• Additional water supplied by dew formation is an important resource for microbes, plants and animals in precipitation-limited habitats, but has received little attention in tropical forests until now. • We evaluated the micro-environmental conditions of tree stem surfaces and their epiphytic organisms in a neotropical forest, and present evidence for a novel mechanism of diurnal dew formation on these surfaces until midday that has physiological implications for corticolous epiphytes such as lichens. • In the understorey of a lowland forest in French Guiana, heat storage of stems during the day and delayed radiative loss during the night decreased stem surface temperatures by 6°C in comparison to the dew-point temperature of ambient air. This measured phenomenon induced modelled totals of diurnal dew formation between 0.29 and 0.69 mm d⁻¹ on the surface of the bark and the lichens until early afternoon. • Crustose lichens substantially benefit from this dew formation, because it prolongs photosynthetic activity. This previously unrecognized mechanism of midday dew formation contributes to the water supply of most corticolous organisms, and may be a general feature in forest habitats world-wide.
© 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.