Ecological Determinants of Species Composition of Biological Soil Crusts on a Landscape Scale
Structure and composition of biological soil crusts varies with substrate, climate, and successional processes (see Chaps. 1–11), and specific factors controlling the distribution and abundance of crustal organisms may vary across different scales (see Ch
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17.1 Introduction Structure and composition of biological soil crusts varies with substrate, climate, and successional processes (see Chaps. 1–11), and specific factors controlling the distribution and abundance of crustal organisms may vary across different scales (see Chap. 16). On a landscape scale (i.e., from meters to kilometers), variation and limitation of soil crusts are mainly related to precipitation, soil chemistry and texture, topography, cover of phanerogamic vegetation, and disturbance. However, because of the problems with taxonomic identifications (see Chap. 1), most relevant studies omit analyses of floristic variation or treat only with a fraction of the flora present. The aim of this chapter is to demonstrate the variation of crusts along ecological gradients, and to present relationships between environmental factors and floristic composition (based on determination of uncultured field material) and distribution patterns of crusts dominated by lichens or cyanobacteria, respectively. The focus is on nonsuccessional studies in landscapes with distinct spatial patterns occurring frequently in semiarid and arid environments, especially ephemeral salt lakes (= playas, Arab.: sebkhas) and adjoining dune systems. Examples are presented from northern Africa and southern and central Australia.
17.2 Ecological Determinants on a Local Scale Soil chemistry and physical properties of soil, runon/runoff rain water displacement, vegetation cover, and topographic microsite variation are the most important nonanthropogenic environmental factors that influence floristic variation in soil crusts on a local scale (e.g., Danin and Barbour 1982; Harper and Marble 1988; Eldridge 1999; Malam Issa et al. 1999). Studies in semiarid areas in both the Northern and Southern Hemisphere (I. Ullmann et Ecological Studies, Vol. 150 J. Belnap and O.L. Lange (eds.) Biological Soil Crusts: Structure, Function, and Management © Springer-Verlag Berlin Heidelberg 2001
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al., unpubl. data) show that at playa margins, there is a similar pattern in shortdistance zonation of both soil crusts and phanerogamic vegetation. Species richness, biomass (expressed as chlorophyll a content), and dominant organisms of the biological crusts are correlated with substrate salinity, carboniferous or gypsic soil components, and texture of surface soil. Comparative studies in late dry season were done in north Africa (Tunisia) and south Australia in winter-rainfall areas of similar latitude (33°), similar annual rainfall (approx. 200 mm), and similar grazing and trampling pressure (moderate to high, sheep and goat versus kangaroos and rabbits, respectively). Soil salinity and soil surface texture apparently limit the occurrence of coherent soil crusts in the sebkhas of south Tunisia. An impressive example of zonation along a salinity gradient in calcareous substrates was obvious within the first 20 m of a transect of 1 m width at the southwestern margin of the small Sebkha Mhabeul (Table 17.1). This transect went from the i
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