Acer rubrum (Red Maple, also known as Swamp or Soft Maple), is one of the most common and widespread deciduous trees of eastern North America. It ranges from the Lake of the Woods on the border between Ontario and Minnesota, east to Newfoundland, south to near Miami, Florida, and southwest to east Texas. Many of its features, especially its leaves, are quite variable in form. At maturity it often attains a height of around 25 metres (80 ft). It is aptly named as its flowers, petioles, twigs and seeds are all red to varying degrees. Among these features, however, it is best known for its brilliant deep scarlet foliage in autumn.
Over most of its range, red maple is adaptable to a very wide range of site conditions, perhaps more so than any other tree in eastern North America. It can be found growing in swamps, on poor dry soils, and most anywhere in between. Elevation is also not a limiting factor in its range, as it grows well from sea level to about 900 m (3,000 ft). Due to its attractive fall foliage and pleasing form, it is often used as a shade tree for landscapes. It is used commercially on a small scale for maple syrup production as well as for its medium to high quality lumber. It is also the State Tree of Rhode Island.
The leaves of the red maple offer the easiest way to distinguish it from its relatives. As with all maples, they are deciduous and arranged oppositely on the twig. They are typically 5-10 cm (2-4 inches) long and wide with 3-5 palmate lobes with a serrated margin. The sinuses are typically narrow, but the leaves can exhibit considerable variation. When 5 lobes are present, the three at the terminal end are larger than the other two near the base. In contrast, the leaves of the related silver maple, A. saccharinum, are much more deeply lobed, more sharply toothed and characteristically have 5 lobes. The upper side of A. rubrum's leaf is light green and the underside is whitish and can be either glaucous or hairy. The leaf stalks are usually red and are up to 10 cm (4 inches) long. Furthermore, the leaves turn a brilliant red in autumn.
The twigs of the red maple are reddish in colour and somewhat shiny with small lenticels. Dwarf shoots are present on many branches. The buds are usually blunt and greenish to reddish in colour, generally with several loose scales. The lateral buds are slightly stalked, and in addition there may be collateral buds present as well. The buds form in fall and winter and are often visible from a distance due to their reddish tint. The leaf scars on the twig are V-shaped and contain 3 bundle scars.
The flowers are generally unisexual, with male and female flowers appearing in separate sessile clusters, though they are sometimes also bisexual. They appear in spring from April to May, usually coming before the leaves. The tree itself is considered Polygamodioecious, meaning some individuals are male, some female, and some monoecious. The flowers are red with 5 small petals and a 5-lobed calyx borne in hanging clusters, usually at the twig tips. They are lineal to oblong in shape and are pubescent. The pistillate flowers have one pistil formed from two fused carpels with a glabrous superior ovary and two long styles that protrude beyond the perianth. The staminate flowers contain between 4 and 12 stamens, often with 8.
The fruit is a 15 to 25 millimeter (.5 to .75 inch) long double samara with somewhat divergent wings at an angle of 50 to 60 degrees. They are borne on long slender stems and are variable in colour from light brown to reddish. They ripen from April through early June, before even the leaf development is altogether complete. After they reach maturity, the seeds are dispersed for a 1 to 2 week period from April through July.
The tree's range ends where the -40°C (-40°F) mean minimum isotherm begins, namely in southeastern Canada. On the other hand, the western range is limited by the much drier climate of the Great Plains. Nonetheless, it has the widest tolerance to climatic conditions of all the North American species of maple. As above, the absence of red maple in the Prairie Peninsula is probably not related to rainfall or other climatic conditions, since it grows healthily in other locations with comparable or even less annual precipitation.
A. rubrum does very well in a wide range of soil types, with varying textures, moisture, pH, and elevation, probably more so than any other forest tree in North America. It grows on glaciated as well as nonglaciated soils derived from the following rocks: granite, gneiss, schist, sandstone, shale, slate, conglomerate, quartzite, and limestone. Chlorosis can occur on very alkaline soils, though otherwise its pH tolerance is quite high. As concerns levels of moisture, the red maple grows everywhere from dry ridges and southwest facing slopes to peat bogs and swamps. It occurs commonly in rather extreme moisture conditions, both very wet and quite dry. While many types of tree prefer a south or north facing aspect, the red maple does not appear to have a preference. Its ideal conditions are in moderately well-drained, moist sites at low or intermediate elevations. However, it is nonetheless common in mountainous areas on relatively dry ridges, as well as on both the south and west sides of upper slopes. Furthermore, it is common in swampy areas, along the banks of slow moving streams, as well as on poorly drained flats and depressions. In northern Michigan and New England, the tree is found on the tops of ridges, sandy or rocky upland and otherwise dry soils, as well as in nearly pure stands on moist soils and the edges of swamps. In the far south of its range, it is almost exclusively associated with swamps.
