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Seeing the Forest for the Trees




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    Finding more options

    If all the cropland in Minnesota’s Morrison County were used to produce biofuels, the county still would need to import energy. While those research results are not surprising and no one believes farmers should supply all our energy, the study illustrates the land use decisions at the heart of our development of renewable energy from biomass.

    How should we allocate our finite land resources to supply food, fiber and fuel as well as space to live, work and play? Research published in April 2007 by professors C. Ford Runge and Benjamin Senauer of the Department of Applied Economics found that using more highly productive cropland for bioenergy crops would drive up food prices, possibly forcing more people toward malnutrition. Their study ignited the so-called food versus fuel debate—a firestorm of discussion that still burns today.

    Stan Hokanson evaluates various kinds of trees.From Stan Hokanson’s viewpoint, there’s an element missing from the debate. “What if we could utilize marginal lands that are too nutrient-poor or too wet to produce food or forest crops?” Adding these un-cropped acres to the potential land-base for biofuels crops might take some pressure off the more productive land.
        
    “As long as we don’t diminish the wildlife and ecological benefits these marginal lands provide,” says Hokanson, an associate professor in the Department of Horticultural Science, “purposely cultivating these woody species on this ‘extra’ land might be really beneficial.”
        
    Hokanson is focused on alder, aspen, hybrid poplar, and willow. Of these, hybrid poplar is most often mentioned as a woody source of cellulosic ethanol. The potential for alder, aspen and willow is virtually unknown. Scientists also have little understanding of which species of these woody plants will thrive on marginal lands, in a far north climate, while producing maximum levels of fermentable sugars.

    A Team is Born

    Those questions helped Hokanson form a research team and attract nearly $650,000 to fund a four-year study. The plan is to evaluate 14 species of alder and 10 willow species and selections, and compare the results with similar tests on two selections each of hybrid aspen and hybrid poplar. The team will evaluate how well each species grows on various sites, the fiber and chemical composition of wood produced by each species, the amount of glucose each species produces, and the efficiency with which the glucose can be released.
        
    As a plant breeder for the Minnesota Agricultural Experiment Station, Hokanson has increased understanding of genetic resistance to disease in rose and strawberry plants. He’s also worked to develop new varieties of landscape trees and shrubs. For this project, he will identify the genetic traits of the alder and willow species that help them survive on marginal sites in northern climates and produce economically viable amounts of cellulosic ethanol.
        
    Andy David considers how trees grow on marginal lands.“If we can identify one or two genotypes of each that are worthy of further testing or that can be hybridized to produce an advanced generation of plant material,” Hokanson says, “this project will be successful. It’s a thrill for me to pursue an opportunity that didn’t exist five years ago, before all the interest in woody biomass.”
       
    Associate professor Andy David of the Department of Forest Resources will evaluate how well all the species and selections grow on marginal lands. From his office at the North Central Research and Outreach Center in Grand Rapids, David has secured two planting sites.
        
    “We have a site in Koochiching County where the growing season is quite short and the winters are quite cold,” David says. “Near Grand Rapids we’re planting on a mine spoils site. We also have a site on the Anoka Sand Plain in central Minnesota where soil fertility is low. If these species can make it on these sites, they’ll do very well elsewhere.”
        
    David is especially interested in studying the survival, growth and eventual regeneration of the aspen selections that have come out of his breeding program.

    Economically viable

    The evaluations of willow also intrigue research associate Dean Current of the Department of Forest Resources because willow bark is a good source of salicylic acid, which is used in skin care products, as a food preservative and as an antiseptic in toothpaste.
       
    “Salicylic acid could be a valuable co-product from willow grown commercially,” Current (’85–M.S., forestry; ’97–M.A., anthropology; ’00–Ph.D., forestry) says, “and that’s vital to making willow competitive in the bioenergy economy.”
        
    If willow or alder have good potential, Current says, landowners will be receptive to planting them. He leads the college’s Center for Integrated Natural Resource and Agricultural Management, which has worked with landowners statewide for over 10 years to develop woody crops that produce income. Current has been working with landowners in central Minnesota to grow willow in a three-year cropping cycle with the plants regenerating on their own for seven cycles before replanting is needed. After three years, the plants are 12 to 15 feet high and are cut down and ground up for easy transport—usually to a facility that burns them as fuel.
        
    Near the end of the study, Current and David will share their evaluations of alder and willow with interested landowners. They’ll also pass along their recommendations for techniques that help the trees survive and thrive.
        
    Growing alder and willow for their cellulosic biomass makes sense in an agricultural setting, says Alan Ek, head of the Department of Forest Resources. “There are fewer environmental issues on the planting, cultivation and harvesting of an agricultural crop as compared to forest crops,” Ek says. “Growing cellulosic biomass is an unlikely scenario in wildlands or existing forest settings.”

    Overcoming obstacles

    Ulrike Tschirner points out that biomass is a great source of glucose and other sugars that can be fermented to ethanol or other products.
        
    The woody parts of plants contain varying amounts of cellulose, hemicellulose and lignin. These chemical compounds are often called cellulosic biomass. Breaking the chemical bonds in cellulose and hemicellulose yields glucose and other sugars, which can be fermented to ethanol. The difficulty is in finding an effective way to break down the cellulose and hemicellulose into their sugar components. Tschirner, an associate professor in the Department of Bioproducts and Biosystems Engineering, will evaluate various methods for extracting the sugars, including various pre-treatment techniques that might improve the process. She also will determine which species will produce the most fermentable sugar.
        
    Another obstacle to Tschirner’s work is that the trees planted in the project’s first year will have grown very little by the time the study concludes four years later. “We’re obtaining samples [pencil lead-size cores removed with a hollow drill] from mature trees in botanical gardens,” Tschirner says, “because the young trees we grow won’t provide good samples for testing.”
        
    Tschirner will know fairly soon which species of alder and willow hold promise for producing ethanol. “The question is, will trees grown here produce similar results?” Samples from the mature trees will be compared with the young trees grown in Minnesota to learn how climate and planting conditions affect the production of cellulosic fiber and lignin in these species.

    Successful options

    The insight the group hopes to gain into each species’ genetic potential for adapting to northern growing conditions and its potential yield of cellulosic ethanol will be key to taking the next step: selecting a species or producing a hybrid specifically for the production of cellulosic ethanol.
        
    Estimates for the ability to produce ethanol from cellulosic sources show promise. The U.S. Department of Energy estimates that an acre of land producing five tons of cellulosic biomass per year would yield about 325 gallons of ethanol. That’s significantly less than the amount of ethanol brewed from an acre of corn. But producing ethanol from cellulosic sources requires fewer energy inputs. And if the team is successful, producing more energy may be possible without taking land away from other uses.

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