured with the Transpiration adding a blue dye to the water and counting balance/Bowen ratio method energy with a microscope the number and area of ves- ranged from 1.6.3 to 4.58 mm-d- At the . 1 sels filled by the solution. The relationship be- beginning of the season, latent heat flux tween the real sap velocity ( Vs) and the heat pulse velocity (Vhpv) is presented in Fig. 1. The was 78% of net radiation corresponding to fitted line has an R 2 regression coefficient of 2.91 mm-d- it reached a maximum in , 1 0.88. July (4.58 mm-d- while during the water ), 1 stress periods it was a much smaller (3’7%), corresponding to percentage 1.66 mm-d- These values are quite simi- . 1 Results and Discussion lar to those reported in another study on the hydrological balance of this forest for the years 1984--85 (Scarascia Mugnozza The seasonal behavior of hpv and energy et al., 1988). balance measurements together with the hpv measurements, integrated over the plant water status (predawn water poten- stand, p!!ralleled the energy bal- tial) are presented in Fig. 2. After an initial whole ance data: 3.42 mm-d- at the beginning 1 increase due to a large evaporative of the season, 4.25 mm in July and 1 d- ’ demand at the beginning of July, transpira- 1.63 mm-d- during the maximum water I tion measured by Bowen ratio and hpv stress period. A calculated regression line techniques decreased continuously fol- lowing the water stress evolution. At the between hpv and energy balance/Bowen
higher than our data (Rauner, 1976) but good correlation be- ratio data shows a 2 these studies mainly relate to more humid techniques, presenting an R tween the 2 climates, where transpiration rates are not coefficient of 0.84 (Fig. 3). limited by water stress and to forests with Transpiration rates of deciduous forests higher LAI. reported in the literature are somewhat
ture and rapid variations of micrometeoro- hpv values ranged from 0 to 50 m-h- . 1 logical parameters. These values are rather high if compared with those of conifers (Lassoie et al., This particular behavior, not evident in 1977), which were in the order of conifer trees, has also been shown by Mil- 0.2-1 m-h- but they are similar to the , 1 ler et al. (1980), who observed fluctuations ones found in other ring-porous species in sap velocities in Q. alba L. and Q. velu- (Miller et al., 1980). tina Lam. of the same magnitude as those we have found in Q. cerris L. A detailed analysis of hpv data taken every 10 min (Fig. 4) revealed several fluctuations throughout the day rather than constant behavior. Since this particular a References pattern could be determined by experi- mental errors, the real sap velocity ( Vs), Gay L.W. (1971) On the construction and use of obtained with the calibration procedure, ceramic wick thermocouple psychrometers. In: Psychrometry in Water Relations Research. was compared with the heat pulse velocity (Brown W.R. & B/an Haveren B.P., eds.), Utah (Vhpv). The coincidence of many of the Agric. Exp. Stn., Logan, Utah, pp. 365 peaks of the 2 fluctuating patterns shows Granier A. (1985) Une nouvelle m6thode pour that the technique is reliable and the la mesure du flux de s6ve brute dans le tronc observed irregular transpiration stream des arbres. Ann. Sci. For. 42, 193-200 could be determined by other factors re- Lassoie J.P., Sc;ott D.R.M. & Fritschen L.J. lated to xylem cavitations, crown architec- (1977) Transpiration studies in Douglas fir using
the heat pulse velocity technique. For. Sci. 23, Scarascia Mugnozza G., Valentini R., Spinelli 377-390 R. & Giordano E. (1988) Osservazioni sul cicio dell’acqua in un bosco di O. cerris L. Ann. Marshall D.C. (1958) Measurement of sap flow Accad. ltal. Sci. For. 37, 89-105 in conifers by heat transport. Plant Physiol. 33, 385-396 Scarascia-Mugnozza G., Valentini R., Giordano Miller R.D., Vavrina C.A. & Christensen T.W. E. & Del Lungo A. (1989) Modelling the coppice (1980) Measurement of sap flow and transpira- stand structure: an ecophysiological approach. tion in ring-porous oaks using a heat pulse velo- Ann. Sci. For. 46 suppl., 519s-522s city technique. For. Sci. 26, 485-494 Rauner J.L. (1976) Decidous forest. In: Vege- Swanson R.H. (1962) An instrument for de- tation and the Atmosphere. (Monteith J.L., ed.), movement in woody plants. U.S. tecting sap Academic Press, London, pp. 241-264 For. Ser. Res. Pap. RM-&8 Dept. Agric.