Wednesday, August 22, 2007

Nitrogen-fixing plants: Self-feeding as a bad thing

One of the important traits of some invasive plants is the ability to make their own fertilizer.

The firetree, Myrica faya, can do it. And as Peter Vitousek and Lawrence Walker, then both of Stanford University, reported in the journal Ecological Monographs back in 1989 (“Biological Invasion by Myrica Faya in Hawai'i: Plant Demoraphy, Nitrogen Fixation, Ecosystem Effects”), it quickly became a problem invader.

The firetree has the additional feature of producing a lot of fruit that birds like—so it has a built-in fast-spreading mechanism.

They found that the leaf litter of the firetree essentially fertilizes the ground.

“We concluded that biological invasion by Myrica faya alters ecosystem-level properties in this young volcanic area; at least in this case, the demography and physiology of one species controls characteristics of a whole ecosystem,” they wrote.

The problem, of course, is that one of the survival mechanisms of some native Hawaiian plants is the ability to survive in poor soils where other plant's can't. If aliens show up and begin scattering fertilizer in these habitats, they create further openings for a mass of other plants that otherwise couldn't compete with the natives.

The invasive two-spotted leafhopper, which attacks native and alien species alike but significantly damages firetree, may have reduced the plant's aggressiveness, although Vitousek in an email said that's more the case in dry areas than wet.

"Unfortunately, it looks green and vigorous across much of its range, despite the leafhopper, Vitousek wrote.

But there's another nitrogen-fixing invader following right on the fire tree's heels.

This time, it's the stately albizia, Falcataria moluccana, which has been fast-invading the back country on several islands in recent years. (It has also been known as Albizia falcata, Albizia falcataria, and Paraserianthes falcataria.)

“Invasion by N2-fixing tree alters function and structure in wet lowland forests of Hawai'i,” was printed in the October 2005 issue of “Ecological Applications,” R. Flint Hughes and Julie Denslow, of the U.S. Forest Service's Institute for Pacific Islands Forestry in Hilo, studied the trees in Kilauea lava flows 48 years old, 213 years old and 300 years old.

Like the firetree, the albizia fertilizes the ground around it. It spreads quickly due to seeds readily blown by the wind.

Among their findings was that native plants like 'ohi'a declined in the presence of the invaders, and other alien species, like strawberry guava increased in population.

“Results provide a clear example of how invasive tree species, by modifying the function and structure of the ecosystems that they invade, can facilitate invasion by additional nonnative species and eliminate dominant native species,” Hughes and Denslow wrote.

They added: “Invasive species pose major threats to the integrity and functioning of ecosystems. When such species alter ecosystem processes, they have the potential to change the environmental context in which other species survive and reproduce and may also facilitate the invasion of additional species.”

The nitrogen-fixing abilities of the albizia has been put to use in Hawai'i. Forester Bill Cowern on Kaua'i interplants albizia with valued hardwood species to reduce or eliminate the need to fertilize them. He has been experimenting with uses for the soft, fibrous wood of the species.

© 2007 Jan W. TenBruggencate

1 comment:

Ken Stokes said...

About Cowern's thinking on albizia...
I like his concept of creating a 'market' for this tree, which he now calls "beyond invasive...a naturalized occupant." His biomass energy strategy is "creating economic incentive to harvest the trees we wish to remove."
Now, if we could just create a market for cats claw...!