SMALL-SCALE STUDY OF 3-DIMENSIONAL DISTRIBUTION OF PHOTOSYNTHETICALLYACTIVE RADIATION IN A FOREST

We use the transport theory to simulate three-dimensional radiation di stribution in a vegetation canopy of a small area (ca. 0.1-0.3 ha). Th is theory is based on two contradictory assumptions (Ross, 1981). On t he one hand, the model resolution have to be so high that input variab les for the transport equation can approximate the given forest stand with necessary degree of accuracy. On the other hand, the transport th eory is based on the assumption that Beer's law can be locally applied to plant canopies that is valid for sufficiently large volumes filled with phytoelements. This sets a limit to the resolution and to the pr edicting accuracy, not only of our model, but also of any other using Beer's law. The aim of our paper is to estimate these limits as a func tion of input variables. A detailed analysis of input variables (canop y structure, optical properties of foliage elements and soil, radiatio n input at the canopy boundary) and of their effect on the radiative f ield underlie our investigations. A comparison of our three-dimensiona l simulation results with field measurements is also included in our p aper, not only to test the model, but also to illustrate the specifica tion of a model resolution and the accuracy of predicted radiative fie ld in a real small heterogeneous experimental site. The forest albedo is an important ecological variable characterising the forest scatteri ng capacity. To measure this variable, two hemispherical sensors are u sually mounted above the forest canopy. The first one records a downwa rd energy flux from the atmosphere, and the second, an upward irradian ce reflected by the forest. The ratio of their responses is usually in terpreted as the forest albedo. As an example, the model is used to qu antify an inadequacy of this interpretation by simulating both the sen sor response and the three-dimensional distribution of the radiation r eflected, and by comparing these results with measurements in the fiel d. (C) 1997 Elsevier Science B.V.