Hybrid trees developed by scientists at the University of Wisconsin-Madison at McKay Nursery Co. in Waterloo in 2012. The trees have since been re-planted in six Wisconsin communities, and future generations could help diversify tree populations in urban settings. Credit: William Hoch / UW-Madison

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Sixteen new varieties of maple and alder trees developed at the University of Wisconsin-Madison have been planted in six communities in an experiment to judge how well the trees stand up against disease, pests and other assaults.

The trees will be getting their first checkup this spring after living in the wild for a year. And in the coming years, researchers will monitor how well they fare and whether any could be used to diversify Wisconsin's tree stock.

The attack of emerald ash borer raises the likelihood of widespread loss of ash varieties in the coming years as it continues to spread across Wisconsin.

Oak wilt is killing oak trees, and Dutch elm disease has left most communities with few, if any, elm trees that once towered over streets.

Maples are widely planted, but there are concerns Wisconsin is susceptible to the Asian long-horned beetle, which has infected tree populations in Ohio, New York, Massachusetts, New Jersey and Illinois. Infestations in New Jersey and Illinois have been eradicated, officials in the two states say.

"What we are looking for are trees that in the future have attributes that people want, sell well and can be added to the list of urban-tolerant varieties," said Dick Rideout, an urban forester with the Department of Natural Resources.

One problem today is that many communities and homeowners plant a relatively small number of tree varieties, "and if another bug hits the landscape that wasn't here before, it could wipe out a lot more of trees," Rideout said.

"The more diversity we have, the better. The idea of this research is to give us a couple of more alternatives."

The most vivid example is the emerald ash borer, an Asian beetle that was discovered in Wisconsin in 2008 in Washington and Ozaukee counties and has since spread to more than 20 counties. Wisconsin has an estimated 725 million ash trees, including 5 million in the state's urban areas.

The 16 new varieties of maple and alder trees were developed by Brent McCown, a professor emeritus of horticulture at UW-Madison, and William Hoch, formerly a graduate student at UW and currently an associate professor of plant sciences at Montana State University.

Hoch said the initial work of crossbreeding seedlings with desirable characteristics started 10 years ago, and then in 2010, the trees were transferred to the McKay Nursery Co., which managed them until they were planted in the spring of 2013. The trees ranged in height from 8 to 12 feet.

Thirty-eight trees were each planted in West Bend, West Allis, Eau Claire, Lodi, Menasha and Port Edwards.

The maples were developed from Norway and Shantung varieties.

The Norway has been widely used on residential streets because of its attractive appearance and ability to withstand soil compaction and pollution. But it produces abundant seeds, and because it is a non-native tree, the Norway is known to invade native woodlands and out-compete sugar maples.

The Shantung, a native of Asia, has an attractive appearance, but is not known to propagate well in the United States. The hybrid will have desirable landscape qualities but won't spread across the landscape on its own.

The alder species included black alder and Japanese alder. The black alder is tolerant to the stress of urban settings from pollution, compact soils and road salt. But it's also prone to pests. The Japanese alder is pest resistant, and when the two are bred together, they are sterile.

Only a few of the 16 trees will make the cut, Hoch said. UW will trademark or patent the best varieties and make them available to companies that propagate trees.

Test of a brutal winter

This year, people in the project are anxious to see how the trees will hold up after the brutal winter of 2013-'14. Hoch said the best varieties won't be known for five or six more years.

In West Bend, "pretty much all our elms are gone," said Melissa Philipps, landscape architect for the city.

The experimental hybrids were planted along older city streets where ash populations are high.

The emerald ash borer was found in the city in 2010. Since then, crews have cut down 300 to 400 trees. The city will remove another 300 to 400 ash trees this year, "because we are starting to lose them faster," she said.

About one-third of the city's 4,000 street trees are varieties of ash. "Now, we are trying to expand our tree diversity," she said.