Albizia Makes Inroads in Native Forests of Puna

posted in: February 2003 | 0

High, broad limbs with graceful compound leaves radiate from the albizia’s massive trunk. At its base, roots the size of elephant legs anchor the tree to the earth. In between those buttresses, a dense network of fine, hair-like roots fills the interstices of the lava rock substrate that the albizia has found so welcoming in Hawai`i.

To the untrained eye, Falcataria moluccana might appear to be all you could want in a tree. Unquestionably lovely. Fast-growing, reaching heights of 20 feet or more after the first year of growth. Shady – but not so much that nothing will grow in the soft light that filters through its lacy canopy. Able to tolerate nutrient poor conditions. More, it positively thrives on them.

And that, in Hawai`i, is precisely the problem, explains Flint Hughes of the Institute of Pacific Islands Forestry, an arm of the U.S. Department of Agriculture’s Forest Service. With its ability to draw nitrogen from the air and to “mine” phosphorus from lava rock, albizia is rapidly invading tracts of low-elevation native forest characterized by nutrient-poor soils and the plants, such as the ubiquitous `ohi`a ( Metrosideros polymorpha) , able to eke out a living on them. Over the last few decades, that hostile, low-rent habitat, the saving grace of the `ohi`a-dominated forest, has been steadily encroached upon by the advancing albizia.

For the last two years, Hughes and colleagues at the institute have been studying what some might call the darker side of albizia in Hawai`i’s native lowland forests, particularly those in the Puna district of the island of Hawai`i. The project, he explains, grew out of an effort to investigate the impacts of clidemia, a melastome that has invaded many areas of native forest in Hawai`i. “We were looking at Lava Tree State Park and noticed a lot of clidemia under albizia trees. Then we started to look more at albizia itself as a facilitator for introduced species,” Hughes says. “We used aerial photos to look at where albizia was” in 1965, 1977, and 1993, the most recent year available.

What he and his colleagues found were “ecosystem-scale impacts.” “Albizia is creating a novel system, increasing nutrient inputs and availability, adding fine root mass, litter, enormous roots,” says Flint. “It’s a transforming tree,” one that has changed the very architecture and functioning of the forest.

A few native trees, including alahe`e and lama, seem to find the albizia’s canopy agreeable. At the Malama Ki Forest Reserve, healthy specimens of these natives are interspersed with the much more common strawberry guava. Hughes speculates that the nitrogen and phosphorous added to the soil by leaf litter from the albizia pave the way for the guava while giving some natives a boost as well.

What you won’t find co-existing with the albizia are `ohi`a. On a 1790 flow of smooth, pahoehoe lava on the makai end of the Malama Ki reserve, albizia forms a closed canopy. In its shade rise thickets of the slim, telltale reddish trunks of strawberry guava, with an accent here and there of alahe`e, lama, noni, and octopus tree. In the mauka portion of the reserve, `ohi`a still dominates, forming an open canopy that, despite the knee-high broomsedge, still hosts such natives as `akia, pukiawe, ulei, and other “pioneers,” as Hughes calls them, on the pahoehoe substrate.

As we approach the boundary of the albizia stand, we see stumps of old, dead `ohi`a more and more frequently. At the boundary itself, and just beyond, a few `ohi`a hang on to life, but not for much longer. Albizia is a merciless killer of the `ohi`a; under its arching canopy, there is little room for this stalwart of the Hawaiian forest.

At the nearby Keauohana Forest Reserve, the story is much the same, although the substrate here is rough a`a lava, made up of a 200-plus-year-old flow overlain with a 50-year-old flow. On the younger lava, small, slow-growing `ohi`a have put down their roots. The trees here are scattered. On the older lava, however, the `ohi`a are large, dense, and form the backdrop for a nearly pristine wet Hawaiian lowland forest. Push your way through a thicket of strawberry guava along the road’s edge, and suddenly you’ve traveled in time back to pre-contact Hawai`i. Moss-covered logs are nurseries for native seedlings. Peperomia rambles over the forest floor, while giant bird’s-nest ferns perch precariously on the highest trees. The vines of `ie`ie are everywhere; those of the maile less in evidence, but by no means rare.

In 1922, at the time the area was included in the territorial forest reserve system, territorial forester C.S. Judd wrote: “The forest, which consists of `ohi`a and kopiko trees and `ie`ie vines É is in excellent condition and does not appear to be in need at present of fencing in Hawai`i.” If Judd were to tour portions of the reserve today, he would probably not notice the changes.

This is about as close as it gets to the forest primeval in Hawai`i – or at least, the wet lowland forest primeval. Hughes says he has never seen signs of pigs or other ungulates here. Human presence is also hard to detect, apart from the occasional pink surveyor’s flag marking placement of a litter trap or resin bag, the tools Hughes and his colleagues use to determine the forest’s productivity and character.

Yet just across the road is the albizia that can change all this, if the state does not move soon to protect this stand. “My concern,” says Hughes, “is that the albizia will keep marching through this forest reserve. And if that happens, its worth as a Natural Area Reserve will be diminished.”

