Influence of canopy structure and light on the three-dimensional distribution of the iconic lichen Usnea longissima

Forest canopies modify microclimates and create habitats for nonvascular epiphytes, but we need to better understand the mechanisms regulating their vertical and horizontal distributions. Here we examine how canopy structure and light environment influence the 3D distribution of Usnea longissima, the world's longest lichen, and associated with old-growth forests. We quantified forest structure, vertical profile of light (PPFD transmittance fraction), and horizontal as well as vertical distribution of the lichen in a 1 ha plot dominated by Picea abies. The forest had a multi-layered canopy with mortality driven by small-scale gap dynamics. The population size of the lichen had an approximate log-normal distribution with host trees showing strong clustering. The lichen extended up to mid-canopy and had a rather sharp upper limit. Population size increased with DBH and upper limit but did not correlate with basal area. The vertical profile of light was steeper in dead than in live trees, with the lichen occurring in a zone with low-intermediate light. The horizontal distribution was linked to the vertical distribution through short-distance asexual dispersal. The lichen's 3D distribution was shaped by various interacting functional mechanisms. Its absence from the upper canopy was mainly explained by sensitivity to high light when desiccated and limited capacity for upward migration. The population dynamics was driven by source trees hosting large populations in mid-canopy. The lichen's strong association with humid, old-growth forests is explained by narrow niche preferences and dispersal limitation, and not by slow growth. Protection of multi-layered forests with long continuity of tree cover is needed to secure substrates and suitable microclimates for development of viable lichen populations. Our study highlights that the 3D distribution of lichens in forest canopies is driven by forest dynamics, canopy structure, microclimate, and lichen functional traits.