Forests are a treasure trove of limbs and bark that can be made into alternative fuels and some worry the increasing trend of using that logging debris will make those materials too scarce, harming the woodlands.
For centuries, forests have provided lumber to build cities, pulp for paper mills and a refuge for hunters, fishers and hikers. A flurry of new, green ventures is fueling demand for trees and the debris leftover when they are harvested, which is called waste wood or woody biomass.
"There simply is nowhere near enough waste wood for all of these biomass projects that are popping up all over the place," said Marvin Roberson, a forest policy specialist with the Sierra Club in Michigan.
Waste wood has become a sought-after commodity, prompting concerns that the demand might overwhelm supply and damage the ecosystem. But government officials say there's plenty available and they point to guidelines that are aimed at maintaining tree debris to give the soil nutrients.
Many biomass projects are tied to the forests that extend across Minnesota, Wisconsin, Michigan and part of Ontario. Among them is Flambeau River Papers, a mill in Park Falls, Wis., that emerged from bankruptcy three years ago and is pinning its hopes for profitability on generating its own heat with woody biomass.
In another Wisconsin town 50 miles away, a power company is switching from burning coal to producing combustible gas from logging leftovers. And in Michigan's neighboring Upper Peninsula, a plant under development called Frontier Renewable Resources will convert timber into 40 million gallons of cellulosic ethanol a year.
Researchers led by University of Minnesota forest expert Dennis Becker reported this summer that many would-be investors are uneasy about supplies of waste wood.
They fear environmental reviews and litigation could make some public woodlands unreliable sources, particularly in the West, where most forest lands are under federal ownership and logging often raises legal tussles, the report said.
Another problem with woody biomass is that much of the supply is in protected areas, or so far from markets that removing and transporting it would be too expensive, Becker said.
He led a separate study that found a realistic estimate of biomass available in Michigan, Minnesota and Wisconsin was 4.1 million tons a year. Annual demand soon could reach 5.7 million tons, it said.
"A lot of folks believe there's a significant amount of woody biomass that can be used for renewable energy," Becker said. "In reality, not everything that's physically available is economically feasible or environmentally sustainable."
State and federal officials say there's enough material left over from harvesting the nation's forests to help reduce dependence on foreign oil, curb greenhouse gases and build a green economy.
A federal report says about 368 million tons of biomass could be removed sustainably from U.S. forests each year. Agriculture Secretary Tom Vilsack in August promised to spend $57 million on 30 projects supporting development of biofuels from trees.
"Emerging markets for carbon and sustainable bioenergy will provide landowners with expanded economic incentives to maintain and restore forests," Vilsack said.
Some schools, hospitals and utilities in the Great Lakes region already use biomass for heat and electricity. Michigan Biomass, a group representing six wood-fired power plants, fears the growing interest will make fuel costlier and more scarce. Closure of lumber and paper mills that were reliable suppliers of biomass has worsened the problem, director Gary Melow said.
"In the past four or five years, where we've seen a dramatic decline in forest products manufacturing and in waste wood availability, we've started to experience significant shortages," Melow said. "We've even had power plants that ran out of fuel."
Roberson, with the Sierra Club in Michigan, said biomass projects will end up using waste wood and logs suitable for paper or other products.
"We advocate getting the best economic return for industrial use of the Midwest's forests, and there is no lower return than throwing it in the furnace," Roberson said. "There are fewer jobs per cord, per acre, from biomass than any other use."
A biomass shortage could bring pressure on forest managers to cultivate plantations of fast-growing species such as willows and aspen at the expense of pines and hardwoods native to the region, Roberson said.
That's also a concern in the southeastern part of the country, which will have to rely heavily on biomass to meet alternative energy goals because it has less potential for wind power, said Jimmie Powell, energy specialist for The Nature Conservancy.
Roberson said biomass incineration, although an alternative to greenhouse gas sources like oil and coal, still pollutes the air. Removing too much woody debris, instead of letting it decay and nourish soils, can damage the health of the forest, he said.
A number of states have developed guidelines for collecting biomass in a way that doesn't harm forests. Minnesota calls for leaving 33 percent of fine woody debris in place, said Anna Dirkswager, state biomass coordinator. Wisconsin recommends 20 to 25 percent remains for most locations.
State officials acknowledge competition for biomass may intensify but predict demand and supply will balance in time. One reason: Not every project on the drawing board will come to pass.
"Any time you have an emerging industry, there's concern that everyone will go rushing into it and there will be too much pressure on the resource," said Cara Boucher, Michigan's state forester. "But the market shakes things out."