ABSTRACT
Biological soil crusts (biocrusts) are formed by soilsurface communities of biota that live within, or immediately on top of, the uppermost millimeters of soil. They consist of cyanobacteria, algae, mosses, microfungi, and lichenized fungi (hereafter, lichens). Cyanobacterial and microfungal ›laments, rhizinae and rhizomorphs of lichens, and rhizinae and protonemata of bryophytes weave throughout the top few millimeters of soil, gluing loose soil particles
together (Figure 10.1). The intimate association between soil particles and organisms forms a coherent crust. A quantitative estimate of global biological crust cover is dif›cult to obtain and not yet available, but the worldwide coverage of the terrestrial surface by biocrusts is very high. In arid and semiarid areas, biocrusts may constitute up to or more than 70% of the living cover and dryland (hyper-arid, arid, semiarid, and polar deserts) ecosystems, where they often dominate, cover ~40% of the terrestrial land mass (Pointing and Belnap 2014).
