RIO DE JANEIRO, Brazil — Chainsaws roared in the deep northern Amazon, chewing into the massive red trunk of a tropical hardwood called roxinho. The tree fell with little fanfare but the moment marked the beginning of an unprecedented logging experiment.

The Brazilian government has begun granting private companies permission to manage a vast swath of the Brazilian Amazon. The first tree was cut last month and by the end of the year, the part of the Amazon available for logging is slated to swell to more than six times its current size.

And in the next five years, Brazil plans to sell logging rights to more than 27 million acres of jungle, the country’s top forest official said last week. Critics call it a dangerous gamble but Brazil’s government says managed logging is an essential alternative to the illegal clear-cutting that has besieged the world’s largest rainforest.

"Everything in this country is an incentive for deforestation,” said Antonio Carlos Hummel, head of Brazil’s forest service, at a summit hosted last week by the Reuters news agency. “So we're having to change the paradigm: finance standing forests.”

The new paradigm, Hummel added, will involve selling logging concessions on 2.5 million more acres of forest by the end of this year and 27.5 million acres by 2015. The move would mean expanding the current 370,000 acres of legal logging concessions — an area about half the size of Rhode Island — to include a swath of forest bigger than Massachusetts, Maryland and Vermont put together.

The plan is simple in theory: Brazilian companies bid for 40-year leases on patches of forest. They submit plans for sustainable logging, employ locals, invest some of their profits in the region and leave the forest healthy when they’re done. Supporters say the system will be a model for sustainable development that improves upon the failed strategy of simply trying to keep loggers out.

“We need to offer alternatives that increase the value of the forest and that turn the forest into a source of benefits, especially of social benefits,” said Marcus Vinicius Alves, a director at the forest service. “It’s not going to be possible to get these types of social gains simply by surrounding the forest with the armed forces.”

The law creating the forest service and authorizing concessions was passed in 2006, but it's taken four years to reach the point where companies can start cutting trees. In the years leading up to the legislation, a significant amount of legal logging happened on private land, often as farms and cattle ranches expanded. But one study suggests nearly half the logs produced in Brazil were cut illegally, at a time when millions of acres of forest were disappearing every year.

Critics, however, say the concessions are unlikely to stop illegal logging or spur local development. They warn the country is implementing a system that has already failed in threatened tropical forests in Africa, Asia and elsewhere.

“I certainly couldn’t point to a country where I’d say it works,” said Rick Jacobsen, a policy adviser with the NGO Global Witness. His organization has served as an independent monitor for forest concessions in Africa, Asia and Central America. “What’s happened is that this has fueled a lot of corruption at all different levels of government, so that very little of this money actually trickles back down and actually contributes to development projects in the communities.”

Global Witness recently surveyed members of 15 different communities living around logging concessions in Cameroon, where international donors invested hundreds of millions of dollars since the 1990s in the hope of developing a sustainable logging program that would help the region develop.

A study they plan to release in November concludes the strategy failed. Only a handful of locals found work in the concession, and it was usually part-time and low-paid. The infrastructure built by the companies turned out to be substandard.

“Schools we saw were falling apart and unusable after less than 10 years, and they had no money for teachers. There were water pumps that never worked, health posts with no staff or medicine, and millions of dollars unaccounted for,” said Peter Wood, a forest policy adviser for Global Witness.

“The benefits local communities had received were negligible and short-lived,” he said. “The impacts of the forest degradation caused by logging were severe and long-term, with locals effectively excluded from the forest they depend on for food and medicine.”

Still, Brazil is more developed and better-policed than Cameroon. And some veteran foresters here say Brazil has a better chance of avoiding these pitfalls.

“We have to be optimistic,” said Johan Zweede, a forester who’s worked in Brazil for 45 years. “They’ve got some excellent professionals in the forest service. And if they make it work as they’ve planned — keep politics out of it — I think it really has a chance. Those industries that want to be legal, right now have no way of being legal.”

The question remains whether logging can be sustainable and yet economically viable. Zweede believes it can. He started an organization called the Instituto Floresta Tropical that teaches reduced-impact logging, a technique companies winning concessions in Brazil will be required to follow.

The method involves things like removing vines to reduce the chances that a felled tree accidentally brings others down with it. Some scientists say it’s less harmful than conventional logging.

Ecologist Greg Asner with the Carnegie Institution for Science has done research comparing the two techniques in the Amazon. In both cases, loggers cut down specific high-value trees, but Asner said the forest they leave behind is dramatically different.

“There are definitely different flavors of selective logging, from very, very high-damage approaches that really leave the forest a terrible mess all the way to the reduced-impact methods — a surgical removal of the high-value trees,” Asner said. “If there’s any hope for sustainability, that’s got to be it.”

But there’s another aspect to sustainability that’s less well understood: how quickly a tract of forest can recover before companies can log it again.

Current rules allowing companies to return to a logged patch after 30 years are “optimistic,” Asner said. “I think that those numbers require much more scientific work.”

Even if companies can devise sustainable management plans, Asner said the concessions’ success will hinge on companies actually sticking to them. In addition, law-abiding companies will have to keep out the less scrupulous — as timber roads often become conduits for illegal logging, hunting and slash-and-burn agriculture.

“Its a huge area,” he said. “It’s spread out, so it’s going to be hard to enforce, but it could be done. The technology is there to support the monitoring and the rest is will.”

On paper at least, the government has tried to ensure the will is there. Under the law, one government agency and another non-profit organization will have the authority to monitor the logging and management of concessions in national forests. In addition, the forest service will monitor aspects of the contract, which include technical capability and investment in local communities. And companies are required to bring in an independent auditor at least every three years.

But doubts remain as to whether the Brazilian government has enough trained staff to oversee the millions of acres it hopes to put up for bid.

Claudio Maretti, conservation director for the non-profit environmental organization WWF Brazil, said Brazil’s federal and state governments aren’t yet up to the task, but he believes they will grow into it.

“The most important thing is to keep implementing, keep improving capacity,” Maretti said, adding the government should continue to move forward with concessions. “It is better to use the forest than to leave it alone. Without use it’s much harder to protect it.”

Others are less optimistic.

“We don’t have enough people, technicians, to monitor and control everything,” said forester Niro Higuchi, with the National Institute of Amazonian Research.

Higuchi has been a vocal opponent of Brazil’s logging concessions since before the law authorizing them was passed. He argues the risk to the Amazon is too great.

“You can see all countries in Africa, Southeast Asia, even in Central America that use concessions for logging,” he said. “Today they don’t have forests anymore.”