Distributional Dynamics in the Hawaiian Vegetation

Abstract

Vegetation ecology is usually divided into two broad research areas, floristic/environmental gradient analysis and studies of vegetation dynamics. The early influential American ecologist Clements combined the two areas into a dynamic system for classifying vegetation. His succession and climax theory, however, was later severely criticized. A new approach to the study of distributional dynamics, called "landscape ecology," focuses on the dynamics of spatial vegetation patterns. There is a spatial hierarchy rule, which implies greater stability of species and community patterns when one considers larger area units versus smaller ones. It is argued that this rule is frequently transgressed in biotically impoverished areas, like the Hawaiian Islands, where certain dominant plant species have become established over unusually broad areas and habitat spectra. A further point made is that with "species packing" successional patterns change from auto-succession, where the dominant species retains dominance by in situ generation turnover (termed chronosequential monoculture), via "normal" succession (i.e., displacement of dominants by other dominants over time [termed chronosequential polyculture]), to small-area patch or gap dynamics (termed chronosequential gap rotation). Examples of the three spatially different succession paradigms are given for Hawaii, and the point is made that chronosequential monocultures cannot be expected to last, but change to chronosequential gap rotation with the invasion of alien dominants. Before the invasion of alien dominants, certain native dominants seem to have segregated into races or varieties by evolutionary adaptation to successional habitats. Finally, the concept of climax is discussed as having two meanings: (I) permanency of community type, which can only be observed for the aggregate assemblage of smaller communities in a larger space, such as occupied by a biome; (2) the mode of organic production in ecosystem development. The mode seems to occur between 1000 and 3000 yr in the Hawaiian rainforest biome on volcanic soils. Thereafter, productivity declines with acidification and soil nutrient impoverishment over a million years and more. This amounts to a retrogression in the course of primary succession.