Researchers quantify impacts of miconia plant
On most weekdays, a few miles above or below the Hana Highway, as it zigs and zags up and down from overlook to streambed and back, there is crew from the Maui Invasive Species Committee hiking through the forest, spread out in a sweep line. The plants here are a mix of non-native and invasive: bamboo, inkberry, guava and white ginger, for example. Somewhere in the midst of this forested area is a miconia plant, another non-native invader, and the crew is searching for it.
Miconia can be found across 45,000 acres of the East Maui Watershed, from Kipahulu to Huelo, but it’s not a continuous infestation. There are dense pockets but it’s mostly patchy, in part due to 27 years of work suppressing and containing the miconia infestation.
Miconia should be an understory plant, growing in the dark subcanopy of a Central American jungle. But, out of place here in Hawaii, in an open canopy rainforest and lacking any insect or fungal partners that keep it in check, it becomes the dominant forest plant. Invasive plants like miconia crowd out the native plants and remove habitat for native animals, but they also have the potential to shape the land and ecosystem processes. A researcher with Japan’s Forestry and Forest Product Research Institute, Kazuki Nanko, and University of Hawaii researcher and professor Thomas Giambelluca led a team of investigating and quantifying the impact of the invasive plant.
First they looked at what’s happening on the forest floor when miconia takes over. Miconia’s leaves are oversized — easily three feet long by one and a half to two feet wide. These large leaves help it collect light in a dark understory in South America. But here it Hawaii, where it has taken over the forest, these leaves “shade out” other plants, reducing the ability of other plants to become established. In comparing the amount of sunlight reaching a forest floor between a miconia-invaded forest, a native ohia-dominated forest, and forest invaded by a diverse mixture of plants, researchers found light levels to be universally low under monotypic miconia stands. Additionally, miconia leaves decompose quickly, and in an invaded forest, there is no leaf litter.
They also measured how the massive miconia leaves affect throughfall — what happens to raindrops as they hit the forest canopy all the way down to the forest floor. The giant miconia leaves act like a tarp in the forest: raindrops collect on the leaves before falling to the forest floor. In fact, miconia produces the largest throughfall drops ever measured.
Finally they looked at what happens when the throughfall hits the ground. A miconia forest is a single layer as opposed to the multi-layered forest found in ohia-dominated forest. Larger raindrops falling from a greater height reach a greater velocity, hitting the ground with more force. These oversized throughfall drops hit the ground with greater kinetic energy than rain falling from the sky. And, without leaf litter or other plants, the water is falling on bare soil.
The intense pounding of throughfall on the bare soil under miconia compacts the soil as tiny particles of dirt are loosened and end up filling in the pores or holes where water should be flowing into the ground. This means that water cannot go down through the soil, rather rainfall travels along the surface. In certain areas of miconia-invaded forest, bare roots and other indications of erosion are not uncommon. The researchers concluded that miconia can contribute to degrading the land through erosion.
The East Maui miconia infestation is an estimated 45,000 acres. Even early on in the effort to address the invader, resource managers realized they could only contain the plant, keeping it out of upper elevation forest and preventing it from spreading farther west — rather than completely eliminating it. But given the destructive potential if this plant, containment was necessary. Meanwhile, research and testing for a safe and effective natural enemy of miconia continues. If you find a miconia plant on Maui, recognizable by its large green and purple leaves, report it to 643pest.org.
* Lissa Fox 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.