Researchers were able to increase the diameter of koa trees by 120 percent in a test plot by fertilizing the trees and thinning competing trees, according to a study to be published in the April edition of Forest Ecology and Management.

The research was done by the U.S. Forest Service scientists and the Institute of Pacific Islands Forestry on the Big Island.

"They found the potential koa crop trees in the test area on the eastern slope of Mauna Loa annually increased their stem diameter at chest height by nearly 120 percent," according to a U.S. Forest Service news release.

"In addition, they found the treatments did not adversely affect the growth of native understory plants and non-native grasses did not grow more where tree thinning had occurred. Scientists even found fertilizers reduced the growth of these alien grasses when compared to unfertilized test plots."

Forest Ecology and Management is a scientific journal. The article is titled, "Understory Structure in a 23-Year-Old Acacia Koa Forest and Two-Year Growth Responses to Silvicultural Treatments."

The study noted that native koa forests in Hawai'i have been greatly reduced because of logging and land clearing for agricultural production.

"Previous studies have shown that a lack of knowledge about koa tree production has hampered commercial forestry investment efforts in Hawai'i," the Forest Service said in its release.

Scientists began to fill the knowledge gap in 2002 when they started measuring how koa trees respond to the thinning of competing trees and the application of fertilizers.

Researchers also were concerned about how the trees and surrounding plants, some of which are endangered, responded to chemical control of non-native grasses.

The study's findings also showed the treatments were either neutral or beneficial to forest bird habitat, an important consideration because many trees in koa forests bear fruit or provide habitat for insects eaten by many Hawaiian birds.

"Our findings indicate the use of low-impact silvicultural treatments in young koa stands not only increases wood production, but also is compatible with maintenance of healthy, intact native understory vegetation," said Paul Scowcroft, an Institute of Pacific Islands Forestry research ecologist and one of the study's authors.