Plant communities of the park, from the mountaintops down to the ocean, are largely tropical rainforest. This forest is the natural characteristic of the wet tropics over all undisturbed land surfaces with hot climate and heavy well-distributed rainfall. It is an evergreen forest and the climax vegetation of the equatorial climate. Unlike the temperate forests of North America dominated one, or only a few tree species, tropical rainforests have high species diversity and the climax forest is dominated by combinations of many species.
Samoa, as a geologically young ocean island, lacks any earlier land connection to continental land masses. Because its native species got here by chance its species diversity is not as rich as Southeast Asia, the source of the islands' plant dispersals.