Exotic trees threaten rainforests


Washington : Hitherto unavailable airborne technologies have identified a new threat to tropical rainforests – invasive, or non-native, trees.

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Researchers at the Carnegie Institution who used an “airborne observatory” to study a Hawaiian rainforest have warned that the invasion of non-native trees can change the basic structure of the forest.

Findings of the study have been published in the Proceedings of National Academy of Sciences.

The team, led by Gregory Asner, used remote sensing aboard the Carnegie Airborne Observatory (CAO) to survey the impact of invasive trees on more than 850 square miles of rainforest in Hawaii.

“These new airborne technologies, sensitive enough to discern saplings and young trees, may make the problem more tractable,” said Flint Hughes, co-author of the study.

And a key problem is that the invasive vegetation renders the rainforest less hospitable to the myriad plant and animal species that rely on it for resources.

Instruments aboard the CAO are capable of penetrating the forest canopy to create a regional “CAT scan” of the ecosystem, identifying key plant species and mapping the forest’s three-dimensional structure.

“Invasive tree species often show biochemical, physiological, and structural properties different from native species,” noted Asner. “We can use these ‘fingerprints’ combined with the 3-D images to see how they are changing the forest.”

In Hawaii, roughly half of all organisms are non-native, and some 120 plant species are considered highly invasive.

Undisturbed Hawaiian rainforests are often dominated by the ohia tree but these slow-growing native trees are losing ground to newcomers, such as the tropical ash (Fraxinus uhdei) and the Canary Island fire tree (Morella faya).

CAO surveys of rainforest tracts on the Mauna Kea and Kilauea volcanoes found that these two invasive tree species form significantly denser canopies than the native ohia trees.

Less light reaches lower forest levels and, as a result, smaller native plants such as tree ferns die out. Invasive trees can also alter soil fertility.

Asner and colleagues plan to expand CAO surveys to other forests on Hawaii and Kauai islands, where premier, remote rainforest reserves remain virtually unmapped.