A revised measurement methodology for spectral optical properties of conifer needles

A procedure for determination of conifer needle spectral optical properties (transmittance, T_λ; reflectance, R_λ; λ=wavelength) was developed to support field measurements acquired in the Boreal Ecosystem-Atmosphere Study (BOREAS). This was a revision of a protocol, C.S. Daughtry et al. (1989), which uses an indirect and labor-intensive step (involving painting of needles) to estimate the inter-needle light transmittance gaps, or gap fraction (GF), in each sample. The present authors procedure uses a direct image capture method to calculate GF, enabling measurements on both dorsal and ventral surfaces of all samples, and is 3-4 times faster than. However, for either method T_λ underestimates often result, including negative T_λ in the visible (VIS) spectrum, especially when GF is large (>25%). The authors performed controlled experiments to evaluate the general effect of GF, and errors in GF estimation, on the calculated T_λ and R_λ spectra. T_λ was found to be inversely related to GF, with larger coefficients associated with VIS than near-infrared (NIR) λ. Consequently, GF overestimates also yielded T_λ underestimates. Using these results, they developed a correction algorithm for their BOREAS measurements. “Corrected” estimates of the fraction of absorbed photosynthetically active radiation (APAR) were ~80-84% for jack pine and 75-78% for black spruce. Correction reduced the Simple Ratio (SR=VIS/NIR) by 40-60% in most cases. SR calculated from corrected T_λ spectra were significantly higher than those determined from R_λ spectra