Hawaii: Climate exacerbates repression in the rainforest

Guavas displace native ironwood trees

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Numerous plant species conquer new habitats as a result of climate change. These so-called "biological invasions" are now regarded as the world's second largest threat to biodiversity. As researchers have now discovered, for example, in the mountain rainforests of Hawaii, the Brazilian guava is spreading and displacing the native ironwood trees.

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In a long-term study, researchers from the Weihenstephan Science Center of the Technical University of Munich (TUM), the University of Hawaii and the US Geological Survey have investigated the complex interaction of natural forest development, global climate change and biological invasions on the flanks of the volcanoes Mauna Kea and Mauna Loa found that the increased occurrence of extreme climatic events and the spread of alien plant species interact and thus change the long-term dynamics of the mountain rainforests of Hawaii. In the study carried out by Hans Jürgen Böhmer with his colleagues since 1999, the population development of key species is being investigated in permanent observation areas.

Since about the mid-1960s, the researchers found an increased frequency of extreme precipitation changes in the mountain rainforest, which is considered as the trigger of a large-scale tree extinction ("ohia dieback") first observed in the 1970s. A rapid succession of climatic extremes such as high irradiation followed by extreme precipitation and again high levels of radiation can lead to a physiological shock in certain rainforest trees at certain sites, resulting in the death of the trees.

The extinct sections of the mountain rainforest, totaling more than 50, 000 hectares, are more susceptible to biological invasions than undisturbed areas. This is exploited by invasive species such as the guava (Psidium cattleianum) that came to Hawaii as a plantation tree. They spread so fast in the diseased forests that the naturally dominant ironwood trees (Metrosideros polymorpha) can not regrow. Where the guava appeared as a single specimen in the 1970s, it was able to build up dense stands after the death of the Metrosideros trees. Here the expected natural rejuvenation of the ironwood trees remains. display

Successful invasive plant species thus have the potential to change the structure and dynamics of the montane rainforest in the long term. Since their influence is no longer expected to successfully rejuvenate indigenous tree species in the long term, a relatively small, guava-dominated rainforest seems to be a realistic scenario for the future, rather than natural forests dominated by ironwood trees.

(Technical University Munich, 08.03.2007 - NPO)