invasive plants — Maui Invasive Species Committee (MISC)

_{Bingabing is an invasive plant found on Oʻahu and Hawaiʻi Island. Documented evidence of its spread in Hawaiʻi helps inform efforts to address invasive species. Photo courtesy of Forest and Kim Starr}

In 2000, Forest and Kim Starr, biologists with the University of Hawaiʻi, accepted an unusual challenge: drive all the roads of Maui at 5-10 miles per hour to look for plants that might be good targets for eradication. On the side of Olinda Road, they spotted a lone tree with large, umbrella-like leaves. It was bingabing, or Macaranga mappa.

If you’ve been to Hilo, you may know the plant; it lines roadsides and low elevation forests. But even after driving all the roads of Maui, the Starrs only found the lone plant upcountry. Why hadn’t it invaded the roadsides here?

Bingabing could have spread, it was just early in the process; the Starrr’s were in search of plants like bingabing, invasive species that were still early in their invasion and hadn’t yet started spreading.

_{An invasion curve illustrates the feasibility of eliminating an invasive species and highlights the importance of finding pests early on. Graphic by University of Florida, IFAS}

An invasion curve shows how a species spreads after its initial introduction and also illustrates the feasibility of eliminating a species. “If there are only a few individuals, it will take time before the population starts to increase dramatically,” explains University of Hawaiʻi professor and researcher Curt Daehler. Daehler studies what factors contribute to a plant’s invasiveness. “There are thousands of introduced species present in Hawaiʻi that aren’t invasive,” he says. “The goal of a plant is to thrive, so what keeps them from spreading? Certain species have a missing pollinator, or the plant is in the wrong location,” explains Daehler.

Stopping an invasive species early— during the lag phase— is an important factor for eradication. But it’s also the most difficult time for detection because their numbers are so low. Having better information about how quickly a species might start to spread can help with prioritizing targets. Estimates of lag time for introduced plants were once presumed to be very long, more than 100 years in some cases. These early estimates were based on indirect evidence and didn’t focus on plants from a tropical environment. To better understand lag times, Daehler says you’d need to intentionally plant a species and watch for its spread. It turns out the Lyon Arboretum in Mānoa Valley on Oʻahu had done exactly that.

*_{Harold Lyon planted hundreds of trees in what is now Lyon Arboretum in an urgent attempt to protect the watershed above Mānoa. Photo courtesy of University of Hawaii.}*

A hundred years ago, the slopes of Mānoa Valley were bare. For decades, feral cattle had grazed the slopes, causing rain to run off the land rather than fill the aquifers; sugar plantation owners were justifiably alarmed. They sought the help of plant pathologist, Harold Lyon. He advocated for protecting the remaining native forests, but also recognized the urgent need to restore vegetation. On a plot of land that eventually became the Lyon Arboretum, he planted hundreds of species from around the world looking for ones that spread swiftly to stabilize the soil and support aquifer recharge. His staff collected detailed information as the plants spread onto nearby land and up low-elevation slopes.

Today, the ability to spread quickly into new areas is a hallmark of an invasive species. Realizing the value of Lyon Arboretum data, Dr. Daehler examined the historic planting records and the documented spread to obtain lag times for 23 plant species known to be invasive in Hawaiʻi. He found that lag times were much shorter than previously estimated. “As soon as they reach maturity, these trees start spreading.” He published his findings to help others address known invaders early.

One of the trees Lyon studied was bingabing. Foresters planted bingabing to support reforestation on Oʻahu; it is now widespread in low-elevation forests. On Hawaiʻi Island, bingabing was seeded by aircraft following a fire in Hilo. It has continued to spread from that initial introduction and today, dense stands crowd the forests surrounding Hilo.

The history of bingabing on Maui appears to be limited to single introductions. Crews from the Maui Invasive Species Committee removed the tree on Olinda Road, and plants from two other sites. Bingabing is one of a dozen species of invasive plants eradicated from Maui before they could spread.

