Planting trees is a way to be green. Yet for one company replanting koa on Hawaii island, some extra green — as in money — could soon be earned from selling credits that represent carbon dioxide captured by the trees.

Hawaiian Legacy Hardwoods is set to announce Friday it has received certified “carbon credits” that it can sell to entities that want to, or must, contribute to greenhouse gas reduction efforts.

Hawaiian Legacy, which has planted close to 300,000 trees over the last five years on part of a former cattle ranch on the slopes of Mauna Kea north of Hilo, estimates it can earn more than $5 million selling the credits, also known as carbon offsets, for sequestering 125,000 metric tons of CO2.

The initiative follows fizzled efforts by other Hawaii forest or tree plantation owners to sell carbon credits in what is still largely a market centered in Europe.

If successful, Hawaiian Legacy claims it would be the first to sell certified carbon credits in Hawaii and the only afforestation project in the United States to earn such credits.

Hawaiian Legacy leaders say what they are doing can be a model for helping restore Hawaii’s native koa forests that have been heavily logged for their expensive, beautiful wood.

“It becomes a profitable economic model to keep the forest in place,” said Jeff Dunster, Hawaiian Legacy CEO.

Added Darrell Fox, the company’s chief operating officer, “A forest is worth more standing up than lying down — that’s where we’re at.”

Hawaiian Legacy obtained certification from The Gold Standard Foundation, a Swiss-based nonprofit that was established in part by the World Wildlife Fund in 2003 and certifies carbon mitigation projects.

The organization said it has issued about 7 million carbon credits to date.

One credit represents 1 metric ton of carbon dioxide, or an equivalent greenhouse gas, not released into the atmosphere.

Consultants and independent third-party verifications were able to show Gold Standard that koa trees can capture carbon at an average rate of 1 ton per 160 trees annually as they mature from seedlings on 1,200 acres leased by Hawaiian Legacy. That equates to 2,500 tons and 2,500 credits per year based on 400,000 trees, the last 100,000 of which Hawaiian Legacy expects to finish planting next year.

Under Gold Standard rules, Hawaiian Legacy may sell 50 years worth of credits mostly upfront, holding just 20 percent of the credits in reserve.

The $5 million total revenue estimate is based on selling roughly 125,000 credits for $40 each.

There are generally two types of buyers: involuntary and voluntary. Involuntary buyers are typically companies in countries outside the U.S. that are required to either stay within established greenhouse gas emission caps or acquire extra cap space through credits.

This compulsory market segment trades credits on a European exchange that at times has been volatile and swamped with a glut of credits that at times have made them worth almost nothing.

Dunster said Hawaiian Legacy aims to sell to voluntary buyers who want to help pay for work that benefits the environment.

The nature of Hawaiian Legacy’s operation, which includes planting native understory plants, should enable the company to sell its credits for a premium, Dunster anticipates.

Gold Standard has reported that developers of its certified forestry projects have sold credits recently for 6 to 9 euros, or about $6.50 to $9.80, per credit.

Buying and selling carbon credits can be arcane, as the credits are unlike ordinary commodities.

Theoretically, carbon credits can be created for almost anything that reduces CO2 emissions — windmills, solar panels and energy-efficient building designs. Capturing methane from a landfill, preserving an existing forest or more efficiently burning coal in a power plant also can qualify.

The value of credits on the voluntary market varies widely depending on the environmental benefit and which entity among a number of certification organizations qualifies a project.

In Hawaii, at least two companies have unsuccessfully tried to break into the market.

Big Island ranch McCandless Land & Cattle Co. entered into a memorandum of understanding in 2010 to market and sell carbon credits for old-growth ohia and koa forests on its land through an affiliate of Canadian-based ERA Carbon Offsets Ltd., now known as Offsetters Climate Solutions Inc.

Ranch manager Keith Unger said work to establish carbon sequestration metrics was transferred to the San Francisco affiliate of Australian-based New Forests Pty. and is kind of on a back burner. “It’s a slow-moving issue,” he said. “We’re still interested.”

Hawaiian Mahogany Inc., a Kauai tree plantation, claims to have obtained certified carbon credits that it gave up trying to sell after difficulties, though two nonprofits dispute the company’s claims over such credits.

Washington, D.C.-based Verified Carbon Standard, formerly known as Voluntary Carbon Standard, refutes Hawaiian Mahogany’s claim that VCS approved such credits for the tree farm. And Arkansas-based Winrock International said it only conducted a preliminary technical assessment of carbon stocks for Hawaiian Mahogany.

“It was a big fizzle for us,” said Bill Stepchew, former operations manager for Hawaiian Mahogany, which supplies wood chips to a Green Energy biomass power plant that produces electricity for Kauai’s publicly owned utility.

Stepchew said the company spent about $20,000 to certify its CO2 sequestration, and estimated that a significant return was possible based on a price of $3 per ton for 130,000 tons of carbon captured per year by more than 1 million albizia and eucalyptus trees the company had planted on 1,800 acres of fallow former sugar cane lands.

Earning $390,000 a year for letting trees grow made the whole carbon credit notion somewhat hard to believe even for Stepchew. “It was a wild idea in the first place to sell clean air,” he said. “We sort of roll our eyes about it now.”

Jeff Mikulina, CEO of local environmental group Blue Planet Foundation, said potential buyers of voluntary carbon credits are challenged with assessing the environmental value of such credits.

For instance, engine maker Pratt & Whitney announced in 2012 that Hawaiian Airlines had received carbon credits certified by the Verified Carbon Standard Association for using an engine wash developed by Pratt & Whitney.

The engine wash dubbed EcoPower helped cut the airline’s fuel consumption by 2.5 million gallons since 2005, saving the airline money and also keeping 22,000 metric tons of CO2 out of the atmosphere.

“We are proud to be the first airline to receive certified carbon credits for reducing emissions,” Mark Dunkerley, Hawaiian Airlines president and CEO, said in a statement at the time. “Importantly, engine washing with EcoPower is helping us mitigate rising fuel costs and significantly reduce Hawaiian’s carbon footprint at the same time.”

Mikulina characterizes credits like Hawaiian’s as “double dipping” if they were sold, though the company could retain them for its own use if it becomes subject to future greenhouse emissions caps.

Hawaiian Legacy’s credits, Mikulina said, are more beneficial because trees are being planted.

“It’s positive,” he said.

Some Hawaiian Legacy trees are slated to be cut down for their wood, which represents about 50 percent of the captured carbon. But Fox said much of the carbon stays sequestered because harvested koa is not totally consumed.

“People don’t buy a koa dining table and then turn it into firewood,” he said. “It’s a long-term storage of carbon instead of it being returned to carbon dioxide.”

Of the 400,000 koa trees slated to fill out Hawaiian Legacy’s forest, only 100,000 will be harvested. These trees represent trees the company sold to investors for roughly $100 per tree. The balance are trees where people or entities paid a $60 sponsorship fee per tree including $20 passed on to charities.

Dunster said Hawaiian Legacy expects to share about 30 percent of carbon credit revenue with investors and use the rest to sustain what it calls its “legacy” forest.

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CORRECTION: A previous version of this story reported Hawaiian Mahogany Inc., a Kauai tree plantation obtained carbon credit certification through two nonprofits — Arkansas-based Winrock International and Washington, D.C.-based Voluntary Carbon Standard — but abandoned the effort after running into difficulty finding buyers for its credits.