By Dan Buckler, Urban Forestry Assessment Outreach Specialist, 608-445-4578
It wasn’t by chance that Norway maple made its way across the pond to our shores. It was, in fact, humble correspondence that invited it here.
John Bartram, the American called the greatest botanist in the world by none other than Carl Linnaeus, sought out Norway maples in 1756 from a supplier in England. By 1762, seedlings were available for sale from Bartram’s nursery in Pennsylvania and another in New Jersey. George Washington had Norway maple planted on his property and, like many things in which the towering figure participated, precedent was set.
Over the next 150 years, Norway maple’s range expanded with alongside that of America’s. Here was a tree that could survive in disparate areas, tolerate urban stresses and grow quickly. And as its reputation for tolerance across many sites grew, so did its availability. But these weren’t days of conquest for the Norway maple; these were days of preparation.
When Dutch elm disease began its assault on streetscapes in 1928, Norway maple was primed to be American elm’s main replacement. The king is dead. Long live the king.
The tree was planted extensively across many American cities and had the unfortunate habit of becoming invasive due to its quick growth, plentiful seed source and creation of deep shade. Despite its invasive tendencies, however, note that Norway maple is not currently regulated under Wisconsin’s Invasive Species Rule, meaning there are no restrictions to buying, selling, planting or transporting this species.
Even as a species with invasive tendencies, the potentially more troublesome situation with Norway maple in Wisconsin is its overabundance in many of our communities, particularly along streets. This leaves these areas with poor species diversity and reduces the resilience of the urban forest to withstand future disturbances. A more diverse forest is a more resilient forest, as an assortment of families, genera and species means that no one pest or pathogen can lead to a catastrophic loss of trees (e.g. emerald ash borer).
It is, however, also important to note that species diversity often improves at different scales. Yes, Norway maple is overrepresented along streets, but if you look at urban land across all ownerships, Norway maple is not dominant. In preliminary data from the Urban Forest Inventory and Analysis project, Norway maple makes up about 3% of all tallied urban trees in the state. Compare that to the species representing 22% of trees in the Wisconsin Community Tree Map, mostly composed of street trees. This is far above the recommendations for appropriate diversity that was written for the DNR’s emerald ash borer toolbox.
Luckily, many communities are already diversifying their streets by careful planning and taking advantage of a diverse suite of trees available from Wisconsin nurseries.
We must continue to be thoughtful with our planting decisions so that we can build a more resilient urban forest. But we can still be thankful for the historic role of this all too American non-native species. For the immigrant Acer is ambitious, tough and adaptable. If a tree can symbolize the melting pot that is the American city, Norway maple might just be the best match.