Harold Lyon was concerned about watershed health; had he known the outcome of some of his plantings, he likely would have chosen differently. Today, we have a choice, and a wealth of information about plants’ invasiveness, thanks in part to Dr. Daehler’s work. He helped launch the Hawaiʻi Pacific Weed Risk Assessment (HPWRA), which serves as a background check for plants by predicting the likelihood a species will become invasive in Hawaiʻi. Resource managers, horticulturalists, and farmers can use the HPWRA before choosing to purchase or plant a new species, helping to prevent the introduction of a new invader. The only thing better than eradicating an invasive species early is preventing its introduction in the first place. Learn more and take advantage of the HPWRA at www.plantpono.org.

Lissa Strohecker is the public relations and education specialist for the Maui Invasive Species Committee. She holds a biological sciences degree from Montana State University. Kia’i Moku, “Guarding the Island,” is prepared by the Maui Invasive Species Committee to provide information on protecting the island from invasive plants and animals that can threaten the island’s environment, economy and quality of life.

This article was originally published in the Maui News on February 10, 2024, as part of the Kia‘i Moku Column from the Maui Invasive Species Committee.

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Miconia escaped cultivation to spread throughout East Maui. Containment rather than eradication is the goal for this invasive species on Maui and crews protect upper elevation watersheds by removing plants they find as they hike through the forest. MISC file photo.

On most weekdays, on the slopes and gulches above and below the Hāna Highway, a crew is hacking its way through the forest, spread out in a sweep line. They make slow progress through a mix of non-native and invasive plants: bamboo, inkberry, guava, and white ginger proliferate in this low-elevation forest. Somewhere in the midst of these forests is miconia, another non-native invader. The Hāna-based crew from the Maui Invasive Species Committee is here to find miconia and pull it from the ground.

Miconia may be found across a broad swath of the East Maui Watershed, from Kīpahulu to Huelo, but not as a continuous infestation. Dense pockets occur, especially in the area above Hāna town and in parts of Nāhiku, but elsewhere it’s patchy, thanks to decades of work to suppress and contain the infestation.

Large miconia leaves act as umbrellas, shading out sunlight. MISC file photo

Miconia should be an understory plant, as it is in the dark subcanopy of South Central American jungles of its native range. But in Hawaiʻi, open-canopy forests and the absence of significant “natural enemies” allow miconia to become the dominant plant. Invasive plants don’t just crowd out native plants and diminish habitat for native animals; they also can alter the landscape and ecosystem processes. A 2013 study on Hawaiʻi Island, by Kazuki Nanko of the Forestry and Forest Products Research Institute in Japan and University of Hawaiʻi professor Thomas Giambelluca, set out to dig a bit deeper into how miconia affects the ‘aina.

The team examined what happens on the forest floor when miconia takes over. Miconia’s leaves are huge – up to three feet long by two feet wide. These monstrous botanical solar panels help collect light in a dark understory in South America, but in Hawaiʻi they cast a deep shade, reducing the ability of native plants to capture the energy they need to thrive. Their study revealed that the amount of sunlight that reaches the forest floor is consistently lower in a monotypic miconia-invaded forest when compared to a native ʻōhiʻa-dominated forest, or forests invaded by a diverse mixture of plants. Additionally, miconia leaves decompose quickly, reducing the amount of leaf litter.

This study also investigated what happens to raindrops as they hit the forest canopy all the way down to the forest floor.

The giant leaves act like a tarp: water puddles before falling to the ground.

Miconia produces some of the largest leaf throughfall drops ever measured, with cascading impacts.

Unlike our multi-layered ʻōhiʻa forests, with their carpets of ferns and mid-canopy plants, miconia can develop into a one- layer stand. Larger raindrops, unimpeded by understory or leaf litter, gain speed, hitting the ground with greater kinetic energy than rain falling from the sky. Giant raindrops pound and compact bare soil, causing water to travel along the surface instead of filtering into the ground to recharge our aquifers. In some areas of miconia-invaded forest, bare roots and other signs of erosion tell the tale of how miconia is washing away our forest floor.

The scale of the East Maui miconia infestation has demonstrated that this plant is a formidable foe. Current efforts focus on keeping it out of upper elevation forest and preventing it from spreading farther west. Research and testing continue to find safe and effective natural enemies of miconia. Help us keep the raindrops in our forests. If you find a miconia plant on Maui, recognizable by its large green and purple leaves, report it to 643pest.org.

This article was originally published in the Maui News on September 12, 2020 as part of the Kia‘i Moku Column from the Maui Invasive Species Committee.

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