Effects of topography on rainforest tree community structure and diversityin American Samoa, and implications for frugivore and nectarivore populations

Aim We investigated the spatial variation of rainforest tree community stru cture and composition to determine if forest structure and diversity varied as a function of topography; and in turn if this could influence patterns of habitat use by native forest birds and pteropodid bats. Location The study was undertaken on the island of Tutuila, American Samoa, located in the South Pacific Ocean. Methods All trees greater than or equal to 10 cm diameter were censused in sixty 200 m(2) plots in ridge, slope and valley forest across the island of Tutuila. Results Forest structure varied significantly across topographical space. R idge forest was shortest and had the highest stem densities, and valley for est was tallest with the fewest stems per unit area. Species richness was h ighest on ridges, and slope and valley forest were more similar in composit ion with each other than they were with ridge forest. Of the fifty-two tree species encountered in the plots, nine showed a statistical affiliation to one of the three topographical positions. Main conclusions We explain patterns of: forest structure and diversity in the context of chronic and catastrophic disturbances. Higher stem densities in ridge forest suggested a higher degree of disturbance on ridges, and th is was supported by the fact that the height/ diameter ratio of the forest was lowest on ridges, which indicated wind-cropping. We hypothesize the pot ential effects of topographical variation and known phenological patterns o n wildlife abundances. We predict that flowering episodes of ridge-affiliat ed, bird-visited species (particularly Syzygium inophylloides (A. Gray) C. Muell.;Myrtaceae) will concentrate honeyeater densities on ridges, and that fruiting of the tree Canarium vitiense A. Gray (Burseraceae) could localiz e populations of the Pacific pigeon (Ducula pacifica). Overall (i.e. net) b at foraging patterns are unlikely to be affected by either flowering or fru iting events. Most of the tree species on Tutuila are generalist in their d emographic patterns, and the island is depauperate in wildlife fauna; the e volutionary and conservation implications are discussed. We conclude with t he argument that conservation of vertebrate species is essential to maintai n the current generalist demographic patterns of Samoan trees.