Pacific Island Forests: Successionally Impoverished and Now Threatened to Be Overgrown by Aliens?1

Indigenous forests in remote islands are generally impoverished of secondary successional tree species. After canopy disturbances, the same indigenous tree species seem to resume dominance by a process known as ‘‘autosuccession’’ or ‘‘direct succession.’’ Primary forest tree species are mostly colonizer species. Mature island forests are difficult to categorize as either pioneer, successional, or climax forests by their canopy species composition. Climax forests, which characterize mature forests in less-isolated areas, are typically of distinctly different canopy species composition than the pioneer forests. In central Canada, for example, pioneer pine forests are replaced in succession by mixed hardwood/softwood forests under exclusion of fire. This process is known as ‘‘normal replacement succession’’ or ‘‘obligatory succession.’’ Another well-known ecological concept distinguishes between ‘‘primary’’ and ‘‘secondary’’ forests in the continental tropics. Secondary forests are formed by fast-growing relatively short-lived second-growth species, which quickly assemble after major disturbances. It usually takes a long time for primary tropical rain forest trees to reappear in secondary forests. In contrast, primary island forests rarely include fast-growing indigenous canopy species that form such secondary forests in the continental tropics. Instead, secondary forests in islands are now made up mostly of introduced species. In this paper I attempt to evaluate alien plant invasion in remote islands in view of these concepts of ecological succession.