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

Mark A. Mesarch, Elizabeth A. Walter-Shea, Gregory P. Asner, Elizabeth M. Middleton, Stephen S. Chan

Research output: Contribution to journalArticle

43 Scopus citations

Abstract

Gaps are unavoidable when compositing small or narrow plant parts (e.g., conifer needles, twigs, narrow leaves, and leaflets) on sample holders in preparation for measuring spectral optical properties. The Daughtry et al. (1989) (A new technique to measure the spectral properties of conifer needles. Remote Sens. Environ. 27:81-91.) method of measuring conifer needle optical 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 gap fraction of the sample is indirectly determined. Following this protocol typically results in distortions in optical properties, including underestimates in transmittance (sometimes negative values), and only one surface of the sample can be measured. The objectives of this article are to: 1) evaluate the influence of gaps between sample elements (conifer needles, twigs, narrow leaves and leaflets) on optical properties calculated with the published equations from Daughtry et al. (1989) and 2) revise the original Daughtry et al. method for optical property measurements by using an image-analysis to directly measure the gap fraction and use both surfaces of the sample. We achieve these objectives by reviewing the theory and investigating the 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 that a relatively small gap fraction (less than 0.20) reduces the occurrence of computed negative transmittance values, reduces the variation in computed values, and yields values expected for the 'true' or 'nongap' transmittance. Employing the image analysis along with reduced gap fractions decreased 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.

Original languageEnglish (US)
Pages (from-to)177-192
Number of pages16
JournalRemote Sensing of Environment
Volume68
Issue number2
DOIs
StatePublished - May 1 1999
Externally publishedYes

ASJC Scopus subject areas

  • Soil Science
  • Geology
  • Computers in Earth Sciences

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