"Albizia trees are a perfect example of the damage an invasive species can cause.”
The Conservancy’s Hawai'i Director of Forest Conservation
By Aja Hannah
Walking through Pahoa today, you wouldn’t realize that nearly six months ago Tropical Storm Iselle hurtled through this quiet Big Island town, causing widespread power outages, road blocks, flooding and crop damage in the larger district of Puna. In the lush green bush of Hawaiian Paradise Park, an area hit hard by the storm, I find remnants of the main culprits for the damage: albizia trees.
I am reminded that a green forest isn’t necessarily filled with healthy native growth, a fact impressed upon me by Dr. Sam ʻOhu Gon, Hawaiʻi senior scientist and cultural advisor for The Nature Conservancy. Puna is one of the last strongholds for lowland wet forest with the largest acreage in the Hawaiian Islands, and albizia are forcing their way in.
In the aftermath of Iselle, the dead arboreal intruders have been dragged off the roads and taken down from the power lines. Saws were put to larger pieces and roots lay upended, splayed like wide and brittle funnel cake. According to authorities, albizia caused 90% of the area’s structural damage—and the threat is not confined to Pahoa.
Trae Menard, who lives on Kauaʻi and is the Hawaiʻi director of forest conservation for The Nature Conservancy, notes that if similar strong winds were to hit Kauaʻi, the damage would be expansive. “There would be sections of this island that would completely cut off,” he says. “Albizia trees are a perfect example of the damage an invasive species can cause. They remind us of why we need to keep these kinds of trees from gaining a foothold in the Hawaiian Islands.”
Fast Growing, Highly Invasive
Fighting albizia is like fighting a war against a growing army. The tree, whose scientific name is Falcataria moluccana, is one of the fastest growing tree species in the world. It has been designated as a highly invasive by the Hawaii-Pacific Weed Risk Assessment.
The roots are broad but shallow so many trees topple over in storms. With significant wind, the branches easily snap and fall on power lines, houses, and roads. Albizia’s weak wood is also prone to “sudden limb drop,” where branches will plummet to the ground without any apparent reason.
Albizia also changes the soil’s composition, pulling nitrogen from the air and adding it to the soil and leaf litter. This high nutrient creates a soil chemistry which favors non-native plants, and can spread into our mountain watersheds.
Additionally, albizia’s seedpods are light and travel on gusts of air. Once they land, seedpods can lay dormant in the earth until they are ready to sprout. So, even when the trees are removed, they are not gone. “I would suspect that there is probably a substantial seed bank waiting to germinate and come back,” says Menard.
Albizia trees originated in Africa and have maintained the savannah-like look as it spread through the Pacific. The species that has invaded Hawaii came from the Molucca Islands in Indonesia—hence the name Falcataria moluccana. While there are several theories as to how albizia ended up in Hawaiʻi, we know that in 1917 Joseph Rock introduced the species as a reforestation tree in Oʻahu’s Mānoa Valley.
Today, the U.S. Forest Service has teamed up with the Hawaiian Electric Light Company (HELCO) to kill and cut down hazardous trees that pose an immediate threat to structures like power lines.
Crews cut or drill holes into the bark and drip a concentrated herbicide, killing the tree from the inside without affecting the surrounding foliage. The approach, while cheap and low in toxicity, is time-consuming and only holds the line of trees at bay. Larger albizia, once dead, still need to be removed and an experienced arborist needs to be called in.
In the back of Oʻahu’s Mānoa Valley, the Lyon Arboretum is utilizing special equipment to take down and dispose of 12 dangerous albizia trees. Elsewhere, the removal process is not so easy. On Kauaʻi, where individual trees grow on cliffs or in remote areas, The Nature Conservancy is strategizing ways to kill the plants from the air without harming other plant life. Another idea being discussed is biological control—finding a natural predator of albizia to bring to Hawaiʻi that would kill the trees without affecting the native growth.
From volunteers and conservationists to big companies like HELCO to government agencies and contracted arborists, people have begun to take action. Time is no ally. The threat is real, present and should not be put off until the next hurricane season.
Of the damage to Pahoa, Menard observes, “I wasn’t surprised. Every time I’ve been in that area, I’ve seen the albizia everywhere. I always thought it was a disaster waiting to happen.”
Aja Hannah is a freelance writer and Na Leo 'O Hawai'i TV producer who lives on Hawai'i Island.