Five Foes of the Hawaiian Rain Forests

posted in: April 2008 | 0

The invasive plants called out for special attention in the Carnegie Airborne Observatory surveys are among the most destructive of all forest invaders in Hawai`i. What follows is a short thumbnail sketch of each of them:

Kahili Ginger, Hedychium gardnerianum

Native to the Himalayas, no one knows when this plant was introduced to Hawai`i, but with showy, fragrant flower stalks up to eight feet tall, presumably it was brought here as an ornamental.It thrives in wet habitats between sea level and 5,000 feet, forming dense mats that crowd out other species. It spreads both by seed and by rhizomes. The wilt-causing bacterium Ralstonia solanacearum may be able to hold its growth in check, but more work needs to be done in developing a dispersal method.

Tropical Ash, Fraxinus uhdei

This shade-tolerant tree from Mexico was widely planted in Hawai`i as a potential source of lumber. Reaching heights of 90 or more feet, it forms canopies that shade out most understory species. Tropical ash colonizes disturbed areas in forests with its wind-dispersed seeds and prevents the re-establishment of native plants. Currently, only chemical herbicide treatment has been effective in controlling its spread.

Faya Tree, Morella faya

Faya, native to the Azores, Madeira, and Canary islands, was introduced to Hawai`i at the beginning of the 19th century, probably as an ornamental. In the 1920s and 1930s, faya was planted by the Territory of Hawai`i for watershed reclamation until its invasive qualities were recognized. A nitrogen fixer, faya adapts to a wide range of habitats. It quickly forms dense canopies, choking out natives. A type of fruit rot already found in Hawai`i, Botrytis cinerea, may be able to suppress the spread of faya. The wounded-tree method, cutting off the bark at the base and applying an herbicide, is another control method being used.

Strawberry Guava, Psidium cattleianum

Introduced in the early 1800s for its edible fruit, strawberry guava, native to Brazil, soon escaped cultivation. It reproduces quickly, with its dispersal aided by wild pigs and birds that eat the fruit. Strawberry guava is found across a wide elevational range. The U.S. Forest Service has petitioned the state Department of Agriculture for permission to release a promising biological control agent, Tectococcus ovatus, against strawberry guava. Tectococcus, a small insect that creates leaf galls, is expected to spread gradually on the plant and reach damaging levels within a few years.

Albizia, Falcataria moluccana and Albizia chinensis

Native to tropical Asia, albizia was introduced to reforest barren lands by botanist Joseph Rock in 1917. Spread by seed and fast growing, albizia thrives in nutrient-poor conditions. It can attain heights of 150 feet, with a crown extending half an acre or more. The roots supporting the tree fan out to cover an area just as large. Albizia is very susceptible to hormone type herbicides and reportedly susceptible to being killed by root damage from heavy equipment. The potential for biological control has not been evaluated, but finding any biocontrol organism to target albizia without damaging native trees is going to be difficult, since so many native tree species are also leguminous.

— Tara E. McCarthy

Tara McCarthy is a graduate student in the Audubon Expedition Institute of Lesley University in Cambridge, Mass. She spent part of her spring break working with Environment Hawai`i.

Volume 18, Number 10 April 2008