The 36 trees planted last week at Boutwells Landing senior living community in Oak Park Heights have a big job: To help revitalize the state’s elm population.

The American elm trees, each about 2 years old and 4 to 6 feet tall, were cloned by University of Minnesota researchers to be resistant to Dutch elm disease, a fungal disease that killed millions of elm trees around the world.

In the late 1970s, there were 1.3 million American elms with diameters greater than 21 inches in Minnesota. Dutch elm disease killed 95 percent of them, leaving behind fewer than 60,000 big elms, according to U researchers.

Ryan Murphy and Ben Held, co-investigators on the U’s disease-resistant elm selection and reintroduction program, want to revive the population. On May 16, they got some help from Boutwells Landing residents David Lime, 84, and Neal Kingsley, 87, both U.S. Forest Service veterans, who participated in the three-hour planting project.

Soon after Lime moved to Boutwells Landing three years ago, he pitched the idea of starting a nursery in the southeast corner of the 100-acre property to help offset the loss of ash trees from emerald ash borer.

Boutwells Landing officials expressed interest, and Lime, who worked for 20 years in the Forest Service’s experiment station on the St. Paul campus and later taught at the U’s College of Forestry, started looking for places giving away trees and people who were researching trees “where we could invite them to plant some trees on our property,” he said.

Lime connected with Kingsley, and the two met with Rob Venette, director of the Minnesota Invasive Terrestrial Plants and Pests Center at the U of M and research biologist with the U.S. Forest Service Northern Research Station. He connected them with Murphy and Held.

“It was just one of those things where they had space, and we were looking for places, and it just was the right connection,” Murphy said.

Hardy elms survived

Dutch elm disease is caused by a fungus that can be spread by a bark beetle or through two trees that have interconnected roots. Beetles reared in infected trees emerge from the diseased wood carrying spores, which they then deposit into healthy trees by feeding on the young twigs. The fungus triggers reactions that block the tree’s vascular system, which prevents it from getting water and nutrients normally, and it becomes wilted, leading to rapid death, Murphy said.

Fortunately, Dutch elm disease didn’t kill every elm tree in the state. Some very hardy “survivor” elms were left behind.

“Oftentimes, they’re the only elm tree left in an area where everything else has died,” Murphy said.

Said Venette: “It’s just a matter of a random mutation that happens to occur in these trees. In general, the species as a whole is highly susceptible, but it’s just these very lucky individual (trees) that have natural resistance.”

Researchers are using the “survivor” elms from around the state – identified by forestry officials, arborists and private landowners – to grow Dutch elm disease-resistant trees.

Here’s how it works: Researchers visit the “survivor” elm in the wintertime and take the branch tips and then grow a tree genetically identical to that elm. “You take that tissue from that twig, and you graft it onto a rootstock,” said Murphy, who also manages the U of M’s Urban Forestry Outreach & Research Lab, which provides education about trees to communities around the state.