J For Res (2006) 11:313–318 © The Japanese Forest Society and Springer 2006 DOI 10.1007/s10310-006-0213-z ORIGINAL ARTICLE Shigenori Karasawa · Naoki Hijii 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 Received: December 6, 2005 / Accepted: April 4, 2006 Abstract We studied the distributional pattern of bird’s Introduction 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 Many researchers have reported that the presence of a Yambaru on the northern part of Okinawa Island, southern well-developed community of epiphytes can enhance the Japan. We located 53 bird’s nest ferns (41 ferns on 27 live nutrient-trapping capacity of forest systems in tropical and trees of 13 species, and 12 on 5 dead trees) in a ca. 4-ha plot, temperate regions, increasing their storage volume and act- collected litter samples from 37 ferns on 25 live trees, and ing as a “nutrient capacitor” (Benzing 2004) to steadily then extracted a total of 11205 microarthropods (Acari and release growth-limiting ions for other flora (e.g., Nadkarni Collembola) from all the litter samples. The ferns preferred 1984a, b; Nadkarni and Matelson 1992; Clark et al. 1998; concave slopes and tended to be distributed on the tree Nadkarni et al. 2002). species that had the typical characteristics of high popula- The bird’s nest fern (Asplenium nidus L. complex) is tion density and/or large basal area in the forest. The ferns widely distributed from tropical to subtropical regions (e.g., were usually established on large trees [≥10m high or Hsu et al. 2002; Ellwood and Foster 2004) and can grow to ≥20cm diameter at breast height (DBH)], although the a fresh weight of more than 200kg (Ellwood et al. 2002). number and size of the ferns were not related to the size of The basket-shaped rosette of long fronds can trap substan- the host trees. The amount of litter accumulated in the ferns tial amounts of leaf litter from the canopy, which contains was correlated neither with the size (height and DBH) of abundant and diverse litter-associated microarthropods the host tree nor with the height and position of the ferns. (Acari and Collembola) in tropical and subtropical forests The amount of accumulated litter had a significant positive (Rodgers and Kitching 1998; Walter et al. 1998; Karasawa correlation only with fern size; this might have caused the and Hijii 2006a, b). These litter-associated microarthropods positive correlations between fern size and the abundance have significant impacts on the decomposition processes of litter-associated microarthropods and the number of spe- in the forest floor and are important components of cies of oribatid mites in the ferns. biodiversity in various types of forest ecosystem (Wallwork 1983; Seastedt 1984; Lavelle and Spain 2001; Coleman et al. Key words Asplenium nidus · Bird’s nest fern · Litter · 2004; Karasawa and Hijii 2004a, b). Thus, clarification of the Microarthropod · Forest of Yambaru factors that determine litter accumulation in bird’s nest ferns and the community structures of the microarthropods associated with the litter could help us to understand the fern’s functions and effects on the diversity of invertebrate communities in forest ecosystems. In subtropical regions, however, few studies have examined how the spatial dis- tribution and other traits of bird’s nest fern, as well as S. Karasawa1 (*) · N. Hijii their associations with host trees, can affect patterns of Laboratory of Forest Protection, Graduate School of Bioagricultural litter accumulation and the community structures of litter- Sciences, Nagoya University, Chikusa-ku, Nagoya, Japan associated microarthropods. Our aim was to reveal the factors that determine litter Present address: accumulation and the abundance of litter-associated 1 Iriomote Station, Tropical Biosphere Research Center, University of microarthropods in bird’s nest ferns in a subtropical forest the Ryukyus, 870 Uehara, Taketomi, Okinawa 907-1541, Japan Tel. +81-98-085-6560; Fax +81-98-085-6830 (the forest of Yambaru) on Okinawa Island, southern e-mail: [email protected] Japan. We first censused the distributional pattern of bird’s 314 nest ferns in the forest and then examined: (1) whether the size [height and diameter at breast height (DBH)] of host trees affected the size and number of bird’s nest ferns; (2) whether the size of the host tree and the location (height and position) and size of the ferns affected the amount of litter accumulated in the ferns; and (3) whether fern size affected the abundance of litter-associated microarthropods in the ferns. Materials and methods Study area We conducted our study in an old-growth, evergreen, broad-leaved forest in the Yona Experimental Forest at the University of the Ryukyus, the forest