Focus Story – Learning From the Past,
Protecting the Future

Beginning in the 1950s, Dutch elm disease (DED) swept across the Northeast and Midwest, leaving ecosystems — especially those in urban areas — devastated by the loss of millions of trees. Milwaukee lost an estimated 200,000 elms during the worst of the epidemic. The city of Milwaukee’s Forestry Services manager, David Sivyer, estimates that 50 years later, the city still has not regained the canopy it had at the time of DED’s outbreak. Unfortunately for Milwaukee, another serious threat to the city’s tree canopy is on the horizon.

Emerald ash borer (EAB) killed its first ash trees in Detroit, Michigan, and Windsor, Ontario, in 2002.[1] Since then, it’s spread throughout the Midwest and even east to Maryland. In July 2012, Milwaukee had its first confirmed cases of EAB, and considering that approximately 17 percent of Milwaukee’s canopy is ash, the effect could be dire. The forestry experts in the area, though, aren’t sitting around, just waiting for it to happen: They’re going on the offensive.

In 2008, the City of Milwaukee utilized the U.S. Forest Service’s i-Tree and UFORE tools to quantify the number of ash trees at risk to EAB in the city limits: a staggering 587,000 trees. Then, in 2009, the forestry team used hyperspectral imaging — the first successful use of the technology in a U.S. city environment — to map the location of those ash trees. The technology uses information from across the electromagnetic spectrum to identify specific objects based on their unique “signatures,” or reflectance in the spectrum. Hyperspectral imaging was an important tool used to identify ash trees at risk of EAB infestation and help Milwaukeeans prepare for its eventual arrival. The combination of the city’s spatial street tree inventory, i-Tree UFORE Ecosystem Analysis and hyperspectral imagery provided the scientific basis for a comprehensive risk-management strategy for EAB.

Injection to protect ash against EAB. Credit: CIty of Milwaukee

Injection to protect ash against EAB. Credit: CIty of Milwaukee

Not long after the imaging was completed, Environmental Services began injecting approximately 13,000 ash trees per year to protect them from infestation. Then, an intern fleet was dispatched to visit all private properties that the hyperspectral imaging had identified as having ash trees to confirm the presence of ash trees and to provide information to the homeowners on the threat that EAB poses to their trees. This canvassing is being followed up in 2012 with a demonstration project showcasing options for EAB management, including chemical injections or removal and replacement of ash trees on private property, funded through a grant with the Environmental Protection Agency’s Great Lakes Restoration Initiative.

The city of Milwaukee hopes that all of this will help mitigate EAB’s effects, but the city is also taking proactive steps to protect its canopy from future threats to specific tree species by diversifying its canopy at the block level. In the past, the city’s tree-diversity goals were achieved by planting uniform species on public land throughout the city based on street or block themes or aesthetic concerns. Now, the goal is for each block to contain four different species. By increasing the diversity of its canopy within each block, Milwaukee hopes to forestall devastation on par with what was experienced with Dutch elm disease, when entire blocks of trees were killed and removed over a short period of time.


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[1] U.S. Forest Service. Northeastern Area. Forest Health Protection — Emerald Ash Borer. (accessed Aug. 9, 2012).