Spectral properties of foliose and crustose lichens based on laboratory experiments Robert Bechtel, Benoit Rivard * , Arturo Sa ´nchez-Azofeifa Earth Observation Systems Laboratory (EOSL), Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 Received 30 October 2001; received in revised form 25 April 2002; accepted 28 April 2002 Abstract Reflectance spectra of rock encrusting lichens were acquired to determine the influence that this vegetation type may have on the reflectance properties of rock exposures located in high latitude and subarctic environments. The samples investigated consist of crustose and foliose lichen species collected from exposures of the Gog quartzite formation in Alberta, Canada. Lichen transmittance was estimated to be < 3% throughout the 350 – 2500-nm spectral region, using spectra measured from the foliose lichen, Umbilicaria torrefacta, as a representative sample of a broader class of lichens. These findings suggest that lichen prevents the transmission of light to the underlying rock substrate. Therefore, the subpixel influence of lichen and rock within a scene can be considered linearly weighted. Discrimination of lichen species is made possible using ratios of reflectance at 400/685 and 773/685 nm. An index using the band ratios 2132/2198 and 2232/ 2198 nm shows the similarity of lichen spectra in the infrared and a distinguishing feature between rocks with OH bearing minerals and lichen. Thus, spectral unmixing of rock and crustose/foliose lichens may be successfully accomplished using a single lichen end-member for this spectral range. D 2002 Elsevier Science Inc. All rights reserved. 1. Introduction Hyperspectral remote sensing systems are becoming increasingly available for regional geological mapping and mineral exploration where cost saving measures are key to commercial competitiveness (Kruse, 1999; Staenz, Szeredi, & Schwarz, 1998). The mixture of several materials within individual pixels can complicate the analysis of multi and hyperspectral information, often masking the diagnostic spectral features of materials of interest and hampering their classification. A widespread example of this problem in high latitude, subarctic regions is the ubiquitous presence of lichens covering exposed rocks that may compromise the ability to map the reflectance signatures of minerals from imaging spectrometer data (Rivard & Arvidson, 1992). In tundra and open woodland habitats, lichens and mosses can cover an area by as much as 70% (Solheim, Engelsen, Hosgood, & Adreoli, 2000), making it difficult to develop comprehensive mapping exercises aimed at resource extrac- tion. Fortunately, the use of spectral mixture analysis (SMA) as described by (Mustard & Sunshine, 1999; Smith, Ustin, Adams, & Gillespie, 1990) addresses the complexity of target identification within mixed pixels and can allow detection of substances exposed at subpixel resolution. Typically this approach assumes that mixed spectra result from the linear combination of spectral end-members (Singer & McCord, 1979). The spectra of end-members are either extracted from the imagery or measured in the laboratory or in the field. Rock coatings (nonbiogenic and biogenic) are scatter- ing/transmitting layers with optical thickness that can vary with material properties and wavelength. Lichens and desert varnish are examples of biogenic and nonbiogenic rock coatings. Desert varnish is a nonbiogenic patina of mixed-layer illite clays and nanocrystalline iron and man- ganese oxides partially covering rock surfaces in Earth’s deserts (Potter & Rossman, 1977; Rivard & Arvidson, 1992; Sultan, Arvidoson, Sturchio, & Guiness, 1987). We know that varnish can, in instances, completely mask the spectral signature of underlying rock material, but in general, it is optically thin (between 400–2500 nm) to the underlying bedrock (Rivard & Arvidson, 1992). The use of a representative spectral end-member for varnish in SMA and the simplifying assumption of linear mixing 0034-4257/02/$ - see front matter D 2002 Elsevier Science Inc. All rights reserved. PII:S0034-4257(02)00055-X * Corresponding author. Tel.: +1-780-492-0345; fax: +1-780-492-2030. E-mail address: benoit.rivard@ualberta.ca (B. Rivard). www.elsevier.com/locate/rse Remote Sensing of Environment 82 (2002) 389 – 396