Arbuscular mycorrhizas influence plant diversity and community structure in a semiarid herbland

The contribution of arbuscular mycorrhizas (AAA) to plant community structure and diversity is reported here in an annual herbland in southern Australia. Mycorrhizal colonization was reduced in field plots by applying the fungicide benomyl as a soil drench. The mycorrhiza-responsiveness of plant species was assessed in intact soil cores containing the indigenous AAA fungi and in a pot experiment using an isolate of Glomus mosseae. Glasshouse experiments showed that Medicago minima, Vittadinia gracilis and Velleia arguta were highly mycorrhiza-responsive, Salvia verbenaca became colonized but exhibited no growth response to AM, and Carrichtera annua remained uncolonized. There was no change in plant species richness in mycorrhiza-suppressed field plots but diversity increased owing to an increase in evenness. Treatment had no effect on community productivity and therefore there was no relationship between mycorrhizal effects on diversity and productivity. Mycorrhizal responsiveness was not a good predictor of species response to suppression of AM in the field. The mycorrhiza-responsive species V. gracilis and V. arguta were not affected by reduced mycorrhizal colonization in fungicide-treated plots, suggesting that competition from the mycorrhiza-responsive dominant M. minima offset the benefits of mycorrhizal association for these species.