REFLECTANCE WAVEBANDS AND INDEXES FOR REMOTE ESTIMATION OF PHOTOSYNTHESIS AND STOMATAL CONDUCTANCE IN PINE CANOPIES

A field experiment determined the reflectance wavebands and indices wh ich corresponded most strongly with photosynthetic capacity in a mixed stand of loblolly pine (Pinus taeda L.) and slash pine (P. elliottii Engelm. var. elliottii). The 5-year-old pines ranged in height from 3 m to 6 m and formed an optically dense canopy. Variation in photosynth etic capacity was amplified by soil application of photosystem II herb icides diuron and bromacil to three of six experimental plots. Field m easurements began on 23 August 1994 and continued through 20 December 1994. Canopy reflectance and leaf physiological data were acquired dur ing mid to late morning for the sides of trees that generally received full sunlight. Net CO2 assimilation rate first regressed significantl y with reflectance on 5 October at wavelengths near 700 nm. In the 5 O ctober to 2 December period, assimilation rates approximated photosynt hetic capacity. When data were combined over this period, the ration o f reflectance at 701+/-2 nm with reflectance at 520+/-2 nm, or a norma lized difference vegetation index (NDVI) computed from these values, r egressed more strongly with photosynthetic capacity than first derivat ives of spectral reflectance or wavelength at the read edge inflection point. The narrow band NDVI accounted for 17-29% more variability in the data than NDVI that were based on simulated TM, MSS, AVHRR, or SPO T bands. As a result of its linear relationship with assimilation rate , stomatal conductance to water vapor also regressed strongly with the narrow band ratio. These results are explained by the high sensitivit y of reflectance near 700 nm to leaf chlorophyll content. The use of a narrow band centered near 700 nm along with a narrow or broad near in frared band in vegetation indices should provide increased accuracy in estimates of photosynthetic capacity and corresponding conductance fo r optically dense canopies. Evaluation of the influence of leaf area i ndex on these relationships will require further study. Published by E lsevier Science Inc., 1998.