Patterns of wind dispersal within a forest canopy: An epiphyte perspective

Marie Hoensbroech , Juliano Sarmento Cabral, Gerhard Zotz

Plant dispersal theory has largely been developed from a terrestrial perspective, in which interception by vegetation is considered a loss, and success is defined by arrival at suitable ground-level habitat. True epiphytes—non-parasitic plants structurally dependent on a host throughout their lives—represent a striking reversal of this framework: their diaspores must be intercepted by suitable aboveground substrates to disperse successfully. Although dispersal is frequently assumed to underlie the spatially patchy distributions common in epiphyte communities, it remains poorly understood. Here, we quantify airflow along tree vertical profiles in a tropical lowland forest and incorporate these measurements into a mechanistic dispersal model to investigate how airflow, vegetation density, diaspore terminal velocity, and release height jointly shape dispersal within a forest. We examine how these factors influence interception by vegetation—particularly bark, a prerequisite for epiphyte establishment—and promote long-distance dispersal. We further introduce a quantitative metric of dispersal limitation that integrates dispersal success, dispersal distance, and fecundity. Our results reveal aerodynamic stratification within the forest canopy, with weak mean vertical airflow but pronounced turbulence between the upper and lower crowns. This trunk zone may operate as a strong yet permeable filter: although occasional updrafts enable escape above the canopy, most diaspores released in the understorey remain confined within it. Only a fraction is intercepted by bark, typically within 5 m vertically and 15 m horizontally from the release point. This results in strong per-diaspore dispersal limitation and may contribute to spatially patchy epiphyte distributions. However, the degree of dispersal limitation is context-dependent and, when accounting for the high fecundity of many epiphytes, may be less severe than commonly assumed, with post-dispersal demographic filters ultimately shaping distribution patterns. More broadly, our findings highlight a gap in within-forest dispersal theory and underscore the importance of local canopy structure and airflow in shaping diaspore movement in complex, stratified environments. Our model is transferable to other systems—from understorey and invasive plant species to pollen, fungal spores, and ballooning arthropods—and can improve our understanding of connectivity and gene flow within and among forests, informing conservation strategies that enhance dispersal pathways.