of Yambaru, on the northern part of Okinawa Island in southern Japan (26°49′ N, 128°05′ E; 250–330m asl). The forest of Yambaru has an area of about 300km2, where more than 1000 plant species have been recorded (Itô 1997). In addition, the for- est supports a large number of endemic and endangered insects, birds, and mammals (Itô et al. 2000), and is also characterized by high species diversities of insects and orib- atid mites (Azuma et al. 1997; Itô et al. 1998; Itô and Aoki 1999). The area is characterized by a subtropical climate and Fig. 1. Projection areas of the rosette at the frond tips and of the litter accumulated in a bird’s nest fern (Asplenium nidus) abundant rainfall throughout the year. The mean annual temperature is 23.0°C, and annual precipitation between 1992 and 2003 averaged 2330mm (Experimental Forest at the University of the Ryukyus). The bedrock is composed From 18 October to 28 November 2003, we counted the of sandstone and slate, and the soil is classified as a yellow number of leaves on each fern; we also measured the pro- soil (Y) according to the Japanese Society of Forest Envi- jection areas of the rosette at the frond tips and of the ronment (1999). A ca. 4-ha plot containing ridges and val- region of each fern on which the litter was accumulated by leys has been established in the forest, which is dominated approximating both of these areas to an oval (Fig. 1). We by Castanopsis sieboldii (Makino) Hatus. with heights chose 37 ferns on 25 live trees of various species in the plot, ranging from 7 to 13m. In the past 50 years there has been removed all the litter from each fern, and placed the litter in no logging or other artificial disturbance in this forest separate plastic bags. (Shinzato et al. 1986; Enoki 2003). Extraction of microarthropods Epiphytes We extracted invertebrates from all samples by using We conducted our measurements and sampling by ap- Tullgren funnels. Samples were first placed in the dark for proaching bird’s nest ferns using a single-rope technique 24h to avoid a sharp rise in temperature, which would have (Perry 1978; Whitacre 1981), so as to minimize the damage killed animals intolerant of desiccation, and then under 40- to the fern on the trunk of the host tree. All our fieldwork W electric bulbs for 72h. We sorted acari and collembolans was conducted between 0900 hours and 1700 hours in sunny from the invertebrates collected and counted them under a weather. Over four days in June 2003, we performed a binocular microscope (SZ6045TR; Olympus, Tokyo) at visual census of the spatial distribution of bird’s nest ferns in a magnification of 30×. Oribatid mites (Acari: Oribatida) the plot, identified their host-tree species, and then mea- were also sorted from acari, and then separated into adults sured the DBH and height of each host tree, as well as the and juveniles. Only the adults were identified (to height and position at which each fern was attached to each morphospecies level) and counted under a binocular micro- host tree. We divided these positions into six categories scope (BX41, Olympus) at a magnification of 400×. After based on Hsu et al. (2002): ground level; base of the trunk extraction of the invertebrates, we dried the litter samples up to 3m above ground level; the trunk; the main branching in an oven (70°C, 72h; DS-62; Yamato Scientific, Tokyo) point; the lower canopy (the primary branch); and the and weighed the dry litter on a microbalance (HF-200; upper canopy. A&D, Saitama, Japan). 315 Table 1. Number of bird’s nest ferns (Asplenium nidus complex) observed, the host trees to which the ferns were attached, and the dominance and distributional patterns of the host-tree species in this forest Tree species Number of Order of the dominance Distributional of host-tree speciesa,b patterna Trees Ferns Density Basal area Quercus miyagii 513154 B Distylium racemosum 463 3 A Castanopsis sieboldii 341 1 A Trupinia ternata 3323 28 C Schima wallichii 242 2 B Schefflera octophylla 234 5 C Diospyros morrisiana 2212 14 C Adinandra ryukyuensis 11<29 <29 – Pileostegia viburnoides 11<29 <29 – Meliosma squamulata 11<29 <29 – Machilus thunbergii 11<29 <29 – Meliosma rigida 11<29 <29 – Neolitsea sericea 11<29 <29 – Dead trees (standing) 3 5 Dead trees (fallen) 2 7 Total 32 53 B, Found in areas with an intermediate microtopography; A, found on gentle convex slopes; C, found on steep concave slopes; –, not investigated a Based on the data from Enoki (2003) b Smaller numbers represent higher ranks in dominance in the forest Analyses Results We compared the mean number of leaves on the ferns Distributional pattern of bird’s nest ferns among three positions by the Kruskal–Wallis test using JMP software (SAS Institute 2002).