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Playing a cardinal role in surface stabilization and in carbon and nitrogen fixation, microbiotic crusts play a crucial role in arid regions where they may serve as useful biomarkers for wind power and wetness duration. This is especially the case on relatively unstable and infertile sand dunes in the Negev Desert where high correlations between the crust chlorophyll content and the daytime wetness duration were found. Yet, only scarce data are available as to the possible link between the chlorophyll content and other physical (color, thickness, strength, crack density, surface roughness and infiltrability) and biological (protein, carbohydrate, organic matter and species composition) factors, which determine, in turn, the crust type and its effect upon geomorphological and ecological processes. No data are available on crust type distribution. These were the aims of the current research.

When a cluster analysis was performed, five types of microbiotic crusts were defined, four of which were cyanobacterial (A-D) and one moss-dominated crust (E). The crusts differed in their physical and biological properties. They showed an increase in chlorophyll content, protein, carbohydrates and organic matter from A to E, with concomitant increase in species diversity, thickness, roughness and strength, but with some variables (crack density and infiltrability) showing a reversed trend at the moss-dominated crust. The increase in the biomass components of the crust and the gradual change of the physical properties are explained by the improved physical conditions (primarily wetness duration), which facilitates longer hours of photosynthetic activity and consequently the introduction of additional, more mesic species such as green algae, lichens and mosses. Extended wetness duration was found to shift the crust type from cyanobacterial to moss-dominated crust. The spatial distribution of the crusts, as verified by crust mapping, coincided with the daylight surface duration, which in turn was controlled by topography (aspect, angle and slope position). It implies that whereas initial physical conditions dictates species composition and thus crust type, the crust type in turn is responsible for characterizing the physical properties of the surface, which may largely affect ecological and geomorphological processes. (C) 2010 Elsevier B.V. All rights reserved.