‘Superweed’ strikes at heart of watershed

Editor’s note: This is the first installment of a two-part story on efforts to eradicate the Australian tree fern from the island of Kaua‘i. Look in tomorrow’s edition for part two.

War has been declared. And the Australian tree fern is the enemy.

Local officials have begun battling the lanky, invasive plant, which they say threatens to push out Kaua‘i’s native vegetation and wreak havoc on the watershed.

“This is an ecological emergency,” said Trae Menard, director of The Nature Conservancy of Hawai‘i’s Kaua‘i program, whose organization has been tasked with finding a solution to the crisis. “It’s the most immediate and most urgent threat we have to deal with. And if we don’t deal with this as aggressively as possible, it’s going to take over.”

A forest restoration effort has been focused on a 12,000 acre area in the heart of the island, a place home to some of the state’s most pristine native forest.

“The Australian tree fern is a superweed that will dominate and change the character of our native forests if its spread is not limited now,” said Chipper Wichman, director of the National Tropical Botanical Garden, a member of the Kaua‘i Watershed Alliance, a consortium of forest landowners and the Kaua‘i Water Department. “We do not have the luxury of being able to wait another 10 years to sort this out. We have an obligation to future generations to do all that we can now to save the core of our watershed.”

The tree fern has become such a problem because of its rapid growth and the fact that it reproduces at a young age by dispersing millions of spores into the air. The microscopic spores easily hurdle natural barriers, floating on the wind for miles. New trees choke out the native trees, shrubs, ferns, lichens and mosses that capture rainfall, maintain stream flow, prevent soil erosion and provide a consistent water source for the island. The end result is a “monotypic” forest of one dominant species.

“If you replace the native forest with something like the Australian tree fern or a few other weeds, you’re going to dramatically impact the way that forest is going to capture and provide water,” said Menard. “You’re going to get more sedimentation, more landslides. It’s all connected.”

Cyathea cooperi, also known as the lacy tree fern, scaly tree fern and Cooper’s tree fern, was widely introduced to Hawai‘i by landscapers and nurseries in the last half century. Many homes and businesses have utilized the hardy plant, which can grow greater than 40 feet tall with fronds stretching 20 feet, to add a tropical look to its landscaping.

But in recent years, as officials realized the damage that the plant was doing to Hawai‘i’s fragile ecosystem, it went from ornamental vegetation to enemy No. 1.

“It has the potential to completely alter the biological community that makes up our native forests,” said Wichman. “It’s multiplying rapidly and if we don’t act decisively now it will be so established and in such great numbers that controlling it in the core of our watersheds will not be possible.”

Though the tree fern is found elsewhere in Hawai‘i, nowhere has it taken a hold as it has on Kaua‘i.

“From a global biodiversity standpoint, Kaua‘i is a very important place. More than 50 percent of the flowering plants in the state of Hawai‘i are found on Kaua‘i. More than 100 of those are ‘single island endemics,’ found only on Kaua‘i.”

In 2003 the Kaua‘i Watershed Alliance contracted The Nature Conservancy of Hawai‘i to assist with coordinating a response to the growing problem. The group began aerial mapping of Kaua‘i’s central mountains, focusing on weeds, disturbances and other threats to the native forest. Menard said by 2005 it was clear the infestation of the Australian tree fern was a major problem.

“When we got to the maps of the Australian tree ferns, our jaws dropped,” said Menard. “It was just everywhere. That’s when we started sounding the alarm. This is not your normal weed problem.”

The group has since begun to implement a three-pronged approach to combat the invasive plant. Ground crews will be sent into the interior on foot wherever possible to apply an herbicide known as Garlon 3A to the tree ferns.

In areas too dangerous or remote to access on foot, or where human contact might further threaten the ecosystem, helicopter application will be used. Using a hanging herbicide application apparatus consisting of a 100-foot-long cable with a weighted spray nozzle known as the “spray ball” or “stinger nozzle,” a quick shot of herbicide is applied directly into the center of the plant. A dye is included in the mix to easily track which plants have been sprayed and monitor any overspray. The same technique has been used fighting miconia on Maui, another place where the invasive plant was introduced as an ornamental.

With both applications, Menard said the goal is to use the smallest amount of herbicide possible while still being effective. Recent tests used between 2 and 17 percent of the allowable rate on the label.

“When we did the calculations, we found that we were only using one ounce of herbicide per plant,” said Menard. “Less than 3 percent of the surrounding vegetation was damaged.”

While some question the spraying of any type of herbicide within the watershed, Wichman said that not spraying may pose an even greater risk to the island.

“Spraying herbicide to control invasive species is a very specialized and powerful tool that we have in our toolbox,” said Wichman. “If we choose to use that tool we must do so in a manner that is demonstrated to be safe and offer a greater benefit to our ecosystems than not using it. In the case of the Australian tree fern, if it can be shown that aerial spraying can be done without significant collateral damage to the biology of our native forests, then we have a responsibility to use it to protect our natural and cultural heritage.”

The NTBG has offered a section of its land for testing spraying techniques.

“The idea of spraying herbicide from a helicopter is a concern to members of the community and they want to know what the impacts of this are,” said Wichman. “I share these concerns and have offered to use our long-term conservation and research site in the upper Limahuli Preserve to conduct tests of the selective helicopter control of the Australian tree fern.”

• Look in tomorrow’s edition of The Garden Island to read about the ongoing battle to eradicate the Australian tree fern through information sharing.


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