Theory suggests that habitats that are structurally complex with a diversity of resources provide more niches for species with specific habitat and resource requirements to partition the habitat and coexist. In tropical trees, there are steep gradients in light, relative humidity, temperature, and the availability of humus substrate, which several species use as a source of nutrients and water, that can influence the distribution of epiphytes within tree crowns. In fact, isotope data reveal that epiphytes in different microhabitats utilize different sources of nitrogen (N). However, little is known about whether these same factors influence the distribution of epiphytes within temperate rainforest trees. We surveyed the epiphyte communities in six large Acer macrophyllum trees in temperate rainforests on the Olympic peninsula in Washington State following Johansson zones. We measured temperature, relative humidity, and canopy cover for each zone in each tree where we conducted our surveys. We tested N source as a potential driver of the distribution of epiphytes using stable isotopic analyses of epiphytes and canopy humus. The lycophyte Selaginella oregana, and bryophytes Rhytidiadelphus loreus, Neckera douglasii, Hypnum subimponens, Leucolepis acanthoneuron, and Metaneckera menziesii were selected for δ15N analyses based on their distribution and abundance in tree crowns.
Results/Conclusions
We found non-random distribution patterns among the tree zones. For example, Rhytidiadelphus loreus was most abundant in the inner crown branch zones while Metaneckera menziesii was restricted to trunk zones. N source varied significantly by species and zone. δ15N values of all bryophyte species confirmed an atmospheric N source. δ15N values of the lycophyte, S. oregana, varied significantly with growing location, and in response to its relationship with R. loreus, which grows above it in the inner crown. Selaginella oregana appears to use canopy humus N in the presence of R. loreus in the crown, and atmospheric N in its absence on the trunk. These findings suggest the role of niche partitioning in the coexistence of S. oregana and R. loreus within the canopy, and illustrate the ability of S. oregana to vary its N source based on competition. We found little variation in temperature, relative humidity, and canopy cover among zones, but epiphyte distribution patterns that related to habitat structure. Thus, physical variations in size and angle of branches, as well as structures on the trunk of Acer macrophyllum trees could create a mosaic of microhabitats that enable epiphytes to partition the tree at finer scales than we measured.