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

The normalized difference vegetation index (NDVI), calculated from ref lected red and near-infrared radiation, has been related to various ve getation properties, including leaf area index, light absorption capac ity, and photosynthetic potential. In an experiment with miniature can opies of 1-m-tall Douglas-fir (Pseudotsuga menziesii) seedlings, we mo dified 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 photosynth esis and light transmission in controlled-environment chambers and the n transferred seedlings to a hemispheric illumination system where we measured canopy reflectance. We found that altering the visible band u sed for computation of a normalized vegetation index substantially cha nged the correlations between the index and canopy properties. For exa mple, the normalized index was best correlated to light absorption cap acity when we used a narrow red band (671-674 nm; R2 = 0.71), and leas t 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 be st correlated with the photosynthetic potential of canopies grown in s unlight (R2 = 0.84), and the correlation was lower when a narrow red b and was used (R2 = 0.37). The cause of these differences is chlorophyl l. The green regions of reflectance spectra were much more sensitive t o changes in chlorophyll concentration compared with the red or near-i nfrared regions. Increased chlorophyll concentration was also related to increased photosynthetic potential when canopies had been grown und er full sunlight. However, we found no statistically significant relat ionship between leaf chlorophyll concentration and canopy light absorp tion.