Alien Trees Destroying Native Hawaiian Forests

Sara Goudarzi
for National Geographic News
March 3, 2008
It turns out not all trees are welcome in rain forests, as invading species can render the ecosystems inhospitable for native inhabitants, a new study says.

Recently, scientists surveyed about 850 square miles (about 220,000 hectares) of rain forest on Hawaii's Big Island using remote-sensing devices aboard aircraft.

The instruments infiltrated the forest canopy to produce what looks like a three-dimensional "CAT scan" of the area.

(See a Hawaii map.)

"Our approach identifies species by their often-unique chemical and structural properties, both of which we can map from the air," said study lead author Gregory Asner, an ecologist at the Carnegie Institution of Washington's Department of Global Ecology in Stanford, California.

From the data, Asner and colleagues determined that invasive trees are changing the structure of the Hawaiian rain forest by denying native species valuable resources, such as sunlight.

Winner Takes All

In Hawaii rain forests are generally populated by the slow-growing ohia tree, which produces the red lehua flower.

But surveys of rain forests carpeting the volcanoes Mauna Kea and Kilauea indicate that the native trees are thinning out as invasive trees—such as tropical ash and firetree—encroach on their habitat.

"The particular non-native species that we studied actually change the structure of the rain forest by shading out native species," Asner said.

"Some of the non-native species physically impede the growth of native plants, [for example] by forming an impenetrable barrier at the soil surface that prevents the seedlings of other species to grow," he added.

By altering soil fertility, invasive species can make the environment more attractive to the nearly 120 plant species in Hawaii that are considered highly invasive.

(See photos of other species invading the United States.)

The study appears this week in the journal Proceedings of the National Academy of Sciences.

Other Impacts

In addition to reducing rain forest diversity, the researchers say, invaders affect the basic life-giving services that forests provide to people.

"For example, we have shown that these particular invasive species change the amount of carbon stored in ecosystems [rather than in our atmosphere]," Asner said.

In other words, more carbon dioxide—a major greenhouse gas—could end up in the atmosphere instead of stored in the forests' natural "sink."

"Even more striking," Asner said, is that the invasive trees "negatively affect the recreational and cultural resources provided by native forests to people living there."

For instance some of the alien trees create an impenetrable layer of vegetation that makes it hard to access the forest for activities such as hiking, Asner said.

Also, invasives "shade out" some of the native tree species that are used in traditional Hawaiian ceremonies.

Julie Denslow, an ecologist at the U.S. Department of Agriculture's Institute of Pacific Islands Forestry, was not involved in the study.

"This is one of the first illustrations of the impacts of invasive species on native, intact tropical forest at a large scale," she said.

"It highlights the vulnerability of protected areas to invasive species and the need for aggressive management to maintain conservation values."

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