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Native forests increasingly have been reduced to remnant fragments on many Pacific islands. Island and continental ecosystems differ in a number of ways that may increase conservation value of such forest fragments on islands. However, few studies have examined performance of tree populations in Polynesian forest fragments. We tested potential conservation value of the largest contiguous patch of lowland lava flow forest on Tutuila, American Samoa, by determining uniqueness, potential vulnerability, and possible viability of the tree community therein. We recorded 1,186 trees ≥10 cm dbh (diameter at breast height) from 37 species in 12 transects (each 10 m wide) running from edge to edge across the forest, as well as 1,332 seedlings and 991 saplings within 62 miniplots (each 25 m2). Locations within the forest of all 462 trees ≥30 cm dbh, of the 10 most dominant species, were then plotted. The most dominant tree species was Pometia pinnata. Similarity indices between the study site and other protected forests on Tutuila were very low. Spatial distributions and abundances of adult trees, as well as the dispersed distribution of seedlings and saplings, suggested low vulnerability to spatially discrete disturbances. We found evidence of potential edge effects in seedling distributions of two species. Abundances of seedlings and saplings indicated a high potential for continued recruitment of characteristic tree species. Species composition of these recruits is largely composed of characteristic primary forest species rather than secondary forest or invasive species. These results show that even very small forest fragments may have substantial value for conservation, because they can combine high within-island uniqueness with a relatively high likelihood of persistence if left undisturbed.