Epiphytes are structurally-dependent plants which grow on other plants without taking nourishment from them. Phylogenetic and ecophysiological differences divide them into non-vascular epiphytes (e.g. mosses and lichens), which are distributed worldwide, and vascular epiphytes (e.g. orchids and bromeliads), which are restricted to the tropics and subtropics. Within their distributional ranges, their abundance is strongly influenced by atmospheric water availability, since they have no access to soil water and are strongly coupled to the atmosphere. Epiphytes are most conspicuous in the tropics, in particular in cloud forests, but they can also be very abundant in cool-temperate wet forests. Their importance for precipitation partitioning rises from their widespread distribution, their location at the atmosphere-biosphere interface, and their adaptations specifically aimed at capturing and retaining atmospheric inputs. The interaction of epiphytes and precipitation partitioning is bidirectional: they deliberately contribute to partitioning and they depend on this partitioning (capture and retention of water and nutrients) for their survival. Additionally, they may be affected by partitioning by other canopy elements, taking advantage of throughfall and stemflow. Stemflow has been shown to be particularly important for non-vascular epiphytes on tree trunks, providing both water and nutrients. The presence of epiphytes increases the effect of forest canopy structure on the vertical and horizontal redistribution of precipitation by diversifying and changing nutrient pathways and by modifying water availability spatially and temporally. These functions are more pronounced in epiphytes than in the rest of the canopy, as epiphytes have developed a diverse array of strategies and mechanisms to cope with intermittent water supplies. Although quantitative information is scarce, it is clear that interception can be substantially increased by epiphytes, in particular by bryophytes and tank-forming bromeliads. In continuously wet environments, however, the potential water-uptake capacity of these groups may not be fully used because of low desiccation rates. Overall, much quantitative and process-oriented research into epiphyte interactions with precipitation interception is still needed to better understand the role of this functionally diverse group of plants in global climate and hydrological cycles. Mutual influence of epiphytes and precipitation redistribution will occur anywhere where epiphytes occur. However, the magnitude and exact mechanisms of the interactions will differ across climate zones and ecosystem types, based on epiphyte abundance, functional composition, and spatial distribution, as well as the frequency and intensity of precipitation as rain, fog and snow.
