THE NORMALIZED DIFFERENCE VEGETATION INDEX OF SMALL DOUGLAS-FIR CANOPIES WITH VARYING CHLOROPHYLL CONCENTRATIONS

The normalized difference vegetation index (NDVI), calculated from reflected red and near-infrared radiation, has been related to various vegetation properties, including leaf area index, light absorption capacity, and photosynthetic potential. In an experiment with miniature canopies of 1-m-tall Douglas-fir (Pseudotsuga menziesii) seedlings, we modified leaf area index, light absorption capacity, and photosynthetic potential by altering the concentration of chlorophyll in foliage and by controlling the density of seedlings. We measured canopy photosynthesis and light transmission in controlled-environment chambers and then transferred seedlings to a hemispheric illumination system where we measured canopy reflectance. We found that altering the visible band used for computation of a normalized vegetation index substantially changed the correlations between the index and canopy properties. For example, the normalized index was best correlated to light absorption capacity when we used a narrow red band (671-674 nm; R2 = 0.71), and least correlated when we used a narrow green band (565-575 nm; R2 = 0.27). On the other hand, the index computed with the 565-575 nm band was best correlated with the photosynthetic potential of canopies grown in sunlight (R2 = 0.84), and the correlation was lower when a narrow red band was used (R2 = 0.37). The cause of these differences is chlorophyll. The green regions of reflectance spectra were much more sensitive to changes in chlorophyll concentration compared with the red or near-infrared regions. Increased chlorophyll concentration was also related to increased photosynthetic potential when canopies had been grown under full sunlight. However, we found no statistically significant relationship between leaf chlorophyll concentration and canopy light absorption.