A spectral analysis of 25 boreal tree species

Abstract

A spectral analysis of 25 boreal tree speciesHovi A., Raitio P., Rautiainen M. (2017). A spectral analysis of 25 boreal tree species. Silva Fennica vol. 51 no. 4 article id 7753. 16 p. https://doi.org/10.14214/sf.7753 Highlights• An extensive spectral library containing leaf and needle reflectance and transmittance spectra was collected.• The spectra openly available in SPECCHIO Spectral Information System.• Effects of tree species, leaf/needle side, canopy position, and needle age on spectra were quantified.• Seasonal variations were measured for four species.• Spectra analysis highlights the importance of shortwave-infrared region in separating tree species. AbstractSpectral libraries have a fundamental role in the development of interpretation methods for airborne and satellite-borne remote sensing data. This paper presents to-date the largest spectral measurement campaign of boreal tree species. Reflectance and transmittance spectra of over 600 leaf and needle samples from 25 species were measured in the Helsinki area (Finland) using integrating sphere systems attached to an ASD FieldSpec 4 spectroradiometer. Factors influencing the spectra and red edge inflection point (REIP) were quantified using one-way analysis of variance. Tree species differed most in the shortwave-infrared (1500-2500 nm) and least in the visible (400-700 nm) wavelength region. Species belonging to same genera showed similar spectral characteristics. Upper (adaxial) and lower (abaxial) leaf sides differed most in the visible region. Canopy position (sunlit/shaded) had a minor role in explaining spectral variation.For evergreen conifers, current and previous year needles differed in their spectra, current-year needles resembling those of broadleaved and deciduous conifers. Two broadleaved species were monitored throughout the growing season (May-October), and two conifers were measured twice during summer (June, September). Rapid changes were observed in the spectra in early spring and late autumn, whereas seasonal variations during summer months were relatively small for both broadleaved and coniferous species. Based on our results, shortwave-infrared seems promising in separating tree species, although it is to-date least studied. The spectral library reported here (Version 1.0) is publicly available through the SPECCHIO Spectral Information System.

Spectral Properties of Coniferous Forests: A Review of In Situ and Laboratory Measurements

“…Coniferous needles have lower transmittance than broad leaves throughout all wavelengths [24][25][26][27], which results in larger absorption for coniferous needles. Several authors have also emphasized that SWIR reflectance for coniferous needles is low compared to broad leaves [11,25,26]. For VIS and NIR reflectance, the differences are not as evident.…”

“…Therefore, it is not surprising that interspecies differences in spectra have been observed [11,21,26,27,[30][31][32]. Drawing general conclusions about interspecies differences and their magnitude compared to other factors is however difficult, because most studies (Table 1, Appendix A) are local, with only few species and sites measured, and data across campaigns cannot be readily compared, due to methodological differences.…”

“…Drawing general conclusions about interspecies differences and their magnitude compared to other factors is however difficult, because most studies (Table 1, Appendix A) are local, with only few species and sites measured, and data across campaigns cannot be readily compared, due to methodological differences. Studies that have examined a large number of species, using comparable methodology for each, clearly show that species is important, although not the only factor explaining spectral differences [11,30,33]. For example, for a set of 12 boreal species belonging to five taxonomic genera, species explained up to 71%, 70%, and 78% of variability in the reflectance, transmittance, and albedo spectra, respectively [11].…”

“…Studies that have examined a large number of species, using comparable methodology for each, clearly show that species is important, although not the only factor explaining spectral differences [11,30,33]. For example, for a set of 12 boreal species belonging to five taxonomic genera, species explained up to 71%, 70%, and 78% of variability in the reflectance, transmittance, and albedo spectra, respectively [11]. On the other hand, a study containing 18 plant species (of which 7 were conifers) in California, USA, reported that species explained 20%, 80%, and 47% of variability in the three first principal components derived from the reflectance spectra [33].…”

“…Thin and long needles, arranged in whorls or shoots, are responsible for the high levels of multiple scattering of photons occurring in coniferous canopies [7,8]-a phenomenon not seen in broadleaved forests. The thin-needle structures have also proven to be a challenge when spectral reference libraries of coniferous species are collected using standard spectral measurement techniques originally developed for broadleaved tree or crop species [9][10][11]. In addition to the three-dimensional (3D) arrangement of foliage in the canopy and optical properties of leaves, a significant contribution to the remotely sensed reflectance of coniferous forests is produced by the understory (or forest floor) layer e.g., [12,13].…”