In the last few years, the 1,600-acre Malama Ki was proposed for Natural Area Reserve status, the highest level of protection the state affords to its rare native ecosystems. Keauohana Forest Reserve – a 261-acre postage stamp of land — has not been nominated as a NAR.

Randy Kennedy, director of the state NARS Commission, says this year his staff will be doing the surveys needed to support the nomination of Malama Ki and other areas when they come up for a vote by the NARS Commission. Malama Ki is one of the two “top-rated” sites on the Big Island, he says, the other being the 40,000-acre Mauna Loa Mosaic.

King of the Hills

By measuring leaves and branches (litterfall inputs) in a given area of the forest floor, Hughes and his colleagues can obtain an idea of forest productivity, with and without albizia, in both old (200- and 300-year-old flows) and new (50-year-old flows), and on both `a`a and pahoehoe substrate.

The litterfall also allows them to measure the nitrogen and phosphorous available to other plants, which gives researchers an idea of the degree to which albizia facilitiates the invasion of other plants.

Their preliminary results, presented at the most recent Hawai`i Conservation Conference, show marked differences in the productivity of native versus albizia-dominated areas, and even greater differences in the available nutrients.

On the 50-year-old `a`a flow, the albizia-dominated areas produce about nine times as much litterfall as the `ohi`a-dominated areas. Over time, the ratio diminishes, but is still dramatic; on a 200-year-old pahoehoe flow, five times as much litterfall is produced under an albizia canopy as under an `ohi`a canopy.

Differences in the amount of nitrogen produced are even greater. Hughes and his colleagues have estimated that quantities of nitrogen in albizia-dominated areas are up to 60 times what they are elsewhere. The albizia forests can produce up to 200 kilograms of nitrogen per hectare per year on a 50-year-old a`a flow, their early results suggest.

The researchers have also come up with estimates of the prevalence of native and alien species, both under and away from the albizia canopies. Here again the changes are noteworthy: In the areas away from the albizia, nearly all the species are native, regardless of the age of the flow or whether the substrate is pahoehoe or `a`a. Under the albizia canopies, strawberry guava accounts for half to almost two-thirds of the individual plants.

Perhaps the most startling of their preliminary findings has to do with the incompatibility of `ohi`a and alibizia. On the young lava flows, in areas away from the albizia canopy, 100 percent of the `ohi`a found were alive. On the same flows, but under albizia, 100 percent of the `ohi`a were dead.

Profile of an Invader

As befits a ruthless usurper, albizia has a number of noms de guerre. The type that is found in Puna and generally along the East Hawai`i coast is Falcataria moluccana, also known as Paraserianthes falcataria, depending on the taxonomy you choose. Before that, it was called Albizia moluccana. On the Kona side of the Big Island, a different type of albizia – Albizia chinensis – is found.

The presence of albizia in the islands goes back to 1917, when Joseph Rock introduced the trees as part of the effort to revegetate denuded slopes with fast-growing species. Between 1927 and 1954, 9,000 albizia seedlings were planted on the Big Island, 138,000 across the archipelago.

Albizia grows at a phenomenal rate. Trees can attain heights of 150 feet, with trunks more than a yard in diameter. A one-year-old tree may reach a height of more than 20 feet, while a 10-year-old tree can be nearly 100 feet tall. The crown of an albizia can extend half an acre or more; the roots supporting the tree fan out to cover an area just as large.

The albizia invests more in size than strength, however. Its wood is soft and has little commercial value. The unmistakable crack of breaking branches and the thunder as they hit the forest floor are the sounds of trees falling in the albizia wilderness. Where albizia are found in developed areas, such as many of the Puna subdivisions, residents are concerned about damage from limbs falling on houses and roads.

According to Fred Bell of the Honolulu-based Institute of Pacific Islands Forestry, in the 1960s, an entrepreneur started up a thriving veneer industry using albizia.

“It did well for a couple of reasons,” Bell explains. The barges bringing goods to Hawai`i from the west coast were going back largely empty, so the entrepreneur was able to obtain cheap shipping rates. “He got his veneer stuff going to the Mainland for a fraction of the going rate,” Bell says, and was exporting several million board feet a veneer of albizia. (The albizia is a high-quality core material for plywood, Bell says, but it doesn’t work as well for the top layer, since “it peels and is a little bit fuzzy. It doesn’t give the nice, smooth plywood people are used to.”)

The bottom fell out of the albizia exports when lumber companies in the Pacific Northwest put the pressure on the barge companies to end their subsidies of the Hawai`i veneer, Bell recalls.

At the moment, he said, a grower on Kaua`i is looking into ways to make albizia into treated lumber products suitable for decking and other outdoor use, Bell says. “Figuring out ways to utilize this beast is part of the solution” to getting it out of places where it is undesirable, Bell says. “The best way to get rid of something is to find a use for it.”

— Patricia Tummons

Volume 13, Number 8 February 2003

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