Definitions

Acer negundo Californicum

Acer negundo

Acer negundo is a species of maple native to North America. Box Elder, Boxelder Maple, and Maple Ash are its most common names in the United States. Other variant names -- some of which are obviously regional - include Ash Maple, Ash-leaf Maple, Black Ash, California Boxelder, Cutleaf Maple, Cut-leaved Maple, Negundo Maple, Red River Maple, Stinking Ash, Sugar Ash, Three-leaved Maple, and Western Boxelder. In Canada it is known as Manitoba Maple. In Russia it is called American Maple (американский клён).

Common names

Indicative of its familiarity to many people over a large geographic range, A. negundo has numerous common names.

The names "Box Elder" and "Boxelder Maple" are based upon the similarity of its whitish wood to that of boxwood and the similarity of its pinnately compound leaves with those of some species of elder. This is the only North American maple with compound leaves.

Other common names are based upon this maple's similarity to ash, its preferred environment, its sugary sap, a description of its leaves, its binomial name, and so on.

These names include (but are not limited to) Ash-, Cut- or Three-leaf (or -leaved) Maple, Ash Maple, Sugar Ash, Negundo Maple, and River Maple.

Common names may also designate a particular subspecies. For example, a common name for A. negundo subsp. interius may be preceded by "Inland" (as in "Inland Boxelder Maple"). A common name for A. negundo subsp. californicum may be preceded by "California" or "Western".

Description

Acer negundo is a small, usually fast-growing and fairly short-lived tree that grows up to 10-25 m tall, with a trunk diameter of 30-50 cm, rarely up to 1 m diameter. It often has several trunks and can form impenetrable thickets.

The shoots are green, often with a whitish to pink or violet waxy coating when young. Branches are smooth, somewhat brittle, and tend to retain a fresh green colour rather than forming a bark of dead, protective tissue. The bark on its trunks is pale gray or light brown, deeply cleft into broad ridges, and scaly.

Unlike most other maples (which usually have simple, palmately lobed leaves), Acer negundo has pinnately compound leaves that usually have three to seven leaflets. Simple leaves are also occasionally present; technically, these are single-leaflet compound leaves. Although some other maples (such as A. griseum, Acer mandshuricum and the closely-related A. cissifolium) have trifoliate leaves, only A. negundo regularly displays more than three leaflets.

The leaflets are about 5-10 cm long and 3-7 cm wide with slightly serrate margins. Leafs have a translucent light green colour and turn yellow in the fall.

The flowers are small and appear in early spring on drooping racemes 10-20 cm long. The seeds are paired samaras, each seed slender, 1-2 cm long, with a 2-3 cm incurved wing; they drop in autumn or they may persist through winter. Seeds are usually both prolific and fertile.

Unlike most other maples, A. negundo is fully dioecious and both a "male" and "female" tree are needed for either to reproduce.

  • Winter buds: Terminal buds acute, an eighth of an inch long. Lateral buds obtuse. The inner scales enlarge when spring growth begins and often become an inch long before they fall.
  • Flowers: April, before the leaves, yellow green; staminate flowers in clusters on slender hairy pedicels one and a half to two inches long. Pistillate flowers in narrow drooping racemes.
  • Calyx: Yellow green; staminate flowers campanulate, five-lobed, hairy. Pistillate flowers smaller, five-parted; disk rudimentary.
  • Corolla: Wanting.
  • Stamens: Four to six, exserted; filaments slender, hairy; anthers linear, connective pointed.
  • Pistil: Ovary hairy, borne on disk, partly enclosed by calyx, two-celled, wing-margined. Styles separate at base into two stigmatic lobes.
  • Fruit: Maple keys, full size in early summer. Borne in drooping racemes, pedicels one to two inches long. Key an inch and a half to two inches long, nutlets diverging, wings straight or incurved. September. Seed half an inch long. Cotyledons, thin, narrow.

Subspecies

Acer negundo is often discussed as being comprised of three subspecies, each of which was originally described as a separate species. These are:

  • A. negundo subsp. negundo is the main variety and the type to which characteristics described in the article most universally apply. Its natural range is from the Atlantic Coast to the Rocky Mountains.
  • A. negundo subsp. interius has more leaf serration than the main species and a more matte leaf surface. As the name interus indicates, its natural range of Saskatchewan to New Mexico is sandwiched between that of the other two subspecies.
  • A. negundo subsp. californicum has larger leaves than the main species. Leaves also have a velvety texture which is essential to distinguish it from A. negundo subsp. negundo. It is found in parts of California and Arizona.

Some authors further subdivide subsp. negundo into a number of regional varieties but these intergrade and their maintenance as distinct taxa is disputed by many. Even the differences between recognized subspecies are probably a matter of gradient speciation

Finally, note that a few botanists treat Boxelder Maple as its own distinct genus (Negundo aceroides) but this is not widely accepted.

Range

As noted, varieties thrive across the United States and Canada. It may also be found as far south as Guatemala

Ecology

This species prefers bright sunlight. It often grows on flood plains and other disturbed areas with ample water supply, such as riparian habitats. Human influence has greatly favoured this species; it grows around houses and in hedges, as well as on disturbed ground and vacant lots.

Several birds and some squirrels feed on the seeds. The Evening Grosbeak uses them extensively. The Maple Bug (also known as the Boxelder Bug) lays its eggs on all maples, but prefers this species.

Cultivation

Although its weak wood, irregular form, and prolific seeding might make it seem like a poor choice for a landscape tree, A. negundo is one of the most common maples in cultivation and many interesting cultivars have been developed, including:

  • 'Auratum' - yellowish leaves with smooth undersides
  • 'Aureomarginatum' - creamy yellow leaf margins
  • 'Baron' - Hardier & seedless variety
  • 'Elegans' - distinctively convex leaves
  • 'Flamingo' - pink and white variegation (very popular)
  • 'Variegatum' - creamy white leaf margins
  • 'Violaceum' - younger shoots and branches have bluish colour

Although native to North America, it is considered an invasive species in some areas of that continent. It can quickly colonize both cultivated and uncultivated areas and it has become naturalized in eastern China. The range is therefore expanding both in North America and elsewhere. It can also be found in some of the cooler areas of the Australian Continent where it is listed as a pest invasive species.

Although its light, close-grained, and soft wood is considered undesirable for most uses, this tree has been considered as a commercial source of wood fibre, for use in fibreboard.

References

  • Maeglin, Robert R.; Lewis F. Ohmann (1973). "Boxelder (Acer negundo): A Review and Commentary". Bulletin of the Torrey Botanical Club 100 (6): 357–363.

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