Lichen hotspots: raised rock temperatures beneath Verrucaria nigrescens on limestone

Field exposure trials and laboratory experiments are integrated to compare surface and internal temperature changes of limestone with an epilithic (Verrucaria nigrescens) and endolithic (Verrucaria baldensis) covering of lichens, and with no lichen cover (control). Blocks with covering of V. nigrescens experience both the highest surface and internal temperatures under a direct heating source during daytime and in laboratory experiments. Spectrophotometer data suggest that the black colour of the V. nigrescens thallus plays a significant role in determining the thermal response of the rock surface. When direct heating (by sun or lamp) is absent (at nighttime and for a diurnal cycle programmed in an environmental chamber), little differentiation occurs between surface and internal temperatures for all surface types compared. For all blocks exposed, maximum rates of daytime surface temperature change ranged from 1.47 to 3.04 °C min−1 and for two of the exposure periods, the highest rates of daytime surface temperature change were experienced by blocks with V. nigrescens cover. The blocks with V. nigrescens cover also experience the greatest thermal gradients in both field and laboratory experiments. The rates of surface temperature change and thermal gradients experienced beneath V. nigrescens in this study could be significant in increasing rock susceptibility to breakdown.