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How Do We Create Resilience in SE Wisconsin's Changing Urban Forests?

Written by: Certified Arborist August Hoppe, WI-0477A

When most people think about forests, they immediate think of wilderness and vast swaths of dense trees in areas far away from people. Many people don’t think about the trees around us in the urban environment as a forest. Yet within our urban environments, trees are truly all around us. The definition of a forest is simply an area that is covered with trees. Trees line our city streets, parks and yards and the benefits from these trees are substantial. We live, work and play with trees surrounding us. Collectively these trees represent our urban forest. The urban forest is a constantly evolving ecosystem, with constant care needed to prune, treat, remove and plant trees.

Here are some interesting statistics about our SE WI urban forest that might surprise you. The average statewide canopy percentage for all municipalities is 28.75% of total area. This includes private and public trees combined. Ozaukee County is the highest canopied urban area in all of WI with tree canopy covering 45.45% of its total area. Milwaukee County is 34% covered by tree canopy. Bayside leads the way in Milwaukee County with 66.4% tree canopy coverage. Suburbs such as Wauwatosa, Shorewood, Glendale and Franklin all fall between 32%-38% canopy coverage. Overall within our urban forests, 22% are maple trees, followed by ash at 11%, and 7% honey locust. Multiple other tree species make up the other 60% of our urban forest. (information provided by WI DNR’s WI Urban Forestry Assessment Program)

Overall the Milwaukee area has a high percentage of canopy cover compared to other parts of the state and the nation. Yet, the makeup of our urban forest is now changing, and under threat. The devastating Emerald Ash Borer (EAB) is a big reason for it. Ash trees are a major part of our tree population. Over the last few years, canopy cover percentages have dropped as hundreds of thousands of ash trees are being removed across the state due to this invasive insect. We expect canopy cover will continue to drop for a few more years as ash trees are cut down and removed from our landscapes. New trees will be planted, but the after-effects of removing so many ash trees will last for at least a generation as we wait for the new trees to grow and mature.

We haven’t seen anything like this since Dutch Elm Disease swept thru the Milwaukee area 50 years ago. Many of us still remember street upon street lined with stately

elms, which created a cathedral-like appearance over roadways. Dutch Elm disease has now wiped out 99% of all the elms. In one year alone the City of Milwaukee lost 16,000 elms to Dutch Elm disease, overall estimates are that over 200,000 elms were removed from city streets over the course of the 1960s, 1970s and 1980s. Replanting efforts took place, with many of them being ash, maple and honey locust trees. Over the last 30 years, our canopy cover has steadily increased as these new trees grew and developed into beautiful shade-creating trees. Now with EAB we are once again losing much of this canopy cover in our wood lots, streets, and backyards.

We learned after Dutch Elm disease to plant multiple types of species to create more variety. We are learning now after Emerald Ash Borer that even more diversity in planting is needed. The more closely related tree species are, the more likely they are to be vulnerable to the same pests and damage. Urban foresters and arborists now look to a formula of 5:10:15 with planting trees. No more than 5% of trees should be of the same species, no more than 10% should be from the same genus and no more than 15% should be from the same family.

We don’t know what the next new pest or disease might be, but we do know that we now have an opportunity and a responsibility to create a new, more resilient urban forest for the next generation. So, if you have a chance to, plant a tree.

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