Diversity and structure of tropical rain forest of Tutuila, American Samoa: effects of site age and substrate

Abstract

We report tree community diversity, guild composition, and forest structure from three 1.2 ha (100 m 120 m) permanent forest research plots on Tutuila, American Samoa, an isolated volcanic island in the South Pacific Ocean. Plots were established in three habitat types of lowland hill forest: two in mature tracts that differed in substrate type (talus vs non-talus), and a third in a 30‐40 year-old abandoned plantation on non-talus soil. We encountered a total of 57 tree species10 cm dbh. Richness was similar across sites, but composition differed substantially. We were able to classify with confidence 24 tree species into four distinct guilds based on foresttype preference: early successional or persistent successional (disturbed forest), generalist (no preference), or mature-phase. Sample size limitations or interactions between site age and substrate precluded categorization of all species. Thirty-eight percent (9/24) of the tree species were successional, a result which contrasts sharply with data from (formerly) continental forest in Panama. Spatial distributions of 33 species revealed 17 species exhibiting clumping or hyperdispersion (i.e., regular spacing) in at least one site. Possible non-anthropogenic mechanisms promoting clumping in the plots were (a) topography (edaphic), (b) gap affiliation, (c) inefficient or altered patterns of propagule dispersal, and (d) lack of natural seedling predators. Forest structure differed across site type, with stem densities highest in regenerating forest; conversely, regenerating forest had the lowest basal areas. Steep talus forest sequestered the most carbon (344.3 Mg ha 1 ), and secondary forest sustained only 42% of levels found in talus forest (145.5 Mg ha 1 ). Mature forest on non-talus soil sequestered the majority of carbon in mid-sized trees (30‐50 cm dbh). Future assessments of land-use cover and biomass will provide for a complete estimate of the carbon budget of Tutuila. Finally, the results of this study suggest that conservation of the native fauna is essential in retaining the potential for regeneration of native forest after large-scale disturbance.