Editor's Note: The following relates one example of how to deal with the introduction of an exotic species.
Norway Maple: Reassessing the threat to natural areas
Norway Maple (Acer platanoides) has been recognized as an occasional invasive exotic in various parts of eastern North America. Toronto is one area where concern has been expressed for this species. It was not until Steve Smith and I had an occasion to examine several sites this fall in early November, after the leaves of the native Sugar Maple (A. saccharum) had fallen, but those of Norway Maple were still retained. As a result it was all too apparent just what a threat this species presents to forested ravines and other natural remnants of the maple-beech and oak forests of Toronto.
A stunning, and horrifying image was seen in many of Toronto's forests, of a partial to continuous understory with bright yellow foliage. On repeat checking, with the hope of finding Sugar Maples with retained leaves, we were disappointed to see only the ovoid buds of Norway, or occasionally Sycamore Maple (S. pseudoplatanus). Often the seed trees could be clearly seen planted along a road in parks, next to a forest remnant, or at the top of a ravine forest in back yards. The seed sources were well within range of the receptive seed beds provided by the shaded forests.
Why should we be concerned about another tree species in our native forests? Norway Maples, as all home gardeners know who try to grow not only makes it difficult for native tree species to establish under the shade of the cohort of developing saplings in the understory, but it also shades out the normal diverse ground flora of woodland wildflowers and sedges. These species are an important component of the biodiversity of the forest and they also provide a very important function by reducing soil erosion. Thus, one can project to the future where the developing understory of Norway Maple will prevent the regeneration of the native forest, in time it will become pre dominantly an exotic forest, and the deep soils typical of southern Ontario forests will be degraded through erosion.
What should we do? This species presents a real dilemma, because it is one of the most popular species for street and garden planting. To suggest a complete ban would cause a great economic impact on the landscape industry. However, there are several current uses that should especially be discontinued: planting in parks which are near natural forests, near ravines, rural areas including roadsides and highway verges, conservation areas, and naturalization areas. Public agencies, park management, and landscape designers should take a more serious look at our native maples, ashes and oaks for framework planting on streets, in parks, and gardens, and communicate this changing need to the landscape industry. Exotics still have a place, but as specimens planted well within the confines of the garden or city square!
What to do if you suspect a Natural Area is being invaded by Exotic Trees:
It is not easy to contemplate removal of trees as part of sensitive environmental management, however the fact is that invasive exotics are causing significant environmental disruption worldwide, resulting in the loss of biodiversity and ecological functioning to natural ecosystems.
First, a survey of the site should be conducted identifying remaining native species of trees and shrubs, and invading exotics such as Norway Maple, European Buckhorn, Tatarian and Amur Honeysuckle. Where there are only a few exotics saplings mixed with native species, these invaders could likely be removed without causing any disruption. The problem arises when the forest canopy is mostly exotic, since removing all of them at once could cause serious problems, including opening the site to further invasions of exotics and soil erosion. Removals around any existing native trees would be a start, as well as removing all exotic seedlings, and saplings where larger trees form the canopy. Where large exotic trees exist, removing the lower limbs to allow light to the forest floor is an initial step to consider where complete removal would open up the forest canopy too severely. If native tree seedlings are not naturally establishing, then planting is called for. As the native forest grows, further thinning of the exotics can take place, until they and the seed source is eventually eliminated. Through this process, a return to a native forest association should occur without opening the forest canopy. Since the ground flora may recover under the lighter shade than Norway Maple provides, it is important that it not be further impacted by invading plants of open conditions that would occur if the canopy were opened too quickly.