Regeneration und Invasibilität des montanen Metrosideros-Regenwaldes auf der Insel Hawaii

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About 50,000 hectares of Metrosideros polymorpha(Myrtaceae) rain forest on the island of Hawaii were affected by a decline of the canopy ("´ohi"a dieback") in the 1970s. A twenty-seven year study of permanent plots has shown that a new cohort of Metrosideros polymorpha trees is replacing the old canopy layer, confirming the predictions of a stand-structure model. The density of healthy trees in former dieback stands has now reached the density in rain forest stands that did not undergo dieback in the 1970s.The large extent of the forest decline is attributed to a uniform age and stand structure ("cohorts") initiated by primary succession following volcanic eruptions. This uniform cohort structure resulted in the simultaneous aging and death of most of the trees in the cohort ("cohort senescence"). We postulate that the decline was a natural demographic phenomenon, whereby synchronized senescence acted as the major predisposing cause. Climatic fluctuations may have acted as dieback triggers, particularly on sites with poor drainage. In these habitats, extreme rainfall events and droughts, eg. during an episodic lifting of the cloud layer in ENSO years, may have caused abiotic stress, particularly a cavitation dysfunction of the xylem.As mentioned above, the density of healthy Metrosideros trees per hectare has almost reached the level of unaffected forest stands again " except for those stands invaded by certain invasive alien plant species. Results show a significant impact of Psidium cattleianum(Myrtaceae) and Hedychium gardnerianum (Zingiberaceae) on the regeneration of indigenous plant species. Both species were not present on the plots in 1976. Today, Psidium cattleianum makes up about 25% of all understorey trees on the study sites, apparently replacing the expected new cohort of Metrosideros polymorpha. Dense stands of Hedychium gardnerianum displace any Metrosideros regeneration as well as numerous other indigenous, endemic, or alien plant species. Both species are probably capable of completely changing structure and species composition of Hawaii´s montane Metrosideros rainforest.