BURLINGTON, Vt. — When it purchased a small hydropower plant on the nearby Winooski River last fall, the municipally-owned electric utility here quietly nudged Vermont's largest city into the sustainability spotlight. With a little creative accounting and the addition of the Winooski facility, Burlington's 42,000 residents were now lighting their homes and running their businesses with a 100-percent renewable mix of wind, water and biomass.

That last resource raised some eyebrows, however, and not everyone was celebrating. Mary S. Booth, a New England ecologist, fired off a letter to the editors of the “PBS Newshour,” which was among many news outlets to herald Burlington's achievement. She pointed to the city's McNeil Electric Generating Facility, which burns wood to produce about one-third of the city's power, noting that the plant emits a variety of pollutants, including greenhouse gases.

Being renewable, Booth suggested, is not the same as being clean and carbon-free.

In an editor's note attached to the segment online, PBS suggested that Booth was not alone in her objections, and it modified its story to reflect concerns raised by several members of its audience. “These viewers argued we were giving an overstated impression of the environmental attributes of the [McNeil] plant,” PBS noted, “and we agree.”

The backpedaling seemed to underscore a variety of lingering uncertainties over the green credentials of biomass power, and the extent to which Burlington's path away from fossil fuels can — or even should — be emulated elsewhere. The benefits of biomass power depend on such a multitude of variables, in fact, that officials from the U.S. Environmental Protection Agency to the European Commission are still trying to sort out the upsides from the down.

Hanging in the balance is the biomass power industry itself, which is considered climate-friendly in some circles and a climate menace in others. Operators of the McNeil facility argue that Vermont's wood-harvesting regulations are such that their plant, and others like it, can provide what is essentially carbon-free electricity. In neighboring Massachusetts, however, a multi-year debate over the issue concluded in 2012 with what are widely considered to be the toughest biomass regulations in the country.

The new rules effectively shut down the industry there.

“It's frustrating,” said Bob Cleaves, the president of the Biomass Power Association, a Maine-based trade group representing the industry. “We are getting swept into a much larger debate about forests and carbon stocks and spatial and temporal metrics and the like,” he said, “and what we're trying to say is that it's not that complicated.”

Deriving electricity by burning wood or agricultural residues to heat water, create steam, and drive a turbine, was long considered to be both carbon-neutral and a sustainable source of energy. That's because while trees and plants do release large amounts of planet-warming carbon dioxide when they are burned — even more than coal, by some measures — they also absorb and trap the gas while they are growing. So long as the harvested biomass is replaced by new plant or tree growth, the thinking goes, no net emissions are added to the atmosphere.

Those baseline assumptions have fueled steady growth in biomass power, both in the U.S., where it is the third largest form of renewable energy, and worldwide. Europe has seen particularly rapid growth in biomass power production, as government incentives and looming deadlines on emissions reduction goals are driving a transition away from coal-fired power.

But the presumed benefits of biomass power — particularly when derived from woody materials taken from forests — have come under increasing scientific scrutiny, and conservation groups are now calling for tighter controls to ensure that policymakers are getting the carbon balance right.

Much of the concern focuses on the time frames needed for new tree growth to soak up carbon dioxide and balance that atmospheric ledger — a question that can hinge on a variety of site-specific conditions, including the local climate, the type of material being used for energy, and what would have happened to that material if a biomass plant hadn't created a demand for it in the first place. There is also little agreement on how long is reasonable to wait for the carbon benefits of biomass to be realized — particularly given the imperatives of global warming. Is 50 years too long to wait? What about a century?

Most experts agree, for example, that generating electricity by burning sawdust and other waste materials collected from the wood-processing and paper-making industries has a low-carbon footprint. Left alone or placed in a landfill, these waste materials would decompose and release their stored carbon back into the atmosphere relatively quickly anyway, so burning them to generate electricity — or even heat — creates little in the way of additional greenhouse gas emissions.