Reflectance Wavebands and Indices for Remote Estimation of Photosynthesis and Stomatal Conductance in Pine Canopies

A field experiment determined the reflectance wavebands and indices which 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 photosynthetic capacity was amplified by soil application of the photosystem II herbicides diuron and bromacil to three of six experimental plots. Field measurements began on 23 August 1994 and continued through 20 December 1994. Canopy reflectance and leaf physiological data were acquired during mid to late morning for the sides of trees that generally received full sunlight. Net CO2 assimilation rate first regressed significantly with reflectance on 5 October at wavelengths near 700 nm. In the 5 October to 2 December period, assimilation rates approximated photosynthetic capacity. When data were combined over this period, the ratio of reflectance at 701±2 nm with reflectance at 820±2 nm, or a normalized difference vegetation index (NDVI) computed from these values, regressed more strongly with photosynthetic capacity than first derivatives of spectral reflectance or wavelength at the red 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 SPOT 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 sensitivity 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-infrared band in vegetation indices should provide increased accuracy in estimates of photosynthetic capacity and corresponding conductance for optically dense canopies. Evaluation of the influence of leaf area index on these relationships will require further study.Published by Elsevier Science Inc., 1998