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

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 wellknown 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. In most islands, the isolation barrier has been broken by the anthropogenic introduction of alien species. Some of the introduced species have become invasive. In spite of successful control efforts, a new natural nonstoppable dynamic has been unleashed by humans through the introduction of invasive species. Questions arising are, Will alien species behave like second-growth species in the continental tropics or will they replace all native island vegetation? What can we predict about forest succession in islands under the added complexities of global change? To elucidate these questions, I will first refer to the island biogeography model with regard to vegetation development. I will then emphasize the effect of isolation on plant succession in remote islands as compared with less isolated areas. Finally, I will try to evaluate the modern threat of alien plant invasion and conclude with a future outlook on island vegetation. the island biogeography model and vegetation development The island biogeography model of MacArthur and Wilson (1967) has captured the generally accepted fact that islands contain fewer species than their biotic source areas when compared on the same land-area basis. The isolation factor has been well demonstrated for mangrove species by Woodroffe (1987). Isolation as reflected in species impoverishment of forest canopy species has also been shown in the island vegetation treatment by Mueller-Dombois and Fosberg Pacific Science (2008), vol. 62, no. 3:303–308 : 2008 by University of Hawai‘i Press All rights reserved 1 Keynote address. Manuscript accepted 20 August 2007. 2 Botany Department, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822 (www.botany.hawaii