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The Mercur Gold Mine tailings pond experienced a dam failure more than 70 years ago, creating a denuded area that was the subject of this study. Arsenic levels in the surface soils ranged from 45 to 3237 mg As kg(-1). Vascular plants, mosses, algae, and lichens all showed a threshold effect to elevated arsenic. Over 2100 mg As kg(-1), chlorophyll a was consistently depressed. Over 1700 mg As kg(-1), grasses, annuals and mosses were absent, and over 1400 mg As kg(-1), cyanobacteria were notably affected. We conclude that the microbiotic crusts will not recover so long as the soil remains contaminated. (c) 2005 Elsevier Ltd. All rights reserved.