Decades ago many of our city streets were planted with American elms. They were easily grown, tough, and well adapted to growing in urban environments. They were also beautiful and created wonderful canopies that arched over streets, and shaded our homes. Many elms were planted. Some streets were lined exclusively with American elms. Dutch elm disease (a fungal pathogen) arrived in the U.S. in 1928 and spread from east to west devastating our elm population. Many communities lost a vast majority of their tree cover. Relatively few American elms remain today.
Following the loss of the elms, other trees were planted as replacements. Unfortunately, we did not fully learn our lesson in the importance of diversity. A large number of ash trees were planted to replace the lost elms. Ash trees were relatively cheap, very adaptable, and fast-growing. Municipal foresters, landscape architects, and nurseryman promoted the use of ash, it was an easy solution.
We are now dealing with emerald ash borer, which will kill all unprotected ash. This will come at a great cost to communities and homeowners. Not only the investment of protecting selected high-value trees, but also the removal and replacement costs of trees that are unprotected. These costs will be staggering. We will also be losing the environmental and aesthetic value the trees provided. If we had only planted a more diverse tree selection in the past, our current challenge would be less daunting.
I shudder to think of what may happen if maples develop a specific pest problem. We have planted a great many maples. Yes, they are wonderful trees, well known and much loved by all. ‘Autumn Blaze’ maple is a good example of our over-indulgence in one tree. Again, adaptable, fast-growing (remember the story of the tortoise and the hare) trees with good fall color too! Norway maples, another overplanted maple, have the propensity to form stem girdling roots, shortening their lifespan. We have planted millions of these two maples across the country. This may come back to bite us. Fortunately, maples come in a great selection of different species and different varieties within species; we can still plant maples but use some constraint!
So what is the answer? Plant a variety of diverse trees on our streets, parks, and yards. Here is a partial list of diverse trees to consider improving biodiversity in your landscape. Yes, we could look at some of these trees and perhaps point out some perceived negative attributes. I suggest you go out on a limb and plant something different; the benefits of planting with tree diversity in mind will pay dividends down the road.
- Native Oaks – many diverse tree species
- Kentucky Coffeetree – a rare native tree
- Hackberry – adaptable
- Ginkgo – choice, pest-free
- Hickory – Shagbark, and Bitternut
- Lindens – many varieties
- Disease resistant elms – many selections now available
- Catalpa – a large tree with huge flowers
- Hybrid London Plane Tree – relatively new introduction
- Buckeyes and Horse chestnuts – colorful
- Katsura Tree – not well known – beautiful
- Turkish Filbert – tough once established
- A great number of small diverse trees too numerous to mention