An Analytical Approach to the Expression of Light Intensity in Plant Communities
1962; Society of Agricultural Meteorology of Japan; Volume: 17; Issue: 4 Linguagem: Inglês
10.2480/agrmet.17.143
ISSN1881-0136
Autores Tópico(s)Greenhouse Technology and Climate Control
ResumoAn instance that plant communities can not always be considered as a homogeneous layer is shown through applying Fresnel's formulae to plant communities in order to obtain albedo from extinction coefficient. Another expression of plant communities is presented with construction of a simple model of plant communities employing vector relations. It is shown from investigation of increase in intensity of light by a leaf that the equation relating amount of increase in averaged intensity with incident averaged intensity is expressed by use of LAI. Similarly, increase in averaged intensity by stem is expressed by use of area of longitudinal section per unit area. Extension of these equations to conditions of plant communities where leaves and stems are continuously distributed in the vertical direction is attempted by employment of probability density functions of leaves and stems in each height. Integrating the differential equation under the assumptions that the distribution of leaves with respect to height is continuous and that the probalility density function of leaves depends only on the angle of leaves making with z-axis, intensity of light is expressed by a simple exponential function in each direction of incident beam. Extinction coefficient of a beam as a function of angle of incidence and of leaf arrangement is not generally monotonic. This property of extinction functions causes a trapping of radiation incident on plant communities with slanting leaves. On the basis of the extinction coefficent for a beam, the extinction coefficient of isotropic incident radiation is computed in the system with horizontal leaves and with vertical leaves. The computed extinction eoefficients in this manner are, respectively, 1 for the former and 0.5-0.4 for the latter system.The value in the former system approximates those experimentally obtained. A difference between the experimental and the computed extinction coefficient is considered as a result of neglect of reflection from and transmission through leaves of light. Reflection from plant communities is computed readily in the system with horizontal leaves, leading to the expression of albedo depending upon angle of incidece. In this case, it is also found that intensities of reflection can exceed those by a leaf in directions of the greater zenith angles of reflection.In order to verify these results on the extinction of a beam, shortwave radiation falling on a horizontal plane is measured in paddy fields with tube pyranometers just above and below the layers where most leaves are of horizontal habit. Observed diurnal variations of the ratios of shortwave radiation in the paddy fields to incident shortwave radiation are similar to those computed from the equation of light intensity. Quantitative comparisons of the observed dependence of extinction coefficients on incident angle with the computed are possible by use of observed shortwave and diffuse incident radiation and of the extinction coefficient for diffuse incident radiation, showing that the expression of light intensity of a beam in plant communities is a good approximation at least for the smaller zenith angles of sun. When the ratio of the averaged intensity of diffuse incident radiation to direct solar radiation is remained small, a linear relaionship between albedo and secant of zenith angle of sun is found from observations of albedo in paddy fields with upright leaves. Validity of the computed results encourages one to analyse communities of lowland rice. The analysis is based on a series expansion of logarithm of intensity ratio in terms of extinction functions. Results of the analyses show some characteristics due to difference in density. An extent, to which the computed result is valid, is inferred through a result of analyzing reflection from paddy field.The detailed description apprears in ISOBE, S. (1962): Preliminary Studies on Physical Properties of Plant Communities. Bull. Nat. Inst. Agr. Sci., se
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