A revised measurement methodology for conifer needles spectral optical properties: Evaluating the influence of gaps between elements

Gaps are unavoidable when compositing small or narrow plant parts (e.g., co nifer needles, twigs, narrow leaves, and leaflets) on sample holders in pre paration for measuring spectral optical properties. The Daughtry et al. (19 89) (A new technique to measure the spectral properties of conifer needles. Remote Sens. Environ. 27:81-91.) method of measuring conifer needle optica l properties utilizes a relatively large gap fraction (approximately 0.3-0. 6) and needles painted black on one surface of the sample from which the ga p fraction of the sample is indirectly determined. Following this protocol typically results in distortions in optical properties, including underesti mates in transmittance (sometimes negative values), and only one surface of the sample can be measured. The objectives of this article are to: 1) eval uate the influence of gaps between sample elements (conifer needles, twigs, narrow leaves and leaflets) on optical properties calculated with the publ ished equations from Daughtry et al. (1989) and 2) revise the original Daug htry et al. method for optical property measurements by using an image-anal ysis to directly measure the gap fraction and use both surfaces of the samp le. We achieve these objectives by reviewing the theory and investigating t he effects of gaps by measurements of an inert photographic film material, fir needles, and mesquite leaflets. Tests to estimate the transmittance of film samples (film) and foliage (fir needles, mesquite leaflets) indicate t hat a relatively small gap fraction (less than 0.20) reduces the occurrence of computed negative transmittance values, reduces the variation in comput ed values, and yields values expected for the "true" or "nongap" transmitta nce. Employing the image analysis along with reduced gap fractions decrease d the variance of measurements and permitted measurements of both surfaces per sample, thus reducing the time required by making half as many samples as originally required by Daughtry et al. Published by Elsevier Science, In c.