Rain, dew, and humid air as drivers of morphology, function and spatial distribution in epiphytic lichens

Abstract This review is a first attempt to combine and compare spatial distribution of the three main water sources, rain, dew and humid air, with water-related traits of mainly epiphytic macrolichens in a conceptual and functional model. By comparing climatic and lichenological knowledge, various effects of dewfall, rainfall and humid air on epiphytic lichen morphology and function are analyzed to search for traits and patterns. Although dew, rain and humid air cause lichen hydration and activate photosynthesis, these atmospheric hydration sources influence and shape lichens differently. In order to visualize hydration patterns, dew, rain and humid air are shown as corners in a triangle exhibiting the various combinations of these hydration sources. The sources of hydration vary on temporal scales, and on the spatial scales: regional, landscape, stand and tree. Lichen growth form, photobiont type, water-holding capacity (WHC) and suprasaturation depression show clear patterns within the hydration triangle. For most lichen species, one average pre-dawn dewfall approximately fills their average internal WHC. This suggests that lichens are optimally designed to utilize dew rather than rain, consistent with literature emphasizing dew as a driver for annual C-assimilation in chlorolichens. However, rain is needed to fill their external WHC and to fully hydrate most cyanolichens. Including the sources of hydration and internal lichen variables, such as water-holding capacity, will improve modelling of local and global future scenarios on lichen distribution and biomass production.