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Norway maple (Acer platanoides), an invasive tree

Acer platanoides Norway maple Aceraceae (sometimes included in the family Sapindaceae)

  Norway maple is a canopy tree up to 65 feet tall native to Europe. The bark of mature trees is black, with narrow, longitudinal fissures and ridges in a fairly regular pattern. The bark of young trees is gray and smooth. The terminal winter buds are large, 0.6-0.9 cm (0.25-0.35 in.), wider than twig tip. They are green, to dark purple, and there are 3-4 pairs bud scales. The twig and leaf stalk sap is milky. These are some of the distinguishing characters that differentiate Norway maple from sugar maple and make identification of young trees easy.

The leaves are opposite, with a blade 8-18 cm (3.2-7.2 in.) wide, wider and darker than A. saccharum (sugar maple) leaves, but otherwise very similar. The autumn color is clear yellow, never red or orange, and the leaves turn color later than native Acer spp.

Norway maple painting by Zelimir Borzan

The flowers are bright yellow-green, radially symmetrical, 5 parted, with a prominent ring-shaped nectary (nectar producing organ) from which anthers arise. The inflorescences are of dense, rounded clusters, while those of sugar maple are pale green and dangling. Norway maple blooms in April, slightly earlier than A. saccharum.

The Fruit is of dry, thin, flat, winged seeds (samaras) joined end-to end. They are larger than those of sugar maple, each 3.5-5 cm (1.4-2 in.) long and the wings are widely divergent, and curved upward. The fruit ripens from Sept.-Oct., and is dispersed by wind. The seeds are probably eaten by some birds and small mammals as are those of other maples.

 Ecology:

            Wetland status: The USDA lists Acer platanoides as UPL (upland). This means that it is almost never found in wetlands or soil that is saturated with water during the growing season.

Habitat: Norway maple is used as a street, park and garden tree. Its large, wind-dispersed seeds easily escape from cultivation into upland woods and edges. It is able to invade intact forests due to the ability of seedlings to tolerant shade. It leafs out earlier than native forest plants and remains green longer into autumn, shading out native trees, shrubs and herbs for habitat. It reproduces freely and shades out all other plants but beech, sugar maple and Norway maple seedlings. The soil under dense stands of mature Norway maple is subject to erosion due to lack of ground cover. It prefers soil pH 5.2-7.2.

Notes: Norway maple is attacked by Verticillium wilt, Verticillium dahlia (Deuteromycotina) a soil fungus that travels in xylem (the vein-like tissue that brings water and nutrients upward from the roots) and causes progressive die off of branches. Anthracnose disease, appearing as dark, necrotic, spots or scorch-like appearance of leaves, caused by Kabatiella apocrypta (Ascomycotina). Didymosporina aceris (syn. Marsonina truncatula, Deuteromycotina) causes two-toned tan and brown spots on leaves. Norway maple is a primary host tree of the Asian longhorn beetle, Anoplophora glabripennis (see post for Acer negundo) and, as such, may help spread this very dangerous, alien insect.

Norway maple is a major pest species in and near urban areas. It is listed as an invasive alien by the Invasive Plant Atlas of New England, the Mid-Atlantic Exotic Pest Plant Council, Inc., and the USDA Forest Service, Northeastern Area, Invasive Plants, among others, but continues to be grown by many Nurseries. Reproductive adults can be killed by girdling or by heavy canopy pruning for a couple of years during the growing season. This also sharply decreases seed production. Hand pulling of seedlings or weed wrenching of saplings is also helpful, but removal of seedlings alone only encourages growth of more Norway maple seedlings. On the other hand, removal of canopy trees decreases Norway maple seedling populations.

 

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