Norway Maple (Acer platanoides) in indigenous not only to its namesake country but to much of Eastern Europe and down to the Caucasus, it was introduced to North America in 1756 were it became a popular street tree in North America because it is a fast growing, hardy species that can withstand city pollution. In Toronto Norway Maple was widely planted to replace the elm trees that were decimated by the spread of Dutch Elm Disease in the the 60s (80% of elms in Toronto were lost). While the Dutch Elm Disease highlighted the dangers of planting street tree monocultures and the devastating effects of invasive pests, Norway Maple is now being recognized as a highly invasive species that out competes native tree species and decreases the biodiversity of our urban forests.
Norway Maples are shade tolerant, they grow very fast and they produce large seed crops; these traits have allowed them to escape the confines of the streets and make their way into Toronto’s parks and ravines. They also leaf out earlier and their leave stay on later in the fall, suck up water leaving behind dry soil and have a very shallow root system whcih can increase soil erosion. We know Norway Maple are in our ravines because a study was commissioned in 1977 to gather ecological data in four ravines and they found that Norway Maple accounted for 10% of the canopy cover; last year a member of our team began to resurvey those ravines and found that Norway Maple now accounted for 40% of the canopy cover. A 30% increase in less then 40 years is a major ecological shift as it comes at the expense of native species like oaks, as and beech, which decreases biodiversity in the ravine systems. The very traits that made it a good street have allowed Norway Maples to spread into one of our most important and fragile ecosystems; we must be vigilante in monitoring and protecting our ravines.