The biomass-is-carbon-neutral story line put forward in the early 1990’s has been superseded by more recent science that recognizes that mature, intact forests sequester carbon more effectively than cut-over areas. When a tree’s carbon is released into the atmosphere in a single pulse, (the Greenfield burner plans to burn a ton of damp, green wood chips each minute) it contributes to climate change much more than woodland timber rotting slowly over decades.

“Biomass is not carbon neutral and will dramatically increase production of greenhouse gases.

The “carbon neutrality” concept states that burning biomass results in net emissions of carbon dioxide close to zero, because biomass grows back and “locks up” into organic carbon the carbon dioxide released by burning.

However, since the average age of trees in forests of Western Massachusetts is 75 – 100 years, it will take a similar amount of time to regrow trees that are burned for biomass and resequester their carbon. Scientists agree that society must reduce our carbon emissions immediately – we don’t have 75 years. The best available science shows that forests lock up atmospheric carbon dioxide best when they are undisturbed by forestry operations.”

(From: http://www.massenvironmentalenergy.org/forestry.html)

In fact, mature temperate forests, like Massachusetts’ may be the some of the world’s most efficient at locking in carbon:

“In tropical forests, dead plant material is rapidly decomposed and carbon dioxide is released into the atmosphere through respiration. By contrast, moist temperate forests are warm enough to encourage good growth rates, dead plant material decays much more slowly and carbon-rich dead biomass lasts much longer…the findings reinforce the role of forests in storing carbon and in mitigating climate change…..the research especially underscores the importance of protecting carbon-dense forests in developed countries.

...another common misunderstanding is that younger growing forests sequester more carbon than mature forests.….while growing forests have a greater rate of carbon uptake, it's more important to look at the total amount of carbon stored in a forest. Since carbon is emitted much more rapidly than it is sequestered …the best way to sequester carbon [in] forests is to protect existing old forests."

(From a report on: “Re-evaluation of forest biomass carbon stocks and lessons from the world’s most carbon-dense forests” by Heather Keith, Brendan G. Mackey, and David B. Lindenmayer)
http://www.abc.net.au/science/articles/2009/06/16/2599532.htm

Online abstract of the study:
http://www.pnas.org/gca?allch=citmgr&gca=pnas%3B106%2F28%2F11635