Tree diversity creates resilient landscapes

Typically in October, as you look west coming into Ellsworth on Main Street, you see the banks of the Union River turned muddy yellow with the fall foliage of Norway maples, a thriving population of non-native invasive trees, escapees from local yards. The scene always makes me feel defeated.

Not this year. Infected with a fungus disease called tar spot, the leaves of these trees went straight to brown before summer’s end. The river banks look desolate in a different way.

Unfortunately, tar spot will not kill the Norway maples. They will leaf out again next spring; produce more seeds to spread the invasion up and down the river.

Driving down to the river on Friday afternoons, I wonder how the riverbanks might have looked in autumns before the invasion. Perhaps I would have been greeted by the crimson red and bright yellow of red maples along the water’s edge and, higher up, the golden yellow, orange and scarlet of sugar maples, or the deep red and yellow brown of red oak.

It is a lesson we find hard to learn: too much of a good thing. We fell in love with Norway maples, particularly the cultivars with dark red summer leaves, long before we recognized their invasive nature. Fast-growing, easy to propagate and disease-free, what else do you need in a street or yard tree? We planted them across the country by the millions.

We did the same thing with American elms in the first half of the last century, lining them along every city street, setting the table for Dutch elm disease. And we did not learn our lesson when thousands of Bradford pears along city streets and in suburban yards fell apart in wind and ice storms because of poor branching structure.

My advice to readers who want to plant a tree: Take stock of what your neighbors are growing and plant something else. Help create diversity by planting a native species that is poorly represented in the neighborhood and suited to local conditions.

For example, instead of another Norway maple, plant a white ash, Fraxinus americana, a shade tree appreciated for the subtlety of its autumn display. The leaf colors are muted and develop slowly, reddish purple at the branch tips grading to yellow toward the center of the tree. ‘Autumn Purple’ is a popular cultivar with leaves that turn completely red to reddish purple in the fall.

Fine specimens of ‘Autumn Purple’ can be found growing on the University of Maine campus in Orono. These trees, surrounded by sidewalks and walking paths, are a testament to the soil compaction tolerance of the species. White ash is known to be drought- and salt-tolerant as well.

Last Sunday I spent two hours walking residential streets in a Bar Harbor neighborhood, visiting the trees on a blue-sky autumn morning. I was impressed with both the diversity of trees and the relative absence of Norway maples.

In one large front yard grew a white pine at the peak of fall color, a mix of fresh green needles at the branch tips and soft brown needles farther back along the branch. A crabapple bearing hundreds of small, bright-red apples, perhaps a ‘Donald Wyman,’ grew in a small front yard, while an old white ash shaded the backyard, extending its branches clad in purple and yellow leaves above the roof. Other white ash trees, along with bright yellow-leafed green ash, red maples, horse chestnuts and red oaks, shaded the sidewalks.

Communities across the country are learning the lesson. Tree diversity creates resilient – and beautiful – landscapes.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@shead.org. Include name, address and telephone number.