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Ben-Gurion University of the Negev (2014)

Identifying and characterizing biocrusts using spectroscopy

Rozenstein Offer

Titre : Identifying and characterizing biocrusts using spectroscopy

Auteur : Rozenstein Offer

Etablissement de soutenance : Ben-Gurion University of the Negev

Grade : Doctor of Philosophy (PhD) 2014

Résumé partiel
Biocrusts are critical components of desert ecosystems, significantly modifying the surfaces they occupy. The mixture of biological components and soil particles that form the crust, in conjunction with moisture, determines the biocrusts’ spectral signatures. This dissertation’s aim is to study biocrusts using spectroscopy applied at close range from remote sensing platforms, in combination with complementary methods. The specific objectives are : (A) To explore how the establishment of biocrusts is affected by the substrate’s sand-grain-size distribution by comparing the rate and dispersal patterns of inceptive cyanobacterial biocrusts on different grain-size fractions. (B) To characterize the spectral signatures of different biocrusts using emission spectroscopy in the TIR region, with respect to dune sand. (C) To develop a spectral crust index based on LWIR data and compare its performance to the original crust index based on reflective data (Karnieli, 1997). (D) To demonstrate the advantage of using high spectral resolution LWIR remote sensing over mono/dual band methods. (E) To explore the potential for data fusion of reflective and TIR data for land-cover mapping. (F) To quantify the diurnal variations in surface emissivity, as detected from space, over bare versus biocrusted sands and to explore the different dynamics between these two ecosystems. To fulfil objective A, reflectance spectroscopy was employed at close range alongside complimentary methods in a controlled experiment. Although the presence of fine soil particles is known to be conducive to the development of biocrusts and their recovery from disturbance, their influence on the inceptive development of biocrusts has not been studied empirically. In this study’s first chapter, the effect of substrate granulometry on the development of biocrusts was explored, under controlled laboratory conditions of light, soil humidity, and temperature. A cyanobacterial inoculum of Microcoleus Vaginatus was applied to five sand fractions in the range of 1 - 2000 µm. The results showed that the biocrusts developed more rapidly on the fine fraction (<125 µm) than on the coarser fractions. While the biocrust cover on the fine fraction was spatially homogenous, it was patchy and discontinuous on the coarse fractions. The difference in the pore size between the different fractions is suggested to be the reason for these discrepancies in biocrust development, since large pores between the particles of coarse soil restrict and regulate the filaments’ spreading. It was found that the spectral indices, namely the Normalized Difference Vegetation Index and the Brightness Index, were more sensitive to the biocrust development than the bio-physiological variables of the biocrusts (polysaccharides, protein, and chlorophyll contents). The faster biocrust development on the fine fractions could explain various biophysical phenomena in Aeolian environments, such as sand dune stabilization by biocrust and formation of vegetation rings. Présentation (BGU)

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