Spectral reflectance of lichens and their effects on the reflectance of rock substrates

Abstract

Rock outcrops in arid and semiarid regions frequently have abundant lichen cover. We collected lichens and their granite, slate, and hornfels substrates in the Extremadura region of Spain and measured spectral reflectance in the 400–2500 nm region in the laboratory. Lichen cover as low as 30 percent masks the spectral features of spectrally flat rocks such as slate and hornfels. The sharp spectral features of granite are obscured at 60–80 percent lichen cover. The strong reflectance peak at 2220 nm in lichens interferes with recognition of the 2200 nm absorption band in hydroxyl‐bearing minerals that is used to locate hydrothermally altered and potentially mineral‐rich areas. However, lichen spectra are characterized by distinctive absorption features that occur in few minerals and can thus be used to identify lichen presence. From computer modeling of laboratory spectra, we conclude that Landsat Thematic Mapper (TM) band ratios of 3 : 4 and 5 : 2 best minimize lichen effects in TM image analysis.

Identification of a Robust Lichen Index for the Deconvolution of Lichen and Rock Mixtures Using Pattern Search Algorithm (Case Study: Greenland)

“…This steady increase is distinctly different from the reflectance curve of vascular plants, characterized by an abrupt increase from the visible to the near infrared and decreases slowly from 800 to 1300 nm (Ager and Milton, 1987).…”

“…The principle of spectral (un)mixing is that the resulting spectrum for an image pixel containing a mixture of several distinct materials will be a linear combination of their signatures (Kruse et al, 1993). The assumption of linear mixture of lichens and rock substrate is reliable as lichens prevent transmission of light to the underlying rock substrate (Ager and Milton, 1987;Bechtel et al, 2002).…”

“…Much of the existing knowledge on the reflectance spectra of lichens corresponds to the spectral resolution of multispectral airborne and spaceborne imaging systems such as Landsat MSS and TM sensors (Ager and Milton, 1987;Leverington and Moon, 2012) and Airborne Thematic Mapper (ATM) (Grebby et al, 2014). Evidently, more information is potentially available from high-resolution spectra of hyperspectral imagery.…”

“…Ager and Milton (1987) suggested band ratios of TM3:TM4 and TM5:TM2 from laboratory modelling to minimize lichen effects in TM images. Bechtel et al (2002) suggested a set of spectral indices to discriminate lichens and a set of spectral indices to guide the selection of a single lichen end-member for use in the Spectral Mixture Analysis (SMA) of rock and lichen.…”

Impact of Environmental Factors on the Spectral Characteristics of Lava Surfaces: Field Spectrometry of Basaltic Lava Flows on Tenerife, Canary Islands, Spain

“…The spectral reflectance of lava flows is also affected by lichen cover [25,26] and chemical weathering [26][27][28][29][30]. Compared to lava spectra, lichens are characterized by higher reflectance with several minima and maxima at specific wavelengths (e.g., [31][32][33]). Lichen cover on lava would therefore alter the pattern of lava spectra [24,33].…”