Nature Conservancy project seeks healthy American elms

Last modified: Monday, April 29, 2013


Once the dominant tree species in Massachusetts, a healthy mature American elm is a relatively rare find these days. That is why scientists from the Nature Conservancy are on the hunt for the few remaining mature elms in New England with hopes of propagating traits that have protected them from the devastating Dutch elm disease.

“You can go into the forest now and see hundreds of American elm saplings, but they rarely live past five years due to this disease,” said Kim Lutz, director of the Connecticut River Program for the Nature Conservancy.

Three years ago, Nature Conservancy ecologist Christian Marks began a project to restore the American elm to Connecticut River floodplain forests using a population of disease-resistant trees.

To accomplish this, Marks travels throughout New England to locate mature healthy elms. His team then harvests 15 to 20 branches from each tree in the spring before the leaf buds are open.

These samples are immediately sent to Jim Slavicek, project leader and research biologist at the Northern Research Station of the U.S. Department of Agriculture’s Forest Service in Ohio. There, Slavicek’s team works to propagate disease-resistant elms by crossing Marks’ samples with trees that the Forest Service has proven to be highly tolerant to the disease. The offspring from these controlled crosses will be planted at floodplain forest restoration sites.

The American elm “survivor trees” that Marks locates are key to re-establishing the elm in forests, floodplains and neighborhoods.

“This year I am focusing on trees in Massachusetts. We did a bunch in Northfield yesterday and we will be going to the Berkshires next,” Marks said last week. “Last year we focused on northern Connecticut.”

Marks and arborist James McSweeney, of Hilltown Tree and Garden in Chesterfield, were in Hatfield to collect samples from an American elm on Main Street.

“We found this tree on a hunch,” Marks said. “I knew that historically, American elms lined the main streets of towns like Hatfield, and the town is also right in the floodplain which is perfect habitat for these trees.”

Marks said that since he began the project, people occasionally contact him with a tip on the location of a large healthy elm. Otherwise, it’s all field work.

As McSweeney prepared to hoist himself high into the large elm, Marks explained the progression of Dutch elm disease in North America.

“The disease was introduced into the United States in 1930’s on a shipment of logs from Europe,” Marks said. “By the mid-1970’s the disease had destroyed millions of elm trees in the United States and Canada.”

Its origins

Interestingly, the disease is not of Dutch origin. It is so named because Dutch pathologist Bea Schwarz conducted the first studies on the disease in the 1920s. It is likely the disease is of Asiatic origin.

The disease is a fungal pathogen that grows within the vascular tissue of the tree, clogging its water-conducting system and causing leaves to become discolored, wilt and drop and eventually killing the tree.

“The disease is spread from infected trees to healthy trees in two ways. One, by the activity of bark beetles and the other through root grafts,” Marks said.

When trees grow close together, their roots often fuse or become grafted, allowing the disease to spread from tree to tree. Trees that become infected through root grafts generally die soon after becoming infected.

This is a significant cause of tree death in urban areas, where elms were often closely planted in long rows.

Disease’s impact

“In a lot of places where silver maple is the dominant tree species now, the American elm would have been the dominant species in 100 years ago,” Marks said.

In his view, the significant loss of the towering American elm has altered floodplain forest ecosystems. Once dominating the forest canopy, the huge elms fulfilled an important ecological niche that combined flood tolerance with shade tolerance. While elms can still be found in the Connecticut River watershed, the disease prevents the trees from reaching significant size.

Marks said that with the continued absence of mature American elms, the floodplain forest will eventually change its composition, allowing it to revert to low, scrubby forest of sumac, vines and golden rod.

“The presence of elms is what helps maintain the floodplain forest structure,” Marks said.

Lutz noted that a healthy, intact floodplain forests not only provide habitat for wildlife, but “act as a sponge helping to control flood levels during heavy rains as well as filtering out sediment and pollution before reaching our rivers.”

Researchers believe the Dutch elm fungus will eventually mutate and attack resistant trees. To combat this, trees need to regenerate so that they can co-evolve with the fungus.

Once Marks’ samples are crossed with trees that the Forest Service has proven to be highly tolerant, the new cultivars are injected with Dutch elm disease when the trunk is about 1 inch in diameter. Those that show no symptoms are used as restore the American elm to its native territories.

Marks says he has already planted samples of disease-resistant American elms on Elwell Island in the Connecticut River in Northampton. The trees are planted well apart from one another and silver maples are planted between each elm to prevent the roots from grafting.

“Christian also has about 50 trees that he planted in Vermont about two years ago,” Luzt said. “It is very exciting project,” she said.


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