VALIDITY OF SURFACE-AREA INDEXES OF PINUS-RADIATA ESTIMATED FROM TRANSMITTANCE OF THE SUNS BEAM

Total surface area indices of forests can be estimated using the transmittance of the direct beam of the sun. Usually, the transmittance is averaged linearly over an area or a line and the surface area index is estimated from this average of the transmittance. However, low values are obtained for surface area indices if the vegetation is clumped or has gaps between rows. In such cases, a procedure is recommended in which the averaging is carried out linearly over a small distance and then logarithmically. It is argued that this correctly accounts for gross discontinuities in canopies. This method gave estimates of surface area indices in a plantation of Pinus radiata which were 88% of the needle or 76% of the total surface area indices obtained from biomass and specific surface area measurements of needles. On the other hand, for the same comparisons, linear averages of the transmittance give values of the surface area indices which were 65% and 56% of the direct measurements, respectively.