Báo cáo lâm nghiệp: "Water movement and its resistance in young trees of Cryptomeria japonica"

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Water movement and its resistance in young trees of Cryptomeria japonica H. Yahata Forestry, Faculty of Agriculture, Kyushu University, of Laboratory of Silviculture, Department Fukuoka, Japan materials to reduce from temperature Introduction errors were estimated gradients. Transpiration rates conductance to by the measurement of leaf water vapor and the ambient vapor deficit be- Information about water flow resistance is tween leaf and air. The water flow rates in the essential to understanding and simulating stem at different heights (0.5-2.5 m) were esti- water movement in trees (Yahata, 1987). mated using the relationships between the There are a number of papers concerned heat-pulse velocity, measured with an automa- tic multichannel recording system (Yahata, with it for some species but few for Cryp- 1984) and the water uptake rates from the tomeria japonica and no data are available severed basal stem at the end of a series of on the gradient of water potential in measurements of the intact tree. Sapwood intact stem. This study was undertaken conducting area was measured by using a dye to examine whether the resistance in (1% solution of acid fuchsine). stems would be regarded as substantially Water flow rate, Q, is customarily expressed constant all day long and to find a simple Ohm’s law analogy with resistance, R, as an equation to predict the effect of stem form and the water potential gradient, Δψ, in the fol- lowing equation (eqn. 1). Q (Δψ-ρgh)/R, on it. = where pgh is the gravitational potential at a height of h (m). By using the above equation with the water flow rate in the stem in place of Q and the gradients of water potential between Materials and Methods soil and leaves, .1" or between rootstock > 1 s- ’ and leaves, dyr the resistance of total path- ,, r- and between root and leaves, R,, way, bRetween sfac! 14 yr old C. japonica trees growing in a planta- soil and root, R were de- and S+n tion of high stand density about 6650 stems per termined, respectively. ha were used. Psychrometer sensors (Wescor Provided that the relative resistance r, (m- ), 2 PCT55-30) were used with an automated defined by Jarvis (1975), is constant recording system for measuring the water as throughout a stem with a length of(m), total potential of soil at a depth of 20 cm and root- resistance R,!a, (Pa can be written as ) 3 s.m- ’ stock at 10 cm, and a Scholander pressure follows (eqn. 2): f?! 1 (r,nlA) d/ r = 1,, /7 chamber for shoots. The sensors were placed = = where l = j J (1M) dl, which can be considered and sealed in small drilled holes in the stem a to be an index of resistance based on stem and rootstock. Diurnal variation of the ambient form, and A is the cross-sectional area of sap- temperature of the sensors was minimized to wood, and ri is the viscosity of water (N ). 2 m- s ’ within less than 1°C by the use of insulating
stock. In order to keep a steady state, it is Results considered that the measuring point for water flow should be located in the middle On the clear day of August 20th, when the of the range of the points for water soil was dry, the predawn water potential potential. Therefore, in the following expe- of leaves was 0.2!.3 MPa lower than the riment, the water potential gradients in the soil. On the other hand, the water potential stem between 0 and 3 m aboveground of rootstock was higher by about 0.1 MPa and water flow rates between the 2 points than the soil and began to decrease after were measured. As a result, no essential sunrise slowing after the leaves and be- diurnal changes of resistance were ob- came lower than the soil about 9:00. It served, and the R, the resistance be- , 3 was confirmed here, too, that water move- tween 0 and 3 m, was 2006 MPa-s-kg- . 1 ment occurred along the water potential The changes in l calculated with the a gradient of soil, rootstock and leaves cross-sectional areas of sapwood indicate during daytime, but reverse gradients of that l is very small in the lower part of the a water potential of about 0.1 MPa were stem and increases with height of the observed at night and in the early morning stem. The value of r,, estimated by eqn. 2 when the water flow declined. using the value of l up to 3.0 m and R,,3, a Fig. 1 shows the relationships between was 2.11 x -2. ! - 1 m 10 the water potential gradient and the water example of the calculation of eqn. As an movement in the tree. Linear regression effect of reducing the water flow 2, an 0.0608 MPa curves intersecting at pgh = pathway on the index of resistance, l , a on the axis of ordinates fitted the observa- was examined, providing that the cross- tions better. The computed resistance of sectional area of sapwood at 1 m high was total pathway, R of stem, R and of, X ,, a p s reduced to 5 cm and the permeability 2 the pathway from soil to rootstock, R S+n,r was lost with a thickness of 10 cm. It is were 8987, 7218 and 1769 MPa-s-kg- 1 clear that the influence was small com- =33 ( x10 MPa-s-m- respectively, by using ), pared to the resistance of water flow. the water flow in place of Q. When transpi- ration was used instead of the water flow, In Fig. 2, using the above equation, the resistances were slightly lower but resistance between stem base and 0.5 m there were no substantial differences in below the top of trees and the water flow the resistance. While the resistance mea- rates to the top shoot when the water sured in the forenoon was larger than that potential gradient was 1 MPa, were calcu- lated. In this calculation, the equation of in the afternoon, especially, for R when , X estimated with the water flow, they were relative stem form and the yield table published for C. japonica were used. At reversed when estimated with transpira- tion. This seems a quite predictable result the beginning of the growth stage, the when taking account of the time lag be- calculated resistance R increased with xcal tween water flow in the canopy and in the height growth up to about 5 m, and sub- stem. Furthermore, using the data of water sequently the increasing rate declined. flow in the stem, there was a larger diurnal The width of sapwood, which was con- variation of R than of R This variation sidered to be almost constant vertically X . sPac is considered to result from the fact that throughout the stem, did not affect the the relative distance of the measuring resistance and the water flow, but the point for water flow was very close to the stem forms did significantly affect the measuring point for water potential of root- resistance and the water flow.
Discussion and Conclusion real active water possibility of the uptake However, further study is by roots. neces- sary to include the possibility. It confirmed that there is a gradient of was Diurnal water potential along the pathway of water changes in the resistance to flow, but there was a reverse gradient be- water flow have been reported, the resis- tween soil and rootstock when the water tance tending to rise in the afternoon flow declined. This result might suggest the (Nnyamah et al., 1978). Nevertheless,
measuring point of water flow in relation to Although eqn. 2 for predicting the effect those of water potential could possibly of stem form and growth on the water cause the diurnal change. It is probable stress to the top shoot should be tested in that the resistance to water flow in the practice, it could provide a simple model stem is substantially constant for C. japo- of the effect on the water movement in nica. stems.
Yahata H. (1984} An automatic multichannel References recording system for a heat-pulse velocity tech- nique. J. Jpn. For Sac. 66 .241-246 Jarvis P. (1&75)Water transfer in plants. In: Heat Yahata H. (1987) Water relations charaotoris- and Mass Transfer in the Plant Environ- tics of Cryptomeria japonica D. Don (Vi). ment. Part 1. (de Vries DA & Afgan N.G., eds). A simulation model of water regime using the Scn Book Co., Washington, D,C., pp. 369 ’ 94 ta p parameters obtained by the P-V curve Nnyamah J.U., Black !;A. & Tan C. (1978) technique. J Fac. ttgrio: Kyushu itruV. 31, 235- Resistance to water uptake in a Douglas fir , 63-76 27 Soil Sel. 1 245 forest.