Development of an Index of Balsam Fir Vigor by Foliar Spectral Reflectance

The potential to measure indices of forest vigor from foliar spectral reflectance was assessed in a range of balsam fir conditions achieved through stand manipulations. In order of increasing vigor, treatments consisted of root pruning, control, thinning, and thinning in combination with fertilization. Concentrations of chlorophyll and nitrogen increased significantly from the low to high vigor plots, and shoot lengths increased by more than three times. Balsam fir shoots from each treatment were measured for reflectance under laboratory conditions by a portable spectroradiometer with a spectral range from 350 nm to 2500 nm. Reflectance decreased consistently with vigor throughout the spectral region analyzed. Most wavebands showed significant effects of the treatments, with the most significant wavebands in the region of chlorophyll absorption between 500 nm and 740 nm. Foliage age class and sample date also affected reflectance in these wavebands. Reflectance indices, defined as the ratio and normalized difference of the most significant waveband at 711 nm and the least significant waveband at 913 nm, were affected by the treatments (P<0.0001). The normalized difference index correlated strongly with concentrations of chlorophyll a (r2⩾0.75), chlorophyll b (r2⩾0.66), nitrogen (r2⩾0.52), and shoots lengths (r2⩾0.69), thereby comprising a spectral index of balsam fir vigor that integrates several aspects of forest physiological condition.