Finding hidden invaders in a Hawaiian rain forest

By applying novel measurement techniques from a high-altitude aircraft, scientists detected two species of invading plants that are changing the ecology of rain forest near the Kilauea Volcano in the Hawaii Volcanoes National Park. Lead author, Dr. Gregory Asner of the Carnegie Institution’s Department of Global Ecology, explained: “We found chemical fingerprints from the plant leaves and used them to tell which species dominated specific areas. We employed the recently upgraded NASA Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) to measure leaf nitrogen and water content from the aircraft, and corroborated the data on the ground. The fingerprints showed where the native dominant tree ’ohia’ (Metrosideros polymorpha) has been taken over by the invading Canary Islands tree, Myrica faya, and more importantly identified areas where Myrica invasion is in its early stages. The aircraft imagery also showed us how the forest canopy chemistry is changing as a result of the invader.” The study is published in the March 7-11, 2005, early online edition of the Proceedings of the National Academy of Sciences.

The new methods are exciting because they detect effects of biological invasions on ecosystems, not just the presence of an invader. Islands like Hawaii are vulnerable to biological invasion; new species can wreak havoc very quickly. The fact that the new techniques allowed the scientists to detect an invader before it dominated the landscape is important to future management strategies. As a result of the findings, the group has expanded to include collaborators from federal, state, and private organizations. Scientists and resource managers from Carnegie, Stanford University, the U.S. National Park Service, NASA, and The Nature Conservancy have teamed up with an unprecedented plan to map the chemical and structural composition of Hawaiian ecosystems and to find invasive species and track their ecological impacts. This month, Carnegie global ecologists and engineers from the NASA Jet Propulsion Laboratory are flying an upgraded version of the AVIRIS airborne spectrometer on a more nimble Twin Otter turboprop aircraft, not only to find invasive species, but to develop the next generation of ecosystem monitoring capabilities.

On Kilauea Volcano, the native Metrosideros tree typically has a low concentration of nitrogen in its leaves ( .6% to .8%), while the invading Canary Islands tree has relatively high nitrogen concentration (1.5% to 1.8%), because it can acquire nitrogen from the atmosphere.

“The high leaf nitrogen associated with the invading tree means that it is basically fertilizing the forest with more nitrogen,” commented Asner. “The leaves turn over faster and there is more nitrogen in the soil. However, the invader shades out nearly all other species, so this excess nitrogen is not available to other species. Although we don’t know exactly what the domino effects of this invasion will be, we are in a good position to predict them as we learn more about the chemical changes the forest is undergoing.”

The scientists also had a big surprise using the new aircraft-based techniques: they located another invader, the Kahili ginger plant (Hedychium gardnerianum), growing under the forest canopy. Ginger cannot be detected from above the forest canopy using traditional aircraft or satellite approaches, but the new methods are sensitive to its high water content. In addition, the aircraft-based analysis discovered that ginger reduces the amount of nitrogen in the Metrosideros forest canopy–a discovery that was later corroborated by ground-based sampling.

Peter Vitousek from Stanford University, who coauthored the paper, commented: “This is the first time where remote sensing showed me something new concerning how an ecosystem works. Up to this point, remote sensing has been invaluable for understanding how features or processes that have been observed in one or a few places are distributed in space and time. These new methods discovered a consequence of biological invasion that had not been detected before AND showed how it varies across the landscape.”

“These findings are valuable to resource managers on two levels,” remarked Tim Tunison, Chief of Resource Management of the Hawaii Volcanoes National Park. “We need to understand the ecological effects of invasions over the landscape to develop effective control strategies, and the Asner/Vitousek work gives us valuable insights about this problem. On a more practical level we need to know the distribution of invasives. Weeds are often difficult to find in dense, wet forest in Hawaii. This study has helped us with a particularly difficult-to-map species with confusing signatures, Myrica faya. This is the first time in my experience that remote sensing has detected an understory species, kahili ginger, one of the most disruptive weeds in Hawaiian rain forests.”

Asner commented on the expanded effort with the multi-institutional team: “Because Hawaii contains so many different types of ecosystems, from desert grasslands to tropical rain forests, Carnegie’s ecological remote sensing program has focused on the area as the ideal outdoor research laboratory for devising the next generation of aircraft and spacecraft observations. Now we’ve added a major focus on the application of our techniques to invasive species problems in the Hawaiian Islands. It is a win-win combination for all involved.”