U.S. FOREST SERVICE PHOTO
Janis Haraguchi, an Institute of Pacific Islands Forestry biologist, measures the diameter of an acacia koa tree. Haraguchi and other U.S. Forest Service scientists found the stem diameter of koa trees with timber potential doubled through a combination of treatments that included thinning, herbicides and fertilizers.
Method for growing koa aids isle biodiversity, study shows
Hawaii scientists have found a way to make koa trees grow faster while increasing biodiversity in Hawaii's forests and enhancing the native ecosystem, according to a new study.
Paul Scowcroft, research forester at the Institute of Pacific Islands Forestry, hopes the findings could lead to a revival of native forests in Hawaii.
"There are active efforts right now going on to restore koa forests in the state. ... Those efforts probably will benefit from what we have found," Scowcroft said. They can get the best of both worlds, he said, improving koa timber production while supporting forest conservation goals, he said.
Scowcroft and two Hawaii scientists will publish their findings next month in the journal Forest Ecology and Management. Their article describes the faster growth of koa and details how that method results in a healthier "understory" of other native plants. The increased lower level of native plants improves the food supply of native birds, enhancing forest biodiversity, Scowcroft said.
The study was paid for by the U.S. Forest Service's research and development branch, which oversees the institute.
Acacia koa, a native hardwood tree that is harvested for crafts and furniture-making, has decreased in Hawaii because of deforestation for agriculture, ranching and logging.
However, the demise of sugar plantations and lower profitability of ranching have led to the opening up of large tracts of land and landowners looking to koa forests as an alternative use of the land, Scowcroft said.
In 2002 the scientists began studying treatments to grow and produce more koa. Scowcroft and colleagues conducted the study in a 23-year-old koa forest owned by Kamehameha Schools, more than a mile high on Mauna Loa's eastern slope.
After selecting trees for a crop, the scientists used three "conservative" techniques to enhance the crop trees: removing alien grass with an herbicide, adding phosphorous fertilizer and thinning surrounding weaker koa trees.
Scientists removed about 32 percent of the crowding koa trees by girdling them -- removing a strip of bark around the tree in a ring, cutting off nutrients to the tree's root system and killing it. The method left decaying trees that supported insects and increased the food supply for native birds.
They found the techniques increased the annual growth of the tree's diameter by 120 percent at chest height.
Koa trees are key to the native ecosystem, Scowcroft said. They provide a canopy for undergrowth and an ecosystem for native species.
In the study, the scientists looked at how cultivating koa affected the lower-level foliage for two years and found the methods benefited native species without negative effects. For example, the naio, a small native tree, had more room to grow and produce fruit for native birds, he said.
Scowcroft said the study results are being tested in two other places on the Big Island: on the Hamakua side of Maunakea and in a dry area of South Kona.