DESERT CRUST MORPHOLOGY AND ITS RELATIONS TO MICROBIOTIC SUCCESSION AT MT. SEDOM, ISRAEL

The upper surface of alluvial terraces at Mt. Sedom, Israel are covere d with a biogenic crust populated by filamentous cyanobacteria. The cy anobacteria expand when wet and shrink when dry, forming when time pas ses cracks in the soil surface in the form of polygons. These polygons are smaller than those formed by shrinking of the mineralogical compo nent of the substratum. Continuing to grow and expand laterally the cy anobacteria cause upfolding of the polygon margins which in time becom e populated with cyanophilous lichens. The lateral growth accompanied by increasing trapping of airborne dust in the cracks of thalli of cya nophilous lichens leads to the development of microridges along the fo rmer upturned margins of cracks. Increasing water storage in the depre ssions of the surface with augmenting roughness ameliorates the moistu re regime by decreasing water runoff from the soil surface. Cyanobacte ria of the early stages of colonization occur at the drought-latent st age below the surface of the flat soil and emerge phototactically when sufficiently wetted. Their fronds are green. The microbionts on the e levated terraces representing progressive, older stages of colonizatio n are situated above the rugged-surfaced soil and have dark thalli or fronds. The number of microbiont species, chlorophyll a, and polysacch aride content of the crust increase from the young to the old terrace. Calcite content, compaction, and linear structure increase too. All t hese quantitative changes lead us to regard the different stands as pa rts of microbiotic succession. We recommend the use of the micro-geomo rphological structures as age indicators which are correlated here wit h the relative time sequence system of alluvial terraces in other plac es, even if they are not in such an obvious chronosequence. (C) 1998 A cademic Press Limited.