Hawai'i's Environment
Single-trunk trees critical to nesting
By Jan TenBruggencate
Advertiser Columnist
Nearly 30 years ago, deep in Kaua'i's mile-high Alaka'i Swamp, I was fortunate enough to see one of the last surviving members of the native forest bird species, the 'o'o 'a'a.
In the company of noted bird expert John Sincock, we tramped to the base of a giant, dead 'ohi'a tree. He had found a pair of the birds nesting in a hole in the tree, and we arrived on the day the chick fledged.
We were able to watch the demanding young 'o'o chick being fed by its parents as it sat on a twig, a few yards from the 'ohi'a tree in which it hatched.
It may have been the last 'o'o 'a'a chick. The species is now considered extinct.
I recalled the story on reading the research of University of Hawai'i zoologist Leonard Freed, who wrote in the journal Studies in Avian Biology No. 22, about the importance of old trees to the nesting of another native forest bird, the Hawai'i 'akepa.
Like the 'o'o, the 'akepa likes to nest in cavities in old trees—primarily koa and 'ohi'a trees.
Freed said his Big Island research indicates that the biggest, oldest trees are being lost faster than they are being replaced, and he calls for the protection of old-growth forest in the Islands.
He said the trees with the best cavities tend to be ones with a single trunk, a growth form botanists term monopodial, as opposed to multiple trunks, or sympodial.
As more light enters a forest that has been thinned by storms, or human activity, trees tend to produce more multiple trunks, reducing the supply of good nesting trees.
Also, grazing animals can browse off the tops of young trees, killing the primary growing tip, and promoting side growth that can create multiple trunks.
Freed argues that cattle ranching has been responsible for both problems—cattle eat seedlings, letting more light into the forest, and they nip off terminal buds, promoting branching. The branched trees "are unlikely to develop cavities when they reach large size," Freed wrote.
Among young 'ohi'a in the Pua 'Akala tract of the Hakalau Forest National Wildlife Refuge, nine out of 10 had already forked before they were a foot tall.
"It is difficult to identify among existing seedlings those that may become the monopodial giants whose height of first forking can occur as high as" 60 feet, he wrote.
Freed said that scientists can make up in the short term for the increasing shortfall of nesting trees by building artificial nesting cavities.
"At least five different pairs of birds have used such cavities at Pua 'Akala and nested successfully. Artificial cavities may be essential if the birds are to persist at high density despite loss of cavity trees and slow regeneration of new ones," he said.
The larger challenge is how to recreate the conditions that let the old single-trunked giants grow as they once did, he said.
Jan TenBruggencate is The Advertiser's Kaua'i bureau chief and its science and environment writer. Reach him at (808) 245-3074 or e-mail jant@honoluluadvertiser.com.
