A new study by scientists has warned 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.
Carried out by a research team led by Gregory Asner of the Carnegie Institution's Department of Global Ecology, the study used innovative remote sensing technology on aircraft to survey the impact of invasives on more than 220,000 hectares (850 square miles) of rainforest on the island of Hawaii.
Instruments aboard the Carnegie Airborne Observatory (CAO) penetrated 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 that are different from native species," said Asner. "We can use these 'fingerprints' combined with the 3-D images to see how the invasives are changing the forest," he added.
Though the 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 and the Canary Island fire tree.
CAO surveys of rainforest tracts on the Mauna Kea and Kilauea Volcanoes found that stands of these two invasive tree species form significantly denser canopies than the native ohia trees.
Less light reaches lower forest levels, and as a result native understory plants such as tree ferns are suppressed.
According to the study, introduced trees can also pave the way for more invaders by altering soil fertility.
For example, the Moluccan albizia "fixes" atmospheric nitrogen, concentrating it in the soil, which speeds the growth of a smaller invasive tree, the Strawberry Guava. The guava trees form a dense, mid-level thicket that blocks most light from reaching the ground and stifles young native plants.
"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," said Asner.
Asner and colleagues plan to expand CAO surveys of the ecological impacts of invaders in other forests on Hawaii and Kauai Islands, where premier, remote rainforest reserves remain virtually unmapped.