Biological soil crusts, known as biocrusts, are communities of microorganisms, fungi, algae, lichens, and mosses. Biocrusts, although they occupy the uppermost soil layer, play a crucial role in dryland ecosystems. Therefore, considerable effort has been put into biocrust mapping by remote sensing means. In this regard, several spectral crust indices were developed and tested in different sites worldwide – the Negev (Israel), Namib Desert, Spain, and Colorado.

Each of these indices relies upon various parts of the electromagnetic spectrum that are created by different physiological properties of the biocrusts and substrates. Among these properties, it is worth mentioning the scytonemin pigments that absorb radiation in the near ultraviolet (NUV) (associated with Scytonemin Crust Index, SCI), phycobilin pigments that reflect in the blue spectral region (associated with Crust Index, CI), chlorophyll absorption in the red (associated with Normalized Different Vegetation Index, NDVI), and cellulose absorption in the shortwave infrared (associated with Satellite Cellulose Absorption Index, SCAI).

An EnMAP and EMIT images were used to calculate and map the four spectral indices on both sides of the border (Fig. 1b,c,d). First, based on field spectrometer (ASD) measurements. Then, two multivariate statistical models were performed to compare multiple responses and explanatory variables: the Partial Least Squares – Discriminant Analysis (PLS-DA) and the Partial Least Squares – Regression (PLS-R). The former was utilized to define the significant difference between sands and biocrusts as well as between different locations. PLS-R and the Variable Importance in Projection (VIP) analyses were conducted to (1) confirm the physiological theories behind the three indices; and (2) refine the width of the critical wavelengths in which the spectra are primarily valuable for the indices.

PLS-DA and PLS-R results from various sites worldwide confirm the robustness of the most important spectral bands for biocrust monitoring and mapping in drylands. Detailed implementations of these spectral indices were demonstrated in the sand dunes of the Northwestern Negev (Israel) and the Namib Desert.