Street trees are falling victim to the emerald ash borer. (Credit: Steven Katovich, USDA Forest Service)

Nationwide, our urban forests are in big trouble. Harsh weather, neglect, and rampant new diseases are causing vast mortality. Arboreta are playing a lead role in addressing this crisis. For example, the Morton Arboretum is working hard to educate the public about the numerous benefits of trees: Tree-lined streets are cooler, quieter, and even stimulate shoppers to spend 12% extra in stores. The 2.1 million urban trees in Philadelphia alone store nearly half a million metric tons of carbon, keeping that troublesome greenhouse gas out of the atmosphere.

At the United States National Arboretum, the Urban Tree Breeding Program seeks to introduce new species to America’s increasingly sunbaked streets. With so many popular types suffering from disease, “We’re starting to run out of trees,” says research geneticist Dr. Richard T. Olsen. So he is experimenting with black gums, catalpas, and hackberries — native species that have never been hybridized for urban use. The idea is to develop new types that are short enough to fit underneath power lines, and suitably small-leaved (to reduce autumn litter), disease-resistant, and attractive.

Windstorms, droughts, and frigid winters are the rule in Nebraska, contributing to a steady decline in street trees over the past 30 years — perhaps a 50% canopy loss. The Nebraska Statewide Arboretum is actively teaching town officials which kinds of trees to plant, and its “Trees for Nebraska Towns” initiative has resulted in the planting of $6 million worth of trees statewide.

Midwestern cities are under assault from the emerald ash borer, a Chinese beetle that has already killed some 25 million ash trees. This plague is proving virtually impossible to stop, and a huge additional number of street trees are expected to succumb soon, including an estimated 20% of those in Chicago. Morton Arboretum is working to retool a defunct power plant in Robbins, Illinois, to burn diseased ash trees, generating much-needed jobs in the process.

To get a better picture of what the threatened urban forest is actually like, Morton undertook the 2010 Tree Census. A computer picked 1,400 random points in the counties surrounding Chicago. Arboretum employees have been visiting all of them to record every tree in sight. The goal: to highlight the urgent need to manage and save our endangered urban forests.

— W. Barksdale Maynard