Structural heterogeneity of trees influences epiphyte distributions in a northern temperate rainforest.

Questions: Large trees provide structural heterogeneity that may influence the distribution and diversity of epiphytes, yet this has not been fully examined in northern temperate rainforest trees. How does epiphyte diversity, percent cover and composition vary among trunk and branch zones within northern temperate rainforest trees? Are there steep gradients in environmental conditions or resources within northern temperate rainforest trees? To what degree do species show specialization to particular zones within northern temperate rainforest trees? Location: Temperate rainforest on the Olympic Peninsula in Washington State, USA. Methods: We recorded epiphyte richness, percent cover, and composition in 78 plots from six tree zones (three trunk zones and three branch zones) in six large bigleaf maple (Acer macrophyllum) trees. At each survey point, we measured temperature, relative humidity, canopy cover, moss depth, height in tree, and branch diameter to examine the degree of habitat specialization along measured environmental and resource gradients and the importance of structural heterogeneity for epiphyte species diversity in entire trees. Results: Rarefied epiphyte richness was 25% higher on the trunk than in the branches, and there was little overlap in species composition between trunk and branch zones. Species composition changed with height as well as decreasing canopy cover along the trunk. Within the branches, epiphyte composition was related to branch diameter and moss depth; while the inner and mid‐branch zone communities were similar, the outer branch community differed. Microclimate variables did not vary significantly among tree zones and were not related to epiphyte distributions. Conclusions: The structural heterogeneity of large A. macrophyllum trees created gradients in canopy cover and substrate characteristics that enabled up to 13 different epiphyte species to coexist. Thus, these trees are critical structures for the maintenance of forest diversity. [ABSTRACT FROM AUTHOR]