Study shows non-native species of trees can harm rainforests
A 3-D image of a Hawaiian rainforest depicts the encroachment of two invasive tree species (represented in pink and red) upon the native trees (in greens and blues). The low vegetation in the foreground is an agricultural field.
To the list of threats to tropical rainforests, you can add a new one: trees.
It might seem that for a rainforest the more trees the merrier, but a new study warns that non-native trees invading a rainforest can change its basic ecological structure, rendering it less hospitable to the myriad plant and animal species that depend on its resources.
A research team led by Gregory Asner, assistant professor (by courtesy) in the new Department of Environmental Earth System Science, used its innovative remote sensing technology on aircraft to survey the impact of invasive species on more than 220,000 hectares (850 square miles) of rainforest on the island of Hawaii. Previous studies of the impact of invasive plants on forests were limited to small areas. Instruments aboard the Carnegie Airborne Observatory (CAO) penetrate the forest canopy to create a kind of regional CAT scan of the ecosystem, identifying key plant species and mapping the forest's three-dimensional structure.
Asner, who is also on the faculty at the Carnegie Institution's Department of Global Ecology, is the lead author on a paper detailing the study in the March 18 issue of Proceedings of the National Academy of Sciences.
"Invasive tree species often show biochemical, physiological and structural properties that are different from native species," Asner said. "We can use these 'fingerprints' combined with the 3-D images to see how the invasives are changing the forest."
It is the first time this approach has been used to track invasives in Hawaii, where roughly half of all organisms are non-native and approximately 120 plant species are considered highly invasive. Most of the invasive plant species in Hawaii were first introduced as agricultural crops or planted around homes as ornamental plantings and spread from these sites into native forests.
"One of the most interesting and alarming things that we found is that some of the invasive species are far from agricultural land or from neighborhoods," Asner said. "They're really in remote places, and they are dispersed by birds and introduced pigs."
Undisturbed Hawaiian rainforests are often dominated by the ohia tree (Metrosideros polymorpha), 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 stands of these two invasive species form significantly denser canopies than the native ohia trees do. Less light reaches lower forest levels, and as a result native understory plants, such as tree ferns, are suppressed.
Introduced trees also can pave the way for more invaders by altering soil fertility. The Moluccan albizia (Falcataria moluccana) fixes atmospheric nitrogen, concentrating it in the soil, which speeds the growth of a smaller invasive tree, the strawberry guava (Psidium cattleianum). The guava trees form a dense, mid-level thicket that blocks most light from reaching the ground and stifles young native plants.
"All of our invasive species detections were made in protected state and federal rainforest reserves," Asner said. "These species can spread across protected areas without the help of land use changes or other human activities, suggesting that traditional conservation approaches on the ground aren't enough for the long-term survival of Hawaii's rainforests."
"These new airborne technologies, which are sensitive enough to discern saplings and young trees, may make the problem more tractable," according to study co-author Flint Hughes of the U.S. Forest Service. "They allow scientists to probe the makeup of forests over large areas and detect invasions at earlier stages."
Asner and his team developed a system combining two technologies. An imaging spectrometer measures sunlight reflected from the leaves of the trees up to the aircraft. "We can measure the reflectance of things in many hundreds of bands, far wider in the spectrum than what we see with our eyes," Asner said. That detail enables the researchers to gather reflection data with a high degree of precision, which enables them to calculate the chemical composition of the leaves doing the reflecting. That data, in turn, allows them to determine the species of plant to which those leaves are attached.
The other part of the system is a special type of laser called a lidar. "We can fire up to a hundred-thousand shots per second," Asner said. "As the laser shoots down, it's percolating through the canopy and every time it hits something, a little bit comes back to the aircraft. From that we can reconstruct the 3-D structure of the canopy, all the way down to the plants that are in the understory."
Based on the success of this study, Asner and his colleagues, including Peter Vitousek, the Clifford G. Morrison Professor in Population and Resource Studies in the Stanford Biology Department, plan to expand CAO surveys of the ecological impacts of invaders in other forests on the island of Hawaii and Kauai, where premier, remote rainforest reserves remain virtually unmapped. "Carnegie and Stanford and a lot of state and federal agencies are working very closely now in Hawaii on these issues," Asner said.
Other co-authors of the paper who are affiliated with the Carnegie Institution are David Knapp, Ty Kennedy-Bowdoin and Roberta Martin.
The study was funded with a NASA Terrestrial Ecology Program Biodiversity Grant. The Carnegie Airborne Observatory is made possible by the support of the W. M. Keck Foundation and William Hearst III.
