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TROPICS Vol. 16 (3) Issued May 31, 2007 Effects of an alien shrub species, Leucaena leucocephala, on establishment of native mid-successional tree species after disturbance in the national park in the Chichijima island, a subtropical oceanic island ＊ Kenji HATA , Jun-Ichirou SUZUKI and Naoki KACHI Department of Biological Sciences, Graduate School of Science, Tokyo Metropolitan University, Minami-Osawa 1−1, Hachioji, Tokyo, 192−0397, Japan ＊Corresponding author: Kenji HATA. Tel; +81−426−77−2585, Fax; +81−426−77−2559, E-mail: [email protected] ABSTRACT Effects of an invasion of an alien & Duggin, 1997), which would change successional shrub species, Leucaena leucocephala , were pathways of native plants in an invaded habitat. The investigated on subsequent establishment of a successional pathways of native species in some tropical native mid-successional wooden species, Schima and subtropical oceanic islands were affected by an mertensiana at early-successional stages after alien shrub species, Leucaena leucocephala (L.) de disturbance in a subtropical oceanic island, Wit (Leguminosae). The species invaded and formed Chichijima in Japan. Changes in basal areas, dense monotypic thickets in disturbed areas in many densities and size distribution of forests at a site oceanic islands, which should have prevented seedlings dominated by L. leucocephala were compared with of woody species and understory herbaceous species those at a site dominated by native shrub species, from germination and/or growth under canopies of L. Trema orientalis at early-successional stages. leucocephala (e.g. Decker 1992, Mueller-Dombois and Effects of L. leucocephala on germination of seeds Fosberg 1998, Yamamura et al. 1999). and growth of seedlings of S. mertensiana were Germination or growth of some plant species in a quantified at the sites. There were few recruited subtropical region were inhibited by aqueous extracts seedlings of S. mertensiana and they did not grow of plant tissues of L. leucocephala under experimental at the site dominated by L. leucocephala. On the conditions (Chou & Kuo, 1986). These allelopathic effects other hand, there were a few recruited seedlings of L. leucocephala could prevent seedlings of endemic and within a 1 m x 1 m area and they positively grew other native plants from establishment in oceanic islands, at the site dominated by T. orientalis. Germination which inevitably influences the native successional rates of seeds and growth rates of seedlings pathway. This process should be quantitatively described of S. mertensiana at the site dominated by L. for conservation of endemic plants in oceanic islands. leucocephala were lower than those at the site Effects of invasion of L. leucocephala on native dominated by T. orientalis. Germination of seeds successional pathways were described by comparison and growth of seedlings of S. mertensiana were of structure of the secondary forests between forests inhibited by L. leucocephala at the disturbed site, dominated by L. leucocephala and those by native species which potentially changed an early successional in the Bonin Islands. In addition, histories of vegetation pathway of the plant community. types of the forests were also analyzed based on aerial photographs (Yoshida & Oka, 2000). There were Key words: invasive plants, Schima mertensiana, differences in the structures and successional pathways Bonin Islands, seed germination, seedling growth, of the forests (Yoshida & Oka, 2000). However, we still Trema orientalis do not know quantitative effects of the invasion of L. leucocephala on establishment of native plants at mid- or late-successional stages. For this purpose, it is necessary INTRODUCTION to compare establishment and growth of native plants at Invasion of an alien plant species often prevents native sites at which L. leucocephala invaded or not. plant species (Myers & Bazely, 2003) from establishment It is hypothesized that native plants of mid- or late- due to shading (Weihe & Neely, 1997), litter accumulation successional species in subtropical oceanic islands would (Walker & Vitousek, 1991) and allelopathy (Gentle be prevented from establishment in a forest dominated 284 Kenji HATA, Jun-Ichirou SUZUKI and Naoki KACHI by L. leucocephala at early successional stages because at the both sites. Forest floors of the study sites were of allelopathic effects of L. leucocephala (Chou & Kuo, covered with herbaceous species, and the most dominant 1986). Based on the hypothesis, we tested the prediction species was Stachytarpheta jamaicensis (L.) Vahl that germination of seeds and growth of seedlings (Verbenaceae). of native mid-successional species under plants of L. leucocephala are lower than those under plants of native Measurements of forests dominated by L. species. leucocephala or T. orientalis In order to test the prediction, firstly, we compared A 10 m × 10 m plot was established at each site in July changes in biomass, densities and size distribution of 2001. The areas from which the broadcasting towers dominant species in a forest dominated by L. leucocephala were removed were so limited that only one plot was with those in a forest dominated by native species at established at the site. All the individuals of woody early-successional stages for three years in the Bonin species in the plots were tagged, and their diameters at (Ogasawara) Islands, subtropical islands in the northern ground level were measured. Subsequent measurements Pacific. Secondly, we compared germination rates of a were carried out in July 2002 and September 2003. mid-successional species of a native tree and growth rates of its seedlings in the forest dominated by L. Field experiments leucocephala with those in the forest dominated by a A field experiment was carried out in order to test native species appeared at early-successional stages by whether established plants of L. leucocephala prevented field experiments. plants of S. mertensiana from germination and growth. Five 1 m × 1 m plots were established under canopies of L. leucocephala outside of the 10 m × 10 m plot at the site MATERIALS AND METHODS dominated by L. leucocephala and other five 1 m × 1 m Plant species plots under canopies of T. orientalis outside of the 10 m × A shrub species, L. leucocephala, was introduced to the 10 m plot at the site dominated by T. orientalis. Each 1 m Bonin Islands in 1862 (Funakoshi, 1989) and spread into × 1 m plot was located at least 1 m away each other. abandoned areas (Shimizu, 1989). A native shrub species, These five 1 m × 1 m plots were divided into three Trema orientalis Blume (Ulmaceae) appears at early categories: three 1 m × 1 m plots of the five 1 m × 1 m stages in a secondary succession in the Bonin Islands plots were allocated to a seedling transplant experiment. (Shimizu, 1989). A wooden species, Schima mertensiana In one of the rest two 1 m × 1 m plots, germinated (Sieb. et Zucc.) Koidez (Theaceae), is endemic to seedlings were counted. Total canopy openness was the Bonin Islands, and dominates at middle stages in measured in the rest 1 m × 1 m plots. secondary succession (Shimizu, 1989). In May 2002, the seedling transplant experiment was conducted. Sixty seedlings of S. mertensiana with Study site ca. 10 cm in height and ca. 0.2 cm in diameter at ground This study was conducted at two sites in secondary level were collected near the 10 m × 10 m plots but forests at Yoakedaira (27˚05 ′ N, 142˚12 ′ E, 220 m above outside of the 1 m × 1 m plots. Ten of the seedlings were sea level) in the national park in the largest island of the transplanted into each of the 1 m × 1 m plots after their Bonin Islands, Chichijima. The ground at the study sites diameters at ground level and heights were measured. (ca. 400 m2) were bared by the removal of broadcasting Each of the seedlings was planted 20 cm away from the towers in 1999. Surface soils and vegetation around the others. All transplanted seedlings were harvested and towers were cleared. One of the study sites was invaded dried at 70 ˚C for 72 h for weighting at the end of the and dominated by an alien species, L. leucocephala, and transplant experiment in May 2003. another site by a native species, T. orientalis after 1999. The initial biomass of the transplanted seedlings was The study sites were surrounded by the secondary estimated based on the relationship between diameters at forest dominated by S. mertensiana (Appendix 1). Many ground level, heights, and dry weights. The relationship mature trees of S. mertensiana could be homogeneously was determined by the 24 seedlings of S. mertensiana dispersed in the secondary forests. Indeed, dispersed collected near the 10 m × 10 m plots but outside of the 1 seeds of S. mertensiana were often observed at the m × 1 m plots. The regression equation was; study sites (personal observation). Therefore, sufficient the initial dry weight (g) = 0.23116 + 1.2608 (diameter numbers of seeds of S. mertensiana could be dispersed at ground level (cm))2 x height (cm), r2 = 0.667, p < 0.0001. Effects of L. leucocephala on S. mertensiana in an oceanic island 285 One of the six 1 m × 1 m plots was abandoned openness of each point. because nine transplanted seedlings died by June 2002. A 1 m × 1 m plot, therefore, was newly established and ten Data analysis seedlings of S. mertensiana were transplanted in the plot Effects of existences of S. jamaicensis on germination of in June 2002. seeds and growth of seedlings of S. mertensiana were not Two hundred seeds that were collected in a detected (data not shown). Therefore, the effects were secondary forest around the study sites were sown in not considered in the further analyses. November 2002 into a pot with 21 cm in diameter and All statistical analyses were carried out using the 17 cm in depth.