High spectral resolution reflectance of douglas fir grown under different fertilization treatments: Experiment design and treatment effects

Douglas fir [Pseudotsuga menziesii (Mirb.) Franco.] seedlings were grown with different fertilization treatments in an experiment designed to investigate the effects of foliar biochemistry on needle and canopy reflectance measurements. Potentially confounding effects of the covariance of canopy structure with foliar biochemical concentration were minimized by fertilizing after leaf expansion. Seedlings showed no significant differences in specific leaf area, % needle moisture, biomass, or LAI, but showed significantly different total nitrogen concentrations, and some differences in chlorophyll concentrations. Measurements were made of needle optical properties, and bidirectional reflectance was obtained of needles and of canopies. Canopy reflectance was acquired under sky illumination using two field spectroradiometers. Needle and canopy treatment differences were highly significant in the visible region. Some treatment differences were also found in broad-band regions in the infrared in the canopy reflectances. Narrow-band infrared differences were detected in needle first derivative spectra, and coincided with known protein absorption features. Needle and canopy bidirectional reflectances were similar, but narrow-band features found in the needle infrared spectra were not found in the canopy data. Two possibilities for the failure to detect these narrow features in the canopy data are a confounding effect from variability caused by illumination angle changes and/or the low signal-to-noise characteristics of the field spectroradiometer relative to the laboratory instrument. The suite of optical and structural measurements taken from the same set of trees should provide a useful data set for parameterizing canopy models aiming to predict high spectral resolution reflectance of canopies.