Profile of the Invasive Norway Maple
by Benjamin Crouch

Walking along a number of wooded areas around Boston in late November, you may have noticed that all of the foliage had long since dropped from most trees, but many trees were just turning yellow and some were still quite green. If this struck you as peculiar, you might also notice that most of these trees are maples (with leaves similar to the one on the Canadian flag) and may have recognized them as a common street and yard tree, the Norway maple.

This anomalous tree, still green with leaves in the late fall, is native to Europe. Scientists consider the length of day to be the primary regulator of the timing of color change in fall. In the latitudes of its home range, fall days grow shorter at a faster rate than they do in New England. Boston receives about nine hours and forty-five minutes of daylight in late November, which is the same amount that Southern Scandinavia, the northern-most reach in the native range of the species, receives in late October. In combination with warmer New England fall temperatures, this is why Norway maples hold their leaves longer than the native trees. The Norway maple reacts similarly when days begin to lengthen in the spring, its buds breaking and leaves emerging earlier than most native trees. Since the tree has leaves throughout a longer portion of the year, it grows faster than kindred native species, such as the sugar maple. This is one of the characteristics that contribute to the species’ ability to invade native habitats.

It is easy to distinguish native maples from Norway maples, but consult with a field guide if you are unsure.

Overall, the Norway maple is a superior competitor against native northeastern hardwoods and is considered an exotic (not native) invasive species. The species has been favored as a street tree and is still planted widely in New England towns and cities. The characteristics that make it desirable as a street tree also make it an aggressive invader in our forests. Besides being fast growing, the tree is highly tolerant of compacted soils, a wide range of soil pH levels, extreme heat, and drought. Its supreme advantages, however, are tolerance of complete shade, lack of insect predators, and its special ability to prevent the reproduction of native trees.

Its ability to tolerate shade allows the Norway maple to seed itself in undisturbed woodland, in which trees have not been cut or otherwise knocked-down. For instance, Norway maples are creeping into the interior of the 90-acre Allendale woods, which has been growing undisturbed in West Roxbury since the late 19th century. Seeds from street and yard trees began sprouting up along the woodland’s edges decades ago. Evidence from other woodlands indicates that this may lead to their eventual dominance over the entire site. As the tree matures, it grows unhindered by the insects that prey upon its leaves, buds, trunk and twigs in its native range, which would otherwise inhibit growth. With little insect damage, it is less vulnerable to pathogens that would otherwise infect some of the trees and reduce its numbers.

Once the tree is established, through a process known as allelopathy it leaches substances called phytoxins into the soil that prevent native seeds from germinating around the tree. Since trees in New England have not evolved alongside the Norway maple, few possess any immunity to its toxins, unlike trees native to Europe. In this stage, the understory of the invaded woodland loses plant diversity and soils start to become bare. Later, when the Norway maple reaches the canopy stage of its life cycle, it can exclude almost all other plant species from growing in its shadow, eventually eliminating most or all ground cover (LINK see restoration “Before Planting” section of restoration photo series). After the mature native trees die off, Norway maple trees are all that remain, growing in a monospecific (single species) stand. Many of these stands exist in Boston’s designated urban wilds and other wooded areas.

At Sherrin Street Urban Wild in Hyde Park, a twenty-five acre natural area comprised of woods, meadow and wetlands, one Norway maple stand spans several acres. In late spring, It is striking how quiet and dark it is underneath the canopy, where the ground bears only a mat of decomposing leaves that barely conceal the exposed lateral roots of the maples. It is a striking contrast to the more diverse areas nearby where many species of birds are singing; light reaches the forest floor; and fragrant sassafrass, fruit-bearing low bush blueberry and native briers grow in abundance. Later in the season, there will be breeding pairs of Eastern Wood Pewees, American Robins and scads of Northern Flickers defining their territory with song and flight in the tall oak canopy adjacent to the Norway Maples, but rarely do their calls or flights spring from the darkened space beneath the boughs of the invasive stand. Also absent are butterflies, bees and other insects that depend on displaced flowering natives. Most wildlife have disappeared, except the occasional grey squirrel or white-tailed deer, both of which need a richer diet than the single course that a Norway maple stand offers.

Planting a Norway Maple in your yard, which is pest and disease resistant, fast growing, and tolerant to all sorts of climatic conditions, may sound like a great idea. However, consider that it will discharge thousands of seeds each year that would send Norway maple seedlings shooting up from your garden, through your bushes and up against the foundation of your house. Also consider that songbirds and other wildlife you might otherwise see would be far less prevalent or absent in the Norway maple. In addition, its phytoxins will make it difficult for you to seed grass or other plants that you may want to plant in your yard. Above all, it is important to recognize the severe consequences that your tree selection can have on nearby natural areas. There are many suitable natives that are fast growing and require minimal care including native oaks, red maple, and American linden.

EarthWorks devotes thousands of hours of staff and volunteer time per year to restore beneficial native species to natural landscapes invaded by Norway maples and other non-native plants. As a resident, a landowner or homeowner, you can help by planting natives and removing invasive species on your property or discouraging their planting in your neighborhood. If you would like more information about planting natives and identifying invasive plants or volunteering for urban wilds restoration, please call or email us.