Plotting ways to save the urban canopy


Community leaders consider strategies to protect the urban forest from looming threats


LINDEN HILLS – A half-dozen neighborhood committee members sat around a table in a park building meeting room last month to discuss their strategy.

What can we do to spread awareness? How can we help residents pay for inoculations? How do we defend the neighborhood’s borders?

They weren’t talking about bird flu.

They were discussing Dutch elm disease, a tree-killing fungus that wiped out tens of thousands of trees in Minneapolis in the 1970s and that urban forestry experts worry is poised for a return.

Thirty years after Dutch elm disease exterminated boulevard trees throughout the city, Minneapolis’ urban forest is now larger and healthier than it was before the crisis. But old and new threats are looming for today’s green canopy, from invasive pests to climate change.

“We really need people to be much more aware of what’s going on and help make sure we keep our city full of trees,” said Dorothy Dahlenburg, development director for Tree Trust, which formed in response to the Twin Cities’ Dutch elm outbreak in the 1970s.

‘Back with a vengeance’

Dutch elm disease is a fungus spread by elm bark beetles. The bugs are attracted to stressed, dying or dead elm wood. They tunnel inside the bark where they lay eggs and hatch larvae. Once inside a tree, the fungus clogs its veins, preventing water and nutrients from circulating.

The disease claimed more than 50,000 trees in Minneapolis between 1977 and 1978. The city responded aggressively, marking infected trees with a spray-painted “T” to indicate they were to be torn down. As crews removed infected trees to prevent the disease’s spread, others started planting new ones. The city has planted more than 200,000 trees in the last three decades. Some of those have died off, but others are now large enough to provide shade and other benefits of a maturing tree.

Dutch elm disease returned to Minneapolis in its highest numbers since then in 2004, when more than 10,000 trees were removed because they were infected. The number of trees removed in 2005 and 2006 fell to about 6,179 and 3,350, respectively.

“Dutch Elm Disease has come back with a vengeance,” said Dahlenburg.

Urban forestry experts aren’t certain about why the resurgence happened, but they have a few theories. One is milder winters, Dahlenburg said. The beetles burrow into bark of healthy stems to survive the winter, but they can still die off in harshly cold temperatures. Fewer deep freezes might be aiding the beetles’ survival in recent years, Dahlenburg said.

A lack of awareness may also have played a role.

“A lot of people assumed it was very much under control, even if they lived through it before,” said Dave Hanson, a research specialist with University of Minnesota Forest Resources.

Recognizing and reporting the early signs of Dutch elm disease is important to preventing the spread of the disease to other nearby elm trees. It’s called a yellow flag – one or several branches in the middle of the tree where leaves are turning yellow. It can indicate the fungus has clogged the transport system to that branch.

Ralph Sievert, director of forestry for the Minneapolis Park and Recreation Board, said he suspects some suburbs have gotten lackadaisical with their efforts to control Dutch elm disease, but he said his department has stayed vigilant. Park staff enforces removal of infect trees on both public and private property, he said.

It’s also taken a lesson from the 1970s outbreak and diversifies the species in plants in parks and on boulevards, which were once predominantly elm.

“The lesson learned was not to put all your eggs in one basket,” Sievert said.

But non-elm species aren’t immune to other threats.

Migrating threat

The most worrisome new threat creeping toward Minneapolis is the emerald ash borer.

It’s an exotic beetle that showed up in southeastern Michigan in 2002, likely carried there by wood packing material from its native Asia. The adult beetles eat ash foliage but cause little damage to the trees. The larvae, however, eat the inner bark, which disrupts the trees’ ability to transport water and nutrients.

The beetle is to blame for killing more than 20 million trees in Michigan, Ohio and Indiana; and last year, it was discovered in the Chicago suburbs. The U.S. Department of Agriculture has placed a quarantine on firewood from those states in hopes of slowing the beetles’ migration, but those watching its path think it’s likely to reach Minneapolis.

“If you listen to the experts, the answer is definitely,” Hanson said. “There’s a camp out there who think it’s already here and we just haven’t identified it.”

Minneapolis is preparing for the emerald ash borer on a number of fronts, Sievert said.

“We’re looking at it as the ‘beat the beetle’ program,” he said.

Park officials are working to revise ordinances that would give the forestry department authority to attack the beetle on public and private property, as they can do for Dutch elm disease. It’s recently stopped planting new ash trees, Sievert said, and have lowered the threshold for when it’s appropriate to remove ash trees.

It’s also working with the U.S. Department of Agriculture to help monitor whether the emerald ash borer has arrived in Minneapolis. Sievert said they select trees in wooded areas and cut off the bark to expose its wood. When the tree dies, it releases pheromones that attract the beetles. The agriculture department then performs an autopsy to test for the presence of the beetles.

If the beetle migrates naturally, it could take 60 years for it to show up in Minneapolis. Residents can help lower the chances of a quicker arrival by not transporting firewood between regions, especially not from areas east of the Twin Cities, Hanson said.

Climate factor

The biggest unknown for the city’s urban forest is climate change, Sievert said.

Milder winters could allow disease-carrying beetles to better survive here. Drier weather could leave trees consistently parched and more susceptible to disease. More intense, severe storms could topple branches and trees.

The forestry department has for several years been experimenting with different species of trees it could plant if winters continue to warm, Sievert said.

Watering trees will become critical if droughts become more common. Sievert recommends residents water their boulevard trees June through September. A good method is to let a hose trickle at its base for about an hour in the evening, he said.

Another way to keep moisture in a tree is to put mulch or woodchips around its base. The park system has several neighborhood wood chip sites where people can scoop them up for free.

New awareness

Even though the Dutch elm disease outbreak in the 1970s claimed thousands of the city’s boulevard trees, forestry experts today see a silver lining from that experience.

“It was very bad,” Hanson said. But “that epidemic really moved us forward in a big way.”

Dutch elm disease got people thinking about the health and importance of the urban forest, he said. He points to the fact the city is already getting ready for emerald ash borer, something that might not be happening if we hadn’t seen firsthand the potential impact of tree diseases.

Just as during the first Dutch elm epidemic, residents have started paying more attention in light of the recent spike, Dahlenburg said. She also thinks growing interest in addressing global warming is drawing attention to trees.

“The positive side is that people have become aware of the fact that trees, especially city trees, are a major deterrent to global warming,” she said.

Urban trees have a double benefit, she said. They absorb carbon dioxide, removing it from the atmosphere, and they also provide cool shade in the summer and wind breakers in the winter.

Kathy Urberg, chair of the Linden Hills environment committee, watched emerald ash borer take hold in Detroit before she moved from there to Minneapolis in 2003.

Shortly after she arrived she took a master gardener and tree-care class. She then started spotting trees in the neighborhood suffering from Dutch elm disease or lack of water.

“I just decided that I wanted to do something,” Urberg said.

The committee and neighborhood board raised $26,000 in private donations last year for forestry projects in Linden Hills. Combined with Neighborhood Revitalization Program funds, they were able to plant 120 trees and pay for elm trees at Southwest High School and the Linden Hills Library to be vaccinated against Dutch elm disease.

Other neighborhoods in the city are becoming more active in helping residents plant, maintain and treat trees, too. All the activity is a sign of what Dahlenburg sees as growing, and necessary, enthusiasm for maintaining the city’s trees.

“It seems to have really blossomed,” Dahlenburg said.