Time and again we've been told that biomass sources, such as switchgrass and forestry by-products, could provide the United States with a potential treasure trove of renewable energy.

But one big challenge remains: Finding a cost-effective way to break down the lignin in these products — a compound that interferes with efforts to convert the cellulose into simple sugars for ethanol.

Plenty of work remains to be done, though that's not to imply that scientists aren't working hard to solve this problem.

One especially promising focus of research involves trees, especially certain types of trees best suited for conversion into ethanol. One of them is black cottonwood.

Scientists announced last year they had completed the genome mapping of black cottonwood or Populas trichocarpa, which, they say, offers huge potential as a biofuel source. The poplars, which grow 12 feet a year, mature in as little as four years and can reach as high as 100 feet.

Scientists are hopeful the knowledge they've gained through gene mapping will enable them to undertake dramatic improvements in plantation productivity associated with the tree, possibly even rivaling the strides made from the green revolution in agriculture.

But in the view of many researchers, this is only the tip of the iceberg. The next inevitable step will involve the genetic alteration of certain tree species so that the lignin can be more readily converted. The New York Times reported Nov. 20 that one researcher, Vincent L. Chiang, co-director of the forest biotechnology group at North Carolina State University, has developed transgenic trees with as little as half the lignin as their non-engineered counterparts.

But challenges remain, some economic, some ethical, the Times reports.

In a manner of speaking, if Mother Nature had wanted low-lignin trees she would have selected for them. And in the view of some researchers and environmental watchdogs, therein lies an ethical issue.

While row-crops, such as cotton and soybeans, have been genetically engineered in a variety of ways to ward off insects and other predators, many experts both within and outside of the forestry industry maintain that trees — in the view of many, the most majestic symbols of pristine nature — should be treated differently.

What, these critics ask, are the environmental stakes involved? What if the genetics of these low-lignin trees eventually find their way into natural trees? What happens then?

Valid questions or not, some researchers contend they are moot. In the end, science ultimately will have to rely on genetically altered trees to stock the growing demand for renewable fuels. It just takes too long to breed and grow conventional trees.

But as one renewable energy expert has cautioned time and again, while the technology may prove inevitable, it likely will turn out to be one of many paths to energy efficiency. In his view, there is no such thing as a silver bullet and likely never will be.

“It probably won't turn out to be a silver bullet insomuch as silver buckshot in a magic shell that may help us profit from renewable energy in a variety of ways,” says Mark Hall, an Alabama Cooperative Extension System renewable energy specialist.

Transgenically altered trees already are being produced in the United States for commercial purposes. The Times reports that one South Carolina-based company, ArborGen, is developing a low-lignin eucalyptus tree that it hopes to sell in South America, where fast-growing trees are needed for the pulp and paper industry. The company also is developing a winter-hardy eucalyptus tree that can be grown more widely within the United States