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Invasive alien woody
species in Pacific
island forests

J.S. Denslow

Julie S. Denslow is Research Ecologist and
Team Leader of the Invasive Species Unit of
the Institute of Pacific Islands Forestry,
United States Department of Agriculture -
Forest Service, Hilo, Hawaii, United States.

The invasive tree Miconia calvescens, originally introduced for its ornamental value, is threatening native forest in Tahiti, with monospecific stands that create barren understoreys and facilitate sheet erosion on steep slopes

- J.-Y. MEYER

In recent years invasive alien species have gained considerable notoriety as major threats to species and ecosystems. Invasive alien species commonly are defined as exotic or non-native species that alter ecosystem processes and threaten the survival of native species in natural ecosystems or cause a significant economic impact in agricultural or other managed lands. By definition, invasive alien species are also naturalized alien species; that is, they have established self-reproducing populations without the necessity of human intervention. While not all naturalized alien species become invasive, the small proportion that do generate substantial costs in lost revenues, in expenses for their control and in lost conservation values and ecosystem services. In native forests, invasive alien plants are able to dominate the understorey, to strangle saplings and to suppress native species. They can promote fire and alter water and nutrient availability. Even natural disturbances such as tree falls facilitate the establishment of exotic species. Since few forests are disturbance-free, all are vulnerable to establishment of invasive alien plants.

The Hawaiian islands in the United States are notable for their high diversity of both endemic and invasive species. About 90 percent of Hawaii's approximately 1 300 species of flowering plants occur only there, often restricted to single islands, mountains or valleys. Hawaii also has as many naturalized exotic plants as it does natives, many of which are known to have negative impacts on native ecosystems. Over the past century, accidental and intentional introductions by gardeners, ranchers and foresters have contributed to the large number of exotic plant introductions into the state. For example, the critical need to revegetate degraded landscapes, protect fragile watersheds and establish potential timber species stimulated extensive planting of a variety of exotic species. Throughout Hawaii, foresters planted over 800 species of trees as well as shrubs, vines, herbs and grasses in a wide range of habitats (Nelson, 1965), undoubtedly providing many opportunities for new species to establish populations.

In Tahiti and Hawaii, the South American tree Miconia calvescens invades the understorey of native rain forests, eventually growing through the canopy and suppressing the native trees. This species was originally introduced for its ornamental value, but its spread has been facilitated by hurricanes, feral pigs and hikers. In Tahiti, the subsequent invasion has homogenized the landscape as monospecific Miconia stands replace native forest, create barren understoreys and facilitate sheet erosion on steep slopes. Hawaii is now spending US$1.5 million per year to protect its native wet forests from a similar fate.

Other invasive trees in Hawaii were introduced for their timber potential or for watershed protection. Grevillia robusta (silk oak), introduced from Australia for its commercial timber potential and for use in reforestation, is now widely invasive in dry forest ecosystems. Its litter is allelopathic, suppressing the establishment of other species. Falcataria moluccana (syn. Albizia falcataria), introduced for shade and its soil-improving properties, is widely interplanted with crop and timber species. It naturalizes easily, however, increasing nutrient supply rates, suppressing native species and facilitating the spread of other alien species. These are but three examples of more than 100 exotic species that threaten native ecosystems in Hawaii and other Pacific islands.

While Hawaii seems like the worst-case scenario, it also exemplifies the vulnerability of many Pacific islands (Space, 2001). For example, the hillsides of Guam and other Pacific islands were sown with the Central American woody legume species Leucaena leucocephala to cover soils laid bare by intensive bombing during the Second World War. Guam remains dominated by Leucaena today, because its dense cover and deep litter accumulation impede establishment of native trees.

In Hawaii, the most carefully designed forest management plans are hampered by the challenges of weed management. The native koa (Acacia koa), a Hawaiian hardwood valued for its beautiful grain and light-reflecting figures, is currently logged from native forests or salvaged from aged and damaged stands. Although koa regenerates well following substrate scarification, several species of invasive exotic grasses compete with koa seedlings for scarce water resources and facilitate the spread of wildfires. In wet forests, any canopy opening or substrate disturbance prompts the establishment of Psidium cattleianum (strawberry guava) and Clidemia hirta, both of which effectively compete with natural regeneration and impede subsequent silvicultural intervention. Programmes to convert old sugar-cane lands to Eucalyptus production must include the costs of controlling the exotic tree Casuarina equisetifolia.

The proliferation of local, national and global initiatives to stop the spread of invasive species is testimony to the impacts of some plant, animal and pathogen populations where they are introduced outside their native ranges. The Convention on Biological Diversity (CBD), the South Pacific Regional Environment Programme (Sherley, 2000), the Global Invasive Species Program (Mooney, 1999), the International Plant Protection Convention and the Invasive Species Specialist Group of the World Conservation Union (IUCN) (IUCN, 2000), among others, promote policies and protocols designed to prevent the introduction of, to control or to eradicate those species that threaten native ecosystems, habitats and species.

Because of the large scale and long time frame of forestry operations and their potential impacts on the composition and health of native ecosystems, those responsible for reforestation of rural and urban landscapes have an obligation to assess the consequences of species choices. Known invasive alien plants should be replaced with native species or with exotics unlikely to spread into native plant communities. "Best management practices" should include removal of known invasives, and their use should be discouraged. Horticultural materials such as grass seed and green mulch should be inspected for their potential to introduce problem species. Nurseries, botanical gardens, extension services and government agencies should work to inform the public of the potential dangers of invasive species and should encourage the use of alternative native or exotic species unlikely to contribute to future invasive species problems. Forest managers are in a good position to provide effective leadership to protect native ecosystems from the introduction, spread and impacts of invasive plants.

The invasion front of a stand of Falcataria moluccana on the Island of Hawaii, United States; the invasive shrub Melastoma candidum has established under the tree canopy

- F.H. HUGHES


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