Abstract
Abstract. This paper addresses the question of transition states in the Hawaiian rainforest ecosystem with emphasis on their initial developments. Born among volcanoes in the north central Pacific about 4 million years ago, the Hawaiian rainforest became assembled from spores of algae, fungi, lichens, bryophytes, ferns and from seeds of about 275 flowering plants that over the millennia evolved into ca. 1000 endemic species. Outstanding among the forest builders were the tree ferns (Cibotium spp.) and the 'ōhi'a lehua trees (Metrosideros spp.), which still dominate the Hawaiian rainforest ecosystem today. The structure of this forest is simple. The canopy in closed mature rainforests is dominated by cohorts of Metrosideros polymorpha and the undergrowth by tree fern species of Cibotium. When a new lava flow cuts through this forest, kipuka are formed, i.e., islands of remnant vegetation. On the new volcanic substrate, the assemblage of plant life forms is similar to the assemblage during the evolution of this system. In open juvenile forests, a mat-forming fern, the uluhe fern (Dicranopteris linearis), becomes established. It inhibits further regeneration of the dominant 'ōhi'a tree, thereby reinforcing the cohort structure of the canopy guild. In the later part of its life cycle, the canopy guild breaks down often in synchrony. The trigger is hypothesized to be a climatic perturbation. After the disturbance, the forest becomes reestablished in about 30–40 yr. As the volcanic surfaces age, they go from a mesotrophic to a eutrophic phase, reaching a biophilic nutrient climax by about 1–25 K yr. Thereafter, a regressive oligotrophic phase follows; the soils become exhausted of nutrients. The shield volcanoes break down. Marginally, forest habitats change into bogs and stream ecosystems. The broader 'ōhi'a rainforest redeveloping in the more dissected landscapes of the older islands loses stature, often forming large gaps that are invaded by the aluminum tolerant uluhe fern. The 'ōhi'a trees still thrive on soils rejuvenated from landslides and from Asian dust on the oldest (5 million years old) island Kaua'i but their stature and living biomass is greatly diminished.
Highlights
The latest information gives the date of colonization of its dominant tree taxon Metrosideros (’ohi’a lehua) as 3.9 Myr (Percy et al, 2008)
When a lava flow cuts through a Hawaiian rainforest, patches of rainforest often survive as so-called kipuka, i.e., islands of remnant vegetation (Fig. 1)
Juvenile Metrosideros trees have rather short branches along their entire stems, and the trees form open stands, which allows for much direct sunlight to penetrate to the ground. This light environment is ideal for the heliophytic fern Dicranopteris linearis, a stoloniferous mat-former, which covers the open bedrock surface, thereby preventing further Metrosideros seedlings to develop into saplings
Summary
In open juvenile forests, a mat-forming fern, the uluhe fern (Dicranopteris linearis), becomes established It inhibits further regeneration of the dominant ’ohi’a tree, thereby reinforcing the cohort structure of the canopy guild. The latest information gives the date of colonization of its dominant tree taxon Metrosideros (’ohi’a lehua) as 3.9 (to 6.3) Myr (Percy et al., 2008) This date range, obtained from phylogenetic methods, puts its island origin intSo tohelidPliEocaenret,hcoinciding with the emergence of the oldest high island Kaua’i. The arrival date of the second main structural component of the Hawaiian rainforest, the tree fern Cibotium (hapu’u), has not yet been established, but is believed to be of ancient origin the e(aRralynkceor,mpmeursnoitnyaTlahscsoeemmCmbluarngyceoiaostinopnnh,ewe20rv1eo2l)c.aEnivcensutrofadcaeys, follows the evolution of plant life from algae and fungi via lichens, mosses, and ferns (Smathers and Mueller-Dombois, 2007).
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