Burning wood was never supposed to be the cornerstone of the European Union’s green energy strategy.

When the bloc began subsidizing wood burning over a decade ago, it was seen as a quick boost for renewable fuel and an incentive to move homes and power plants away from coal and gas. Chips and pellets were marketed as a way to turn sawdust waste into green power.

Those subsidies gave rise to a booming market, to the point that wood is now Europe’s largest renewable energy source, far ahead of wind and solar.

But today, as demand surges amid a Russian energy crunch, whole trees are being harvested for power. And evidence is mounting that Europe’s bet on wood to address climate change has not paid off.

Forests in Finland and Estonia, for example, once seen as key assets for reducing carbon from the air, are now the source of so much logging that government scientists consider them carbon emitters. In Hungary, the government waived conservation rules last month to allow increased logging in old-growth forests.

And while European nations can count wood power toward their clean-energy targets, the E.U. scientific research agency said last year that burning wood released more carbon dioxide than would have been emitted had that energy come from fossil fuels.

“People buy wood pellets thinking they’re the sustainable choice, but in reality, they’re driving the destruction of Europe’s last wild forests,” said David Gehl of the Environmental Investigation Agency, a Washington-based advocacy group that has studied wood use in Central Europe.

The industry has become so big that researchers cannot keep track of it. E.U. official research could not identify the source of 120 million metric tons of wood used across the continent last year — a gap bigger than the size of Finland’s entire timber industry. Researchers say most of that probably was burned for heating and electricity.

Next week, the European Parliament is scheduled to vote on a bill that would eliminate most industry subsidies and prohibit countries from burning whole trees to meet their clean energy targets. Only energy from wood waste like sawdust would qualify as renewable, and thus be eligible for subsidies.

But several European governments say that now is no time to meddle with an important energy industry, with supplies of Russian gas and oil in jeopardy. In the Czech Republic, protesters have mobbed the streets, furious with rising energy costs, and the French authorities have warned of rolling blackouts this winter.

Internal documents show that Central European and Nordic countries, in particular, are pushing hard to keep the wood subsidies alive.

The debate is an acute example of one of the key challenges that governments face in fighting climate change: how to balance the urgency of a warming planet against the immediate need for jobs, energy and economic stability. The European Union has been a leader in setting green policies, but it is also racing to find energy sources as Russia throttles back its supply of natural gas.

In documents circulated among lawmakers about the proposed rule change, Latvia warned of a “possible negative impact on investment and businesses.” Denmark argued that these decisions should be left to national governments. A winter without reliable Russian gas looms over the debate.

Scientists have warned of this moment for years.

To have a chance of fighting climate change, countries must reduce the amount of carbon dioxide they release into the air. That will require a shift away from fossil fuels. The European Union has required countries to meet aggressive renewable-energy targets. Wood qualifies as renewable energy, on the logic that trees ultimately grow back.

In 2018, the last time the subsidies came up for a vote, nearly 800 scientists signed a letter urging lawmakers to stop treating logged trees as a green source of energy. While trees can be replanted, it can take generations for a growing forest to reabsorb the carbon dioxide from burned wood.

“Using wood deliberately harvested for burning will increase carbon in the atmosphere and warming for decades to centuries,” the scientists wrote.

One of the authors of that letter, Tim Searchinger, a Princeton environmental science scholar, said European lawmakers were understandably eager to find green energy, but they incorrectly lumped all renewable sources together. “I’m not sure people were thinking much about wood at all when they passed these laws,” he said.

Even one of the godfathers of the policy, the former European Union environmental official Jorgen Henningsen, went to his death bed last year regretting pushing so aggressively for wood energy.

Today, as the debate intensifies, environmental advocacy groups are using new tools to argue that it is time to change course.

Experts with the Environmental Investigation Agency, working with a loose network of forestry conservationists, have spent nearly a year hiking into some of the continent’s oldest forests and attaching tracking devices to trees. They have scraped government truck-location data and tracked trees from natural parks and conservation areas to wood mills. They have linked loggers to companies marketing wood pellets as carbon-neutral fuel.

They found that pillaging Europe’s last standing wild forests to make pellets has become a widespread practice in Central Europe.

The New York Times supplemented the group’s data with publicly available records. A reporter and photographer spent four days hiking through Romania’s forests, which represent two-thirds of the European Union’s virgin woodlands. There, they documented clearcutting and followed trucks from ecologically sensitive forests.

