Determinants of litter accumulation and the abundance of litter-associated microarthropods in bird's nest ferns (Asplenium nidus complex) in the forest of Yambaru on Okinawa Island, southern Japan

We studied the distributional pattern of bird's nest ferns (Asplenium nidus complex) and the factors that determined litter accumulation and the abundance of litter-associated microarthropods in the ferns in the forest of Yambaru on the northern part of Okinawa Island, southern Japan. We located 53 bird's nest ferns (41 ferns on 27 live trees of 13 species, and 12 on 5 dead trees) in a ca. 4-ha plot, collected litter samples from 37 ferns on 25 live trees, and then extracted a total of 11 205 microarthropods (Acari and Collembola) from all the litter samples. The ferns preferred concave slopes and tended to be distributed on the tree species that had the typical characteristics of high population density and/or large basal area in the forest. The ferns were usually established on large trees ≥10 m high or ≥20 cm diameter at breast height (DBH)], although the number and size of the ferns were not related to the size of the host trees. The amount of litter accumulated in the ferns was correlated neither with the size (height and DBH) of the host tree nor with the height and position of the ferns. The amount of accumulated litter had a significant positive correlation only with fern size; this might have caused the positive correlations between fern size and the abundance of litter-associated microarthropods and the number of species of oribatid mites in the ferns.