Sensor spots
forest invaders

Researchers using a sophisticated sensor aboard an aircraft flying at the edge of space were able to spot an invasive tree species starting to take over native forests near the Big Island's Kilauea volcano, according to a study published yesterday.

The sensing instrument pinpointed where Myrica faya trees, originally from the Canary Islands and the Azores, are starting to take over native ohia trees in and around Hawaii Volcanoes National Park.

Previous studies located the Myrica trees, but mostly after they had proliferated, scientists said. The study indicates the remote sensor can spot infestations at their very beginning stages when there is still a chance of controlling them.

"That's the key piece to the puzzle," said Gregory Asner, an ecologist with the Carnegie Institution's Department of Global Ecology. "Once the infestation has taken its course, it's easy to find, it's easy to see with traditional remote sensing methods."

The study, by Asner and Stanford University biological sciences professor Peter Vitousek, was published yesterday in the early online edition of the Proceedings of the National Academy of Sciences.

By using NASA's Airborne Visible and Infrared Imaging Spectrometer from an ER-2 aircraft flying up to 12 miles above the earth, Asner and Vitousek were able to measure the concentration of leaf nitrogen and water content in the plants on the ground.

The Myrica trees, which unlike the ohia are able to draw nitrogen from the air, gave off much higher levels of nitrogen than the native plants.

The effects of this are severalfold: Not only do the Myrica trees grow larger than the native trees and plants, blocking their sunlight, they change the way the soil works by fertilizing the forest with more nitrogen, Vitousek said.

"When the Myrica dies out or is replaced, a whole set of other invasive plants comes in," he said. "So it doesn't just do itself some good, it does good for other invaders in comparison."

The remote sensor also located another hidden invasive plant near the volcano: the Kahili ginger plant, which cannot be detected by conventional methods from the air.

There are several species of ginger that are considered highly invasive in Hawaiian ecosystems because they shade out other plants, use a lot of water and change soil conditions, Asner said.

The NASA instrument was able to locate the invasive ginger by measuring its high water content, and also discovered that the ginger reduces the amount of nitrogen in ohia forests. That discovery was confirmed later by a ground-based sampling.

Tim Tunison, chief of resource management for Hawaii Volcanoes National Park, said the information is valuable to resource managers because it will help them to locate the invasive species before they take hold, and it will also help them understand the invaders' effects on the native ecological system.

Asner is currently working on more studies in Hawaii, this time using the instrument aboard a smaller plane that flies slower and lower to get a higher-resolution look at the invasive plants.

He is also working with the National Park Service on Maui to detect the beginnings of miconia infestation near Hana.