COS 194-1 - Community assembly of epiphytic bryophyte communities in a northern temperate rainforest

Abstract: Understanding the factors that drive the assembly of communities is central to understanding patterns in community structure. Colonization patterns could be driven by deterministic factors, such as niche specialization, or stochastic factors, such as dispersal limitation. Epiphytes, structurally-dependent plants, are a conspicuous and important component of rainforests yet little is understood about how these communities form. Recent theoretical debate over whether colonization patterns of epiphytes are driven by deterministic or stochastic factors motivated us to use an experimental approach to test the drivers of community assembly. We hung cleared and cleaned branch substrates (400 cm²) at different heights and orientations (horizontal and vertical) in 12 large ( >60 cm dbh) Acer macrophyllum trees in the Hoh rainforest in Olympic National Park, WA. These trees harbor an abundant load of epiphytic bryophytes. We surveyed the substrates and examined patterns of colonization every 3-6 months with the last survey occurring after 2.5 years (the study is still ongoing). We measured temperature and relative humidity with dataloggers hung under each branch in 6 replicate trees for 1 year. Here we examine patterns of community composition, richness, abundance, and microclimate among the branches to test whether community assembly is determined by deterministic or stochastic factors.

Only a few species colonized the branches despite a large and rich species pool. The early colonizing species were those species most abundant on small branches and included Isothecium myosuroides, Neckera douglasii, and Porella navicularis. Interestingly, most of the colonizers were from propagules of thallus of bryophytes that landed on the branches rather than from spores suggesting that bryophyte communities assemble through both asexual and sexual reproduction. Colonization through spore dispersal occurred only after 1.5 years and was limited to one species. Height and orientation influenced richness and abundance with horizontal branches at lower heights having the most abundant and rich communities. These data suggest that both dispersal and recruitment limitation are important drivers of community assembly because lower and horizontal branches likely had a higher propagule rain and yet only a subset of the species in the pool successfully colonized the branches. Preliminary analyses indicate that there was no predictable variation in temperature and relative humidity among the branches. We conclude that colonization of new substrate in this system is quite slow, and assembly is governed by a combination of recruitment and dispersal limitation.