Interestingly, it is thought that the pre-European forest of eastern North America contained far fewer red maples than at present. Most diversity surveys conducted in eastern forests prior to their large scale exploitation showed the red maple representing under 5% of all tree species and it was furthermore mostly confined to poorly drained areas. The density of the tree in many of these areas has increased 6 to 7 fold and this trend seems to be continuing. A series of disturbances to the oak and pine forests since European arrival, such as the suppression of forest fires and global warming, are most likely responsible for this phenomenon. Concern has been expressed, as the ongoing spread of the red maple is changing the nature of eastern forests by reducing the number of oaks and pines that would otherwise dominate.
A. rubrum is one of the first plants to flower in spring. A crop of seeds is generally produced every year with a bumper crop often occurring every second year. A single tree between 5 and 20 cm (2 and 8 inches) in diameter can produce between 12,000 and 91,000 seeds in a season. A tree 30 cm (1 ft) in diameter was shown to produce nearly a million seeds. Fertilization has also been shown to significantly increase the seed yield for up to two years after application. The seeds are epigeal and tend to germinate in early Summer soon after they are released, assuming a small amount of light, moisture, and sufficient temperatures are present. If the seeds are densely shaded, then germination commonly does not occur until the next Spring. Most seedlings do not survive in closed forest canopy situations. However, one to four year old seedling are common under dense canopy and though they eventually die if no light reaches them, they serve as a reservoir, waiting to fill any open area of the canopy above.
Red maple is able to increase its numbers significantly when associate trees are damaged by disease, cutting, or fire. One study found that 6 years after clearcutting a 3.4 hectare (8.5 acre) Oak-Hickory forest containing no red maples, the plot contained more than 2,200 red maple seedlings per hectare (900 per acre) taller than 1.4 m (4.5 ft). One of its associates, the black cherry (Prunus serotina), contains benzoic acid, which has been shown to be a potential allelopathic inhibitor of red maple growth. Red maple is one of the first species to start stem elongation. In one study, stem elongation was one-half completed in 1 week, after which growth slowed and was 90% completed within only 54 days. In good light and moisture conditions, the seedlings can grow 30 cm (1 ft) in their first year and up to 60 cm (2 ft) each year for the next few years making it a fast grower.
The red maple is a used as a food source by several forms of wildlife. Elk and white-tailed deer in particular use the current season's growth of red maple as an important source of winter food. Several Lepidoptera (butterflies and moths) utilize the leaves as food; see List of Lepidoptera that feed on maples.
Due to A. rubrum's very wide range, there is significant variation in hardiness, size, form, time of flushing, onset of dormancy, and other traits. Generally speaking, individuals from the north flush the earliest, have the most reddish Fall color, set their buds the earliest and take the least winter injury. Seedlings are tallest in the north-central and east-central part of the range. The fruits also vary geographically with northern individuals in areas with brief frost free periods producing fruits that are shorter and heavier than their southern counterparts. As a result of the variation there is much genetic potential for breeding programs with a goal of producing red maples for cultivation. This is especially useful for making urban cultivars that require resistance from verticillium wilt, air pollution, and drought.
Red Maple is a good choice of a tree for urban areas when there is ample room for its root system. The red maple is excellent at withstanding harsh urban conditions, including tolerance of both dry and wet soils, and a higher tolerance of pollution than sugar maple. Like several other maples, its low root system can be invasive and it makes a poor choice for plantings in narrow strips between a sidewalk and a street. It attracts squirrels, who eat its buds in the early spring, although squirrels prefer the larger buds of the silver maple.
In the lumber industry A. rubrum is considered a soft maple. The wood is close grained and as such it is similar to that of A. saccharum, but its texture is softer, less dense, and has a poorer figure and machining qualities. High grades of wood from the red maple can nonetheless be substituted for hard maple, particularly when it comes to making furniture. As a soft maple, the wood tends to shrink more during the drying process than with the hard maples.
Red maple is also used for the production of maple syrup, though the hard maples A. saccharum and A. nigrum, the black maple, are more commonly used. One study compared the sap and syrup from the Sugar Maple with those of the red maple, as well as those of the Silver Maple, boxelder (A. negundo), and Norway maple (A. platanoides), and all were found to be equal in sweetness, flavor, and quality. However, the buds of red maple and other soft maples emerge much earlier in the spring