Denver tree canopy in danger

Emerald ash borer and spruce ips beetle are killing Denver trees

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Denver’s tree population is suffering from drought, climate change and insects.

Denver residents have likely witnessed extremely dry, crackling and withered looking trees throughout Denver city parks, streets and the landscape this year. Drought, climate change, and insects are having a severe effect on much of Denver’s beloved tree population.

“September was one of the state’s hottest on record, and we’ve had statewide drought for the last year,” said Mike Swanson, Denver’s acting city forester.

Swanson said that climate change was certainly having an ill effect on the trees too. He gave an update on the tree canopy to the Denver Inter-Neighborhood Cooperation and other citizens’ groups the evening of Oct. 15 in University Park.

Swanson also updated the group on harm done by two deadly pests that are infesting and killing Denver trees: the emerald ash borer and the spruce ips beetle. Emerald ash was first detected in Michigan in 2002. It has since spread to 33 states and was confirmed in Boulder in 2013. The Colorado Department of Agriculture then began a quarantine with a ban on transporting infected trees out of Boulder. This may have helped slow the spread, but the insect was confirmed in Broomfield in late August, and Westminster in September. Most recently, it has been found in Berthoud in Larimer County. Ash trees may be infested for up to four years before signs of decline are visible.

It is estimated that Denver has more than 30,000 ash trees along the public right of way, 4,500 of which are within parks and parkways, according to Denver Parks and Recreation. Within the Denver metro area there are approximately 1.45 million ash trees, primarily on private property. To date this year, the Denver Forestry Field Operations team has removed nearly 1,000 dead or in very poor condition ash trees from the city. The forestry team has also treated over 2,000 ash trees through the city’s four-year health treatment cycle plan.

“We have also recently brought in 800 trees that are being planted in underserved areas of the city,” said Swanson.

Denver has also started a “Be a Smart Ash Program,” which is an educational and planting program geared toward residents and private landowners that teaches them how to share in the preservation and renewal of Denver’s urban forest. The city is planting trees that it finds tolerate the arid climate which include Arizona cyprus, Bosnian pine trees, ball cyprus, and “100 sequoia trees,” said Swanson.

“It is tough to grow many trees in Colorado,” he said, “so whatever works we keep doing.”

Signs and symptoms of the ash borer in trees include sparse leaves or branches in the upper part of the tree, D-shaped exit holes approximately 1/8 inch wide, new sprouts on the lower trunk or branches, vertical splits in the bark that are winding, S-shaped tunnels in the bark, and increased woodpecker activity.

“We are spreading the word to private owners that they need to remove trees that are diseased, and please plant new ones to contribute to new growth,” said Swanson.

This year, the city has also seen an uptick in the spruce ips beetle, first detected in Longmont and Fort Collins, and then Denver. Last month the city removed 18 trees in central Denver that were infested or dead from the insect, and Swanson said this was not an isolated incident.

“We really have no clear answer as to the uptick,” he said. “It is possible it was related to the timing of our past treatments not being effective, or a lack of snow cover in 2019, or the occurrence of statewide drought in 2018.”

Normally, these beetles limit their attacks to trees that are already in decline or otherwise stressed, but under current weather conditions, the beetle has been allowed to improve its survival and build up a large population. No chemical treatment exists for trees already infested.

The city’s phloem reduction of affected trees within the parks system and along the public right-of-way and on private property to treat and/or remove trees is not unlike the strategy used to fight Dutch elm disease in the 1980s to 2000s. Swanson estimates the city has 250 to 1,500 remaining Dutch elm trees.

Helene Orr, a resident of Denver since 1971 in the Overland North area and a member of the Denver Inter-Neighborhood Cooperation, said she believes many factors are stressing the trees.

“The (emerald ash borer) and climate change are serious threats.”

Orr also believes there is too much runaway development in the city, and that the health and safety of trees is not being considered.“The city is letting developers do whatever they want, with no concern for the trees,” she said of the Denver building boom over the last several years.

Orr is working on INC’s parks committee to address these issues, including trying to get Denver parks and the city to better regulate, and even ban, events in the park that she said harm them and lead to overuse.

The extreme drop in temperature going from 80 to 9 degrees nearly overnight in early October also hurt the trees. Those temperatures caused many of the trees’ leaves to die early, because they were not yet ready for the extreme cold. Hence the “crunchy” look to many trees’ leaves this year.

“We had a terrible hail event in the spring that also stripped the trees of many of their leaves,” Swanson said. “That too is contributing to the horrible look of Denver trees this year.”

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