7 FEBRUARY 2008
Investigating invasives
Curious whether invasive species can wreck a landscape? Then book passage to Hawaii, ground zero for the ecological havoc caused by imported plants and animals. In these remote islands, entire landscapes are under attack by exotic plants, often aided by feral pigs that root through the soil and leave a perfect seedbed for the newcomers.
Hedychium gardnerianum, the invasive herb Kahili ginger, was photographed in Maui. When this critter is present, the Carnegie researchers found three times the normal volume of vegetation at the bottom of the forest, but the middle canopy had only one-third its usual amount of vegetation. Photo: Forest & Kim Starr
Now we hear of an airborne measurement system that can not only produce a three-dimensional picture of a forest, but also put some numbers on the species-invasion problem. Invasives can distort the entire structure of a forest, according to Gregory Asner, a research scientist at the Carnegie Institution, who was lead author of a survey covering about 850 square miles of protected forest on the island of Hawaii.
Asner admits that you don't need an airplane to see the dramatic effect of invasives. "Some of the invasives we are working on have progressed rapidly just in the time I have been working here, since 1993. It's amazing how fast some of the invasions have occurred."
Invasive plants can shoulder out native plants that animals need for food or shelter. Lacking diseases and predators, these fast-growing plants can convert a diverse landscape into a virtual monoculture -- an ecological desert.
Outlined areas were covered in the study of forest structure on the Big Island of Hawaii. Although these areas all have legal protection, lines on a map do not stop the 120-odd exotic plants that are causing big problems in Hawaii's natural areas. Image: Carnegie Institution
Introduced trees can also pave the way for more invaders by, for example, altering soil fertility. The Moluccan albizia, Falcataria moluccana, for example, moves nitrogen from the atmosphere into the soil, feeding strawberry guava, a smaller invasive tree known as Psidium cattleianum . The guava trees then form a "you-have-to-see-it-to-believe-it" thicket that blocks light from reaching the ground and stifles native plants.
Ecologists have traditionally explored plant invasions by mapping their locations from the ground, but stumbling around Hawaii's steep, remote, muddy and weed-infested landscape is not the ideal situation for measuring the exact extent of infestation. "It's been very difficult, we've not had any quantitative information on how invasives impact the structure of the forest," says Asner. "A lot of information is anecdotal, or what you can see on foot; it's not systematic. What we have done is mapped it out and quantified it."
The airborne surveyor can measure the spread of invasives, and document how they are changing the 3-D structure of the forest -- how the branches and leaves are arranged in space. The instrument can even measure how much light reaches various depths of the forest.
Light level is a key determinant on which plants may live at a particular height in the forest. "With the new system, we are seeing details of the forest canopy, layer by layer," says Asner.
The Carnegie instrument looks straight down on the invasion front between fast-growing Moluccan albizia -- the big pink trees, and native forest, shown in blue and green. Without some help from the tree-huggers, the invaders win this battle every time. Photo: Carnegie Institution
Instrumentally speaking
The research focused on four trees and one ground-cover plant, all major ecological pests in Hawaii, using the Carnegie Airborne Observatory (CAO), a flying laboratory with three major components:
a spectrometer that can read up to 280 individual bands of light reflected from the forest;
a light-operated lidar, or laser system, that measures the 3-D structure of the canopies; and
an inertial-guidance system -- a fancy gyroscope -- that can locate the airplane in space to within centimeters (much more accurate than a GPS).
3-D image of invasive tree species (reds-pinks) and native Hawaiian lowland rainforest (greens), taken from the Carnegie Airborne Observatory. Image: Carnegie Institution
The key to the operation is correlating particular patterns on the spectrometer to particular chemicals, and then using that data to identify plant species. "Not all species have these unique signatures," Asner says, "but it turns out that in Hawaii, some of the most important invasive species do have them."
The crew of the Carnegie Airborne Observatory come from the Jet Propulsion Laboratory, the U.S. Forest Service, Twin Otter International, and the Carnegie Institution. Photo: Carnegie Institution
The study found, for example, that tropical ash allows only 2 to 4 percent of sunlight to reach the forest floor, much less than the 9 to 13 percent found in a nearby patch of native forest. As light is a critical factor in species growth, the composition of the mid-canopy and ground species both change.
Remotely qualified
Despite progress in a few areas, Asner says Hawaii is losing the fight against invasives. Eventually, the airborne instrument could improve the deployment of weed-whacking resources. Money is always short, and it's more cost-effective to clean out the first invaders than to tackle dense infestations. Land managers, he adds, "really want to penny-pinch, to send people ... where time and money can be spent most effectively."
While scanning for weed outbreaks, Asner says, the observatory can examine 12 acres per second -- when the land is not obscured by clouds.
Fraxinus uhdei (tropical ash) is a light-hogging exotic tree that darkens this gloomy new trail in Maui, Hawaii. Photo: Forest & Kim Starr
It's no secret that invasive trees "are really changing the fundamental makeup of these forests," Asner says, but the new scanner provides more evidence of how the notorious strawberry guava is spreading. "We detected outbreaks in very remote areas, where there was no foot traffic, no human facilitation. Ecologically it was really interesting that these plants can reproduce, move across the landscape, without our help, being dispersed by birds, wind or pigs."
- David "Weed Warrior" Tenenbaum
Related Why Files
• Migration
• Tracking Butterflies
• It came from the ground: Hidden cicadas
Bibliography
• Juvenile Hormone Regulates Butterfly Larval Pattern Switches, Ryo Futahashi and Haruhiko Fujiwara, Science, 22 Feb. 2008.