Although logging is not forbidden in Europe’s protected forests, governments are required to conduct environmental assessments to ensure the land is being conserved. But experts say that such assessments are rare. Last year, the European Court of Auditors raised the alarm on these supposedly protected forests, finding many of them in “bad or poor conservation status.”

Today, after a 2,200-foot hike up Ceahlau Mountain in Romania, a trail of wooden carcasses is visible below, a scar across one of Europe’s last old-growth forests where 200-year-old trees once stood.

Further down the mountain, logs were loaded on a truck branded for Ameco, one of Romania’s biggest pellet producers. “Pellet production offers the possibility to use waste from agriculture and forestry,” the company says on its website. Its pellet bags are labeled as coming exclusively from sawdust and wood chips.

Times journalists saw trees from Romania’s protected forests being fed into Ameco grinders.

The company also says its product does not emit greenhouse gas when burned. Scientists have calculated that, per unit of energy, burning wood actually releases more greenhouse gas emissions than burning gas, oil, or even coal.

Ameco declined interview requests. In an email, an Ameco sales manager denied that the company logged from ecologically sensitive forests. When The Times replied that reporters had witnessed six truckloads from these sites, and that Ameco’s own shipping data showed hundreds more, a second company representative responded, acknowledging the shipments but saying they were all legal.

According to the Environmental Investigation Agency’s data, most major pellet plants in Romania have received whole logs from protected forests. The group calculated that about a third of wood shipments to these factories originated in protected areas.

“Once you cut down these old trees, you degrade ecosystems that took centuries to form with little human intervention,” said Dan-Catalin Turiga, a forest engineer who accompanied Times reporters. Mr. Turiga is also an investigator with an environmental organization called Agent Green, which collaborated on the tree-tracking initiative.

Mr. Turiga pointed to logging roads cut on very steep slopes, causing erosion and runoff into waterways. “Planting new trees won’t restore the biodiversity that existed,” he said.

Forest owners, state or private, are supposed to replace cut trees with new ones within two years to help balance the carbon cycle. But environmental groups have shown over the years that this is not always done. Times journalists saw wide swaths of land that had been registered as logged years ago, yet no replanting was done. In some cases, saplings died, possibly because of the degraded soil.

Reporters also saw an unregistered truck hauling wood, the sort of logging that helps explain why European researchers are unable to identify the source of so much timber. The Environmental Investigation Agency found repeated examples of unregistered shipments. In some cases, shipments were understated or identical load weights were recorded several days in a row.

This could qualify as illegal logging in Romania. The country’s environmental ministry did not respond to written questions about these shipments, the proposed law change and the pellet industry in general.

The Environmental Investigation Agency tracked logs from ecologically significant forests to 10 pellet mills and three power plants in Romania, Bulgaria, Slovakia and Poland.

Bioenergy Europe, a trade association, said problems were rare. When harvested correctly and sustainably, wood remains important at a time when Europe is desperate to find domestic, renewable sources of energy, said Irene di Padua, the group’s policy director.

“We still can increase capacity in Europe in a sustainable way,” she said.

The association opposes cutting subsidies or changing the way clean energy is defined. If the European Union no longer considers energy from burnt wood to be carbon-neutral, it would immediately throw many countries off track to hit renewable-energy targets.

That would have major consequences for countries like Italy, the continent’s largest consumer of wood pellets. More than a third of its renewable energy comes from burning plant material. For years, the Italian government has offered tax deductions to encourage buying pellet stoves.

Similar tax breaks are in place in other countries, along with financial incentives for wood producers. Those incentives could be unlawful if the new proposal comes into force.

Even if the European Parliament endorses a change, however, the details must be worked out in negotiations with national governments.

The governments of Germany, Netherlands, Belgium and Luxembourg have signaled support for ending the subsidies. Other countries have stayed largely silent.

While environmental groups are still optimistic, even the most strident supporters of the rule change acknowledge that the Russian energy crisis has made the politics challenging. Natural gas prices have increased tenfold over the past year, and many Europeans fear being unable to afford to heat their homes this winter.

“We need more domestic renewable energy and self-sufficiency, not less,” Antti Kurvinen, the Finnish minister for agriculture and forestry, wrote on Twitter in May. “I will fully promote forest energy.”