How do you track the relative contributions of a plant species in an ecosystem? When you are talking about thousands of square miles of land area this can be an incredibly daunting task, but it is very important because it provides important information related to invasive species that may be displacing their native counterparts in an ecosystem.
I remember in a biology course a Stanford we were shown how you perform experiments like this using trees in a nearby nature reserve. In order to a get a sense of the geographic distribution of different types of tree, we would walk around the park with a GPS and a computer marking the points where we found trees. Considering this was chaparral, this is a time-consuming but not completely impossible task. There really aren't that many trees. You spend a day or two trucking around in the heat. No harm, no foul.
Now imagine trying to perform a similar experiment in a rain forest. Trying to mark the locations of plant species in a rain forest would take a bagillion years, even if you could get to all the points in the forest -- which half of the time you can't because they are too remote. Even if you use sampling, this is a nearly insurmountable task.
Thinking of this difficulty, I was struck by the cleverness of an alternative approach employed by Asner et al. publishing in the journal PNAS. Asner et al. used a combination of light spectroscopy, radar, and aerial surveying to map out the plant species in rain forest reserves in Hawaii.
I don't know if you have ever been to Hawaii, but it is a categorical example of the problem of species mapping. One, Hawaii has a huge issue with invasive species. The islands have numerous unique and highly evolved species that are being displaced by mainland invaders. Two, a lot of the islands are rain forest, and a lot of those rain forests are in -- let us say -- precarious positions on the sides of cliffs and volcanoes and such. I pity the grad student who they would have shimmy down a rope to count tree life on the sides of those cliffs.
The researchers, however, were determined to get some sort of head count on the invasive species of plant life in the Hawaiian islands. They use a new technology developed at the Carnegie Institution for Science at Stanford University. The technology -- called the Carnegie Airborne Observatory -- uses an aircraft to sweep over the areas of forest to be surveyed. Then using a combination of light measurements reflected off the forest and radar measurements to determine the relative contributions of different levels of canopy (and really, really accurate measurements of where the aircraft is in the air), the researchers can calculate the different types of plant life in the forest to a reasonably high level of resolution:
We deployed an airborne remote sensing system designed to measure the composition, physiology, and structure of ecosystems. The Carnegie Airborne Observatory (CAO) integrates high-fidelity imaging spectrometers (HiFIS) with light detection and ranging (LiDAR) sensors for regional-scale ecological research. The HiFIS subsystem provides detailed canopy spectroscopic reflectance signatures that express plant chemistry, physiological status, and taxonomic composition. The LiDAR subsystem provides 3D structural information on canopies, underlying vegetation, and the terrain below. The CAO HiFIS and LiDAR are physically and digitally coaligned and packaged with a high performance inertial navigation system that provides highly accurate determinations of aircraft position and the location of ground targets in three dimensions. (Citations removed.)
The theory behind this technique -- as I understand it -- is that different plants because of different relative concentrations of chromophores in their leaves reflect light differently. This different in light reflectance can be used to establish the identity of the plant.
The authors validate that their technique by going back on the ground to see whether the vegetation they saw from the air using was actually there in about the same proportion on the ground. They found that the error rates using this technique to identify species were all lower than 10% and in some cases error free.
Below is a sample of the images that this technique produces. The top shows the color of the forest observed from the air. The middle shows how this was classified into invasive and native species. The bottom shows a cross-section of the radar image of the canopy densities of plants. (Figure 1 in the paper)
What did the researchers find about invasive species in Hawaii?
Well, here is the distressing part. The researchers identify at several invasive species that are making large changes to the ecosystems. (From news coverage in Science):
The worst invader, Asner says, is strawberry guava (Psidium cattleianum), which was imported for its fruit. It can grow up to 15 meters tall and is "fundamentally changing" the forest architecture, he says, altering the hydrology and nutrient balance. Guava also restricts the light reaching the forest floor, preventing other plants from taking root. The tree has rapidly encroached into the protected areas established by Hawaii's Department of Land and Natural Resources to preserve indigenous forests, the team reports online this week in the Proceedings of the National Academy of Sciences. As a result, Asner says, P. cattleianum is "destroying native forests on the Big Island." He says the team has detected similarly severe invasions by kahili ginger (Hedychium gardnerianum), an ornamental tree whose rhizomes, or hard root caps, create a barrier in the topsoil that no other plants can penetrate.
These invaders are causing macro-changes in the ecosystem that can be best illustrated by showing the radar images of changes in the canopy. (From Figure 2 in the paper)
For example, Fraxinus uhdei is an invasive tree species depicted in the radar image above. Not only does it change the tree height for the entire forest where it has taken over, it starves the ground cover by reducing the light that can get to the ground cover:
Field measurements guided by the remotely sensed data confirmed that only 2-4% of incoming light penetrates the canopies of mature F. uhdei or M. faya vs. 9-13% light penetration in adjacent native forests. We also identified the midcanopy species potentially missing from invaded sites, which included native tree ferns (e.g., Cibotium glaucum) and smaller native woody species such as Cheirodendron trigynum and Ilex anomala. The understory of these invaded forests contained few native ground-covering ferns such as Dicranopteris linearis or native seedlings, as were usually present in their native counterpart forests. In sum, these tall, high-leaf-volume invaders create a biologically impoverished environment beneath their canopies. (Emphasis mine. Citations removed.)
Why should we care about this paper?
This paper is important for both its good news and bad news. The bad news is that whole ecosystems are changing in Hawaii as a result of invasive species. These changes if left unchecked will undoubtedly cause the extinction of species that are found nowhere else in the world. Hawaii has a long history attempting to counteract these invaders with only mixed success.
On the other hand, if you are going to seriously try to interdict invasive species, the first step is accurate measurements of where they are and what changes they are causing. To this end Asner et al. have given a great gift to the scientific community in showing how large scale measurements of ecosystem changes can be performed relatively efficiently.
Hat-tip: Science
Asner, G.P., Hughes, R.F., Vitousek, P.M., Knapp, D.E., Kennedy-Bowdoin, T., Boardman, J., Martin, R.E., Eastwood, M., Green, R.O. (2008). Invasive plants transform the three-dimensional structure of rain forests. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0710811105
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