Effects of host tree size on the species richness and abundance of epiphyte assemblages in a Bornean lowland tropical forest

This study clarified the effects of host tree size on the numbers of epiphyte species and individuals on a tree, based on observation of 122 species and 2,162 individuals of epiphytes on 430 target trees in a Bornean primary lowland dipterocarp forest. We analyzed relationships of the diameter at breast height (DBH) of target host trees with the probability of epiphyte presence, number of epiphyte species on each target tree (species richness), number of epiphyte individuals on each target host tree (abundance), and cumulative number of epiphyte species. The probability of epiphyte presence was estimated by a generalized linear model (GLM) with a logistic distribution to be higher than 50 % on trees with DBH ＞ 40 cm. A GLM with a quasi-Poisson distribution showed that both species richness and epiphyte abundance increased exponentially with DBH. The most rapid increase was found for trees with DBH ＞ 40 cm. Also, about half of all epiphyte species recorded in this study were only found on trees with DBH ＞ 60 cm. These results suggest that in lowland tropical rainforests in the Southeast Asian tropics, large trees play an important role in maintaining the species diversity of epiphyte assemblages by providing them with habitats. Meanwhile, several epiphyte species were found only trees with DBH ≤ 40 cm, suggesting that the small trees also provide essential habitats with such epiphyte species. Primary forests with various sizes of trees are necessary for conservation of the epiphyte assemblages.