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A foundational concept in ecology is the positive relationship between habitat heterogeneity and species diversity. Epiphytes demonstrate microhabitat specialization to particular areas within a tree; thus, epiphyte communities are potentially influenced by the structural heterogeneity of host trees. We evaluated the relationship between structural features of Acer macrophyllum and epiphyte distributions and abundance in a temperate rainforest in Washington, USA. Epiphytes and structural features of three Acer macrophyllum trees were systematically surveyed using the point-intercept method from the base to the crown and on three branches for each tree. Rarefied species richness increased with structural richness. Species richness along the trunk differed significantly among types of structural features (i.e., broken branches, burls, holes, rivets, branches, and trunks); broken branches had the highest species richness and branches had the lowest, followed by trunks. Rarefied species richness increased with height and peaked at 12 m, but the relationship between structural diversity and height was not significant. The species that occurred on branches were different to those that occurred on trunks, and species composition varied significantly among trunk features. The high structural diversity in northern temperate rainforest trees influenced the fine-scale distribution of nonvascular epiphytes and may explain their coexistence in tree crowns. [ABSTRACT FROM AUTHOR]

Plant-plant interactions change through succession from facilitative to competitive. At early stages of succession, early-colonizing plants can increase the survival and reproductive output of other plants by ameliorating disturbance and stressful conditions. At later stages of succession, plant interactions are more competitive as plants put more energy toward growth and reproduction. In northern temperate rainforests, gap dynamics result in tree falls that facilitate tree regeneration (nurse logs) and bryophyte succession. How bryophyte-tree seedling interactions vary through log succession remains unclear. We examined the relationships of tree seedlings, bryophyte community composition, bryophyte depth, and percent canopy cover in 166 1.0 m² plots on nurse logs and the forest floor in the Hoh rainforest in Washington, USA, to test the hypothesis that bryophyte-tree seedling interactions change from facilitative to competitive as the log decays. Tree seedling density was highest on young logs with early-colonizing bryophyte species (e.g., Rhizomnium glabrescens) and lowest on decayed logs with Hylocomium splendens, a long-lived moss that reaches depths >20 cm. As a result, bryophyte depth increased with nurse log decay and was negatively associated with tree seedling density. Tree seedling density was 4.6 x higher on nurse logs than on the forest floor, which was likely due to competitive exclusion by forest floor plants, such as H. splendens. Nurse logs had 17 species of bryophytes while the forest floor had six, indicating that nurse logs contribute to maintaining bryophyte diversity. Nurse logs enable both tree seedlings and smaller bryophyte species to avoid competition with forest floor plants, including the dominant bryophyte, H. splendens. H. splendens is likely a widespread driver of plant community structure given its dominance in northern temperate forests. Our findings indicate that plant-plant interactions shift with succession on nurse logs from facilitative to competitive and, thus, influence forest community structure and dynamics. [ABSTRACT FROM AUTHOR]

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]