Hybrid Cottonwoods and Poplars: Faster than Fir

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In the 1960s, UW forestry professor Reini Stettler began working on the reproductive biology of black cottonwood trees. On the face of it, that might not seem out of the ordinary for a professor of forest genetics, except that he was working in the Pacific Northwest where the Douglas fir was the most economically important tree species. Stettler was considered to be something of a heretic in those days for not establishing a breeding program in Douglas fir.

Over the years, the experiments of Stettler and colleagues to hybridize different species of cottonwoods and their cousins, the poplars, proved that the hybrid trees were remarkable indeed. Not only did they have characteristics superior to the pure species, the hybrids were demonstrated to be among the fastest-growing plants known, approximately 10 times as productive as Douglas fir during the first ten years of growth.

By the time Stettler retired in September, 1995, his hybrid poplars were being grown for paper fiber by many Northwest forest products companies: James River, Boise Cascade, Potlatch, MacMillan Bloedel, Scott Paper, and Georgia-Pacific West. And since 1978, the U.S. Department of Energy has funded work on the hybrid trees as a potential source of biomass to produce ethanol for automobile fuel.

The work "demonstrates how one man's basic research, vision, and leadership can result in the creation of an entire industry," observes Toby Bradshaw, research faculty member of the College of Forest Resources and collaborator on the project. "This is a classic example of high-quality basic research, carried out on a global scale with its epicenter here at the UW, that has been taken from the most fundamental level of biology to the commercial--and very profitable--scale."footnote 1

Bradshaw notes that the U.S. forest products industry is moving rapidly toward the culture of fast-growing hardwood trees, such as the hybrid poplar, for the sustainable production of wood, for paper fiber, and for biomass in the production of fuel. Such trees offer a long and impressive list of advantages. For starters, hybrid poplars are typically harvested for pulp after just 6 years of growth, rather than the 20 to 60 years required for pine and Douglas fir. After those 6 years, the poplars average more than 60 feet in height and more than 6 inches in diameter. On good sites, the hybrid poplars are the fastest-growing trees in the world's northern temperate zone.

The yield of fiber from hybrid poplar over such short rotations is 5 to 10 times higher than the most productive native Douglas fir. Moreover, in eastern Washington and Oregon, 1 acre of hybrid poplar can spare up to 100 acres of native forest without a reduction in the output of a pulp/paper mill. The poplar pulp is naturally lighter in color than other kinds, and so requires very little bleaching in the papermaking process. The discharge of dioxin to waterways is therefore less. And the trees can be cultivated on marginal agricultural lands lying fallow or planted to pasture, eliminating the need for further clearing of native forest.

Fuels produced from biomass offer many environmental advantages. They have little or no sulfur, so they won't produce the acid-rain precursors that fossil fuels do. And they produce few nitrogen oxides, the culprits in photochemical smog. Unlike coal, oil, and natural gas, fuel from biomass does not cause a net increase in atmospheric carbon dioxide when burned, and so offers a way to reduce global greenhouse gas emissions. The DOE suggests that within about 30 years, technology may be available to replace half of the gasoline used in cars and buses in this country by biomass fuels, such as ethanol from hybrid poplars. footnote 2comma footnote 3

In a 1991 article in the Oregonian, Stettler explains that in the 1960s he did not foresee ethanol production as an application of his work. He picked the trees as "an experimental system full of promise for basic research."footnote 2

Besides its fast-growing properties, many poplar species offer a large variability in traits such as disease resistance or drought tolerance. Those characteristics can be developed by crossing wild trees with cultivated stocks. Moreover, poplars can be clonally propagated, meaning that many genetically identical trees can be made from a single tree, reproducing all of that tree's genetic traits. That property allows researchers to conduct carefully controlled experiments on identical copies of a tree to determine the clones' responses under a variety of growing conditions. And once an especially promising clone is identified, industry can then propagate the tree into many more identical clones for cultivation in plantations on a commercial scale.

Work on the multidisciplinary project continues, as researchers use genetic mapping techniques to determine, at the molecular level, which genes are responsible for critical traits such as disease resistance and rapid growth. The project involves eleven senior investigators, housed in eight different departments of the UW and Washington State University.

  1. Information provided by Toby Bradshaw is gratefully acknowledged.
  2. "Fast-growing trees fuel dreams," Carol K. Yoon, The Oregonian, July 25, 1991, p. F1.
  3. "Improved Yields of Biomass," Philip H. Abelson, Science, 252, 1469 (1991).

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