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This paper examines the links between moisture, epilithic lichens, rock surface temperatures and weathering. Field and laboratory experiments were conducted in order to investigate the combined effect of surface wetness and Verrucaria nigrescens (an epilithic lichen) on limestone temperature fluctuations. Exposure blocks with and without a lichen cover were subjected to short-term temperature fluctuations and two types of moisture events: artificial showers (8 ml rainfall) and larger storms (40 ml rainfall). The experiments showed that larger storms are more likely to dampen temperature fluctuations at the rock surface, especially in the presence of an epilithic lichen cover, providing evidence for a mechanism of bioprotection. The conclusion is that the epilithic lichen retains moisture, which reduces thermal stress in the surface of the limestone. (C) 2003 Elsevier Ltd. All rights reserved.