Acer is a genus of trees or shrubs commonly known as maple. There are approximately 128 species, most of which are native to Asia,with a number also appearing in Europe, northern Africa, and North America. Only one species, Acer laurinum, extends to the Southern Hemisphere. The type species of the genus is the sycamore maple, Acer pseudoplatanus, the most common maple species in Europe.
Most maples are trees growing to 10–45 m (33–148 ft) height. Others are shrubs less than 10 meters tall with a number of small trunksoriginating at ground level. Most species are deciduous, and many are renowned for their autumn leaf colour, but a few in southern Asia and the Mediterranean region are evergreen. Most are shade-tolerant when young and are often riparian, understory, or pioneer species rather than climax overstory trees with a few exceptions such as Sugar Maple. Many of the root systems are typically dense and fibrous, inhibiting the growth of other vegetation underneath them. A few species, notably Acer cappadocicum, frequently produce root sprouts, which can develop into clonal colonies.
Maples are distinguished by opposite leaf arrangement. The leaves in most species are palmate veined and lobed, with 3 to 9 (rarely to 13) veins each leading to a lobe, one of which is central or apical. A small number of species differ in having palmate compound, pinnate compound, pinnate veined or unlobed leaves. Several species, including Acer griseum (Paperbark maple), Acer mandshuricum (Manchurian maple), Acer maximowiczianum (Nikko maple) and Acer triflorum (Three-flowered maple), have trifoliate leaves. One species, Acer negundo (Box-elder), has pinnately compound leaves that may be simply trifoliate or may have five, seven, or rarely nine leaflets. A few, such as Acer laevigatum (Nepal maple) and Acer carpinifolium (Hornbeam maple), have pinnately veined simple leaves.
Some species of maple are extensively planted as ornamental trees by homeowners, businesses and municipalities due to their fall colour, relatively fast growth, ease of transplanting, and lack of hard seeds that would pose a problem for mowing lawns. Particularly popular are Norway Maple (although it is considered invasive in North America), Silver Maple, Japanese Maple, and Red Maple. Other maples, especially smaller or more unusual species, are popular as specimen trees.