As the research has gone forward, the scientists have had to deal with a different set of risks. The locations of the research projects are usually not disclosed because of the possibility of violence, such as the firebombing two years ago of the University of Washington's Center for Urban Horticulture. The Earth Liberation Front claimed responsibility.
"That act of terrorism cost the university millions of dollars and destroyed valuable research," said UW geneticist Toby Bradshaw, who was targeted for the vandalism by the ELF based on a mistaken belief that he worked with genetically altered poplars. Bradshaw's trees were genetic hybrids, but created by traditional cross-pollination methods.
The tree scientists say the critics don't understand how science, even biotechnology, also can give Mother Nature a fighting chance against ravages natural and manmade.
Biotechnology, they say, may provide just what's needed to help reverse global deforestation and industrial pollution while satisfying increased demand for wood and paper products. Fruit-tree farmers could also benefit from the creation of hardier crops.
Already, biotechnology has been credited with saving Hawaii's $14 million-a-year papaya industry. A virus had wiped out 40 percent of the crop and threatened to destroy the rest before seeds engineered to resist the virus were introduced in 1998.
Now, advocates of biotechnology say, Hawaii's papaya industry is thriving again. Critics of the technology, however, contend the altered papaya tree is weaker and requires heavier use of pesticides and fertilizer to survive.
Papayas are the only approved engineered tree for market. The rest are still experimental.
About 230 notices of genetically engineered tree experiments have been filed with the U.S. Department of Agriculture since 1989, with about half coming since 2000.
"There is a lot of value in genetic engineering," said Oregon State University researcher Steven Strauss, who tends to a few thousand engineered trees.
Some researchers are infusing trees with genetic material taken from viruses and bacteria that helps them grow faster and fatter and yield better wood. Others are splicing mercury-gobbling bacteria genes into trees, enlisting nature to help clean polluted soil.
Still others are inserting foreign genes that might reduce the amount of toxic chemicals needed to process trees into paper.
Poplar, eucalyptus, apple and coffee trees are among those being engineered.
Researchers even hope to revive the cherished American chestnut, devastated by blight a century ago. Few of the trees, once a dominant feature of the eastern United States, grow higher than shrubs before succumbing.
The experimental tree plots are much smaller in scale than the 100 million acres of genetically modified food crops planted last year.
Except for the Hawaiian papaya, no genetically modified tree is expected to be commercialized for the next five to 10 years. Trees grow much slower than crops, and genetic researchers need years to compare and contrast generations.
But could biotech trees crossbreed with their natural brethren and ruin forests' genetic diversity? The Sierra Club fears that, among other ecological consequences.
Researchers hope to placate critics by engineering sterility into their designer trees, so their effect on the environment can be contained. But that technology remains elusive.
Many field trials are backed by paper and timber companies hoping to design trees that yield more wood and paper.
Numerous projects are aimed at growing more wood on less land or making it cheaper and less environmentally harmful to process trees in mills. Fruit-tree farmers, such as those in Hawaii, are looking for hardier trees with less reliance on chemical bug and weed killers.
The Pentagon's Defense Advanced Research Projects Agency even awarded Colorado State researchers $500,000 this year to develop a pine tree or other plants that can change colors when exposed to a germ or chemical attack.
All this is being done now because of better understanding of tree genomes. The Dendrome Project at the University of California-Davis, mimics the Human Genome Project, offering detailed genetic information on 100 trees on its Web site.
Forestry researchers are proud of their work, but have learned to be circumspect about disclosing where their genetically engineered trees are growing.
In June, three protesters were arrested after chaining themselves inside a UC-Davis science building to protest tree research.
Oregon State's Strauss says the protesters' legitimate concerns are virtually identical to those of scientists. After all, he is working to engineer sterility into poplars.
"The violent guys just don't understand the science," Strauss said. "Genetic engineering is not one thing; it's a thousand things. But the extremes want to stop it all."