Moisture and substrate stability determine VA-mycorrhizal fungal community distribution and structure in an arid grassland

The arid central dune held of the Namib Desert is characterized by a pronounced rainfall gradient across its west-east, 160 km breadth, and a correlated increase in sand stability and grass community complexity. In addition to these macro-gradients, micro-gradients of sand stability and available moisture across each dune slope result in stratified grass communities on the dunes. The effects of abiotic factors and plant associations on the community structure of VA-mycorrhizal fungi in a naturally arid and unstable grassland could thus be investigated. Mycorrhizal fungal communities associated with five grass species were sampled at sites located across the gradients. Diversity and abundance of spores, as well as percent mycorrhizal colonization of plant roots, were used to characterize the fungal communities and their plant specificity. Five Glomus species (Glomales) were associated with grasses at all sites, but no plant specificity was observed. Rather, the fungal communities varied in diversity and abundance both within a dune site and across the dune field. Regression analyses showed that spore abundance and colonization levels were significantly affected by abiotic factors. Sand stability affected spore abundance and thus determined the limits of distribution of the fungal community in the dune grassland. In contrast, colonization levels were primarily affected by moisture availability, and fungal growth and spore production following an isolated rain event were closely associated with moisture availability. A rapid and opportunistic growth response to moisture, production of resilient spores in response to declining moisture, and lack of plant symbiont specificity are characteristics which allow mycorrhizal fungal communities to function under hyperarid conditions. (C) 1997 Academic Press Limited