Forest Structure, Composition, and Distribution on a Pacific Island, with Reference to Ecological Release and Speciation

Native forest and scrub of Chichijima, the largest island in the Bonins, were classified into five types based on structural features: ElaeocarpusArdisia mesic forest, 13-16 m high , dominated by Elaeocarpus photiniaefolius and Ardisia sieboldii; Pinus-Schima mesic forest, 12-16 m high , consisting of Schima mertensiana and an introduced pine , Pinus lutchuensis; RhaphiolepisLivistonia dry forest, 2-6 m high, mainly occupied by Rhaphiolepis indica v. integerrima; Distylium-Schima dry forest, 3-8 m high , dominated by Distylium lepidotum and Schima mertensiana; and Distylium-Pouteria dry scrub, 0.3-1.5 m high , mainly composed of Distylium lepidotum. A vegetation map based on this classification was developed. Species composition and structural features of each type were analyzed in terms of habitat condition and mechanisms of regeneration. A group of species such as Pouteria obovata, Syzgygium buxifolium, Hibiscus glaber, Rhaphiolepis indica v. integerrima, and Pandanus boninensis, all with different growth forms from large trees to stunted shrubs, was subdominant in all vegetation types. Schima mertensiana , an endemic pioneer tree, occurred in both secondary forests and climax forests as a dominant canopy species and may be an indication of " ecological release," a characteristic of oceanic islands with poor floras and little competitive pressure. Some taxonomic groups (Callicarpa, Symplocos, Pittosporum, etc.) have speciated in the understory of Distylium-Schima dry forest and Distylium-Pouteria dry scrub. Speciation seems to have occurred exclusively where there are comparatively small numbers of component species, historically stable habitats, some opportunity for regeneration without large-scale disturbance, and the occasional occurrence of canopy gaps