As is true throughout the Pacific, there is considerable contrast between the vegetation of the coral islands and the volcanic islands. The indigenous flora is of Indo-Malayan origin, although there are a few endemic species, and many new plants have been introduced since the coming of the Europeans to the Pacific. In general the low coral islands with their poor soil offer a very limited environment and have a much more restricted plant life than is found on the volcanic islands that have a much more varied physical environment.
The plants of the low coral islands are limited to a small number of species that tolerate high salinity. The flora of the low islands has been so changed by human occupancy that it is impossible to establish the nature of the original vegetation cover. Even the characteristic coconuts were probably introduced by man.
The high islands of the eastern and western Carolines fall into two general groups of natural vegetation. The eastern islands of Kusaie, Ponape, and Truk have fairly heavily forested interiors whereas the western islands of Yap and Palau have interiors that are mainly covered with savanna grass, the trees being concentrated near the coast and along streams. The more even rainfall in the eastern islands has been advanced as a reason for this difference, although there is a possibility that the savannas were culturally induced.
In both the Yap and Palau islands the practice of burning off the sword grass in order to produce tender shoots for grazing cattle and to clear the ground for agriculture frequently results in fires escaping from control and burning large areas. Since the American occupations some forested areas have been replaced by sword grass.