The Colorado Springs tree canopy could be thinned significantly in coming years when an invasive green beetle arrives. 

The emerald ash borer kills all species of ash trees unless the trees are treated with pesticides. It has spread through the Denver metro area rapidly in recent years, moving into Broomfield and Westminster last year, and  Arvada and Louisville this year, according to a Colorado State Forest Service map. The expected loss of the ash trees will reduce shade over hot urban areas and many of the other environmental benefits trees provide in Front Range cities. 

For example, the Colorado Springs tree canopy covers 17% of the community and provides the city with an estimated $100 million in air filtration, $900 million in stormwater retention benefits and $2 million in carbon sequestration, according to the city's new Urban Forest Management Plan. 

However about 15% of the trees in Colorado cities are ash, making the beetle a formidable menace for parks departments and residents, who have to choose which trees to save. Trees need insecticide injections for the rest of their lives to live through a beetle infestation, said Vince Urbina, Colorado State Forest Service urban and community forest manager. If communities don't treat for emerald ash borer once it starts to attack trees, the trees eventually have to be removed, which carries its own costs. 

"I would say communities that have opted to treat trees are being very selective in the number of trees," Urbina said. 

While Colorado Springs has yet to detect the borer, its losses to the beetle could be heavy because the trees account for 10% to 20% of the canopy in some parts of town, said Dennis Will, city forester. The trees were popular because they did well in the city's heavy clay soils, he said. 

The city doesn't know exactly how many public ash trees are at risk or how many trees it has total because its tree inventory is about 13 years old, Will said. However, the city estimates it has about 250,000 street trees and about 20,000 park trees, he said. 

The city expects to use about $108,000 of an extra $200,000 dedicated to its 2021 budget to start updating its tree inventory, but it's not expected to cover the full cost, Will said. The additional $92,000 is going to pay for buying 150 new trees and their maintenance over three years.

The extra boost to the 2021 budget, made possible by the voters' approval of question 2A in November, was helpful because city forestry has been underfunded since budget cuts in 2008 and 2010, Will said. However, the city Forestry Department remains understaffed with only seven arborists and two staff foresters to take care of 200 square miles, he said. A consultant recommended an additional 16 arborists to take care of the city's urban forest, he said. 

The small staff keeps the city reacting to the most urgent tree needs and not as prepared as they could be to deal with the emerald ash borer once it arrives, Will said. 

"If we had more help, we could maintenance those trees better, faster, more efficiently," he said. 

The city could also create a pruning schedule across the city, he said. 

Boulder's ash borer battle

The emerald ash borer arrived in Colorado in 2013 when it invaded Boulder. The beetle arrived far earlier than anyone expected. It was first detected in the U.S. in 2002 in the Midwest, Urbina said. The invasive species was imported unintentionally from China. The beetle's larvae typically are transported in firewood, pallets and other wood products, he said. 

"It showed up and we were just frantic," he said. 

The city of Boulder set out to save about 25% of its public ash trees through ongoing injections and removes the rest as they become symptomatic, said Kathleen Alexander, city forester. The injections have been successful, but the city now only expects to save 22% of the public ash trees after some were lost to heavy, wet snow and other weather events, she said. 

Boulder has also introduced some tiny parasitic wasps native to China that feed on the emerald ash borers to help control their population, she said. Downy and hairy woodpeckers also love feeding on the beetles and can help slow them down, she said. 

Efforts to keep the beetle populations lower are key, because otherwise cities can see ash trees die so quickly it can be tough to keep up with removals, Alexander said. 

Boulder is also planting about 450 trees representing 35 different species annually to increase diversity of its tree canopy, she said. However, the city is still removing more trees than it's planting and it expects to see a net loss in its canopy because of the beetle and weather events that have killed off trees, she said.

Alexander recommends that communities try to increase the diversity of their tree populations to increase chances for long-term resiliency, as this is not the first time cities have seen an epidemic among trees, she said. Dutch elm disease killed off rows and rows of American elms in the Midwest, she said. 

"What did they do? They planted rows and rows of ash trees," she said. 

Saving and replacing trees

Residents interested in saving their ash trees should wait until the pest is detected before starting to treat them and make sure they are healthy to begin with, Urbina said. Underperforming ash trees should be replaced with different species, he said. 

He urged residents to think beyond the Colorado favorites of aspen and blue spruce. 

"If you are going to five trees, plant five different ones. ... That’s part of the diversity formula that every community should take," Urbina said. 

Ahead of the beetle arriving, communities can plant young trees near the ash trees, a strategy known as shadow planting, so that the new trees are already in place to replace the ash, he said. 

The city of Colorado Springs keeps an approved street tree list and a list of prohibited trees.  

Find the lists at ColoradoSprings.gov/planttree

Contact the writer at mary.shinn@gazette.com or (719) 429-9264.

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