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

20世纪70年代，夏威夷岛上大约5万公顷的Metrosideros polymorpha（桃金娘科）雨林受到了树冠下降的影响。一项为期27年的永久性地块研究表明，一个新的队列Metrosideros polymorpha树木正在取代旧的冠层，证实了林分结构模型的预测。森林衰退的主要原因是火山爆发后原生演替形成的年龄和林分结构的统一。这种统一的队列结构导致队列中大多数树木同时老化和死亡（“队列衰老”）。我们假设这种下降是一种自然的人口现象，同步衰老是主要的诱发原因。气候波动可能是枯梢病的触发因素，特别是在排水不良的地点。在这些栖息地，极端降雨事件和干旱，如。如前所述，Metrosideros每公顷健康树木的密度几乎已达到未受影响的林分的水平，除了那些被某些外来入侵植物物种入侵的林分。结果表明，番石榴（桃金娘科）和姜花（菊科）的乡土植物物种的更新显着的影响。这两个物种在1976年都没有出现在地块上。今天，Psidium cattleianum占研究地点所有下层树木的25%，显然取代了预期的新一代Metrosideros polymorpha。密集的姜花取代任何Metrosideros再生以及许多其他土著，特有或外来植物物种。这两个物种可能都能够完全改变夏威夷山地Metrosideros雨林的结构和物种组成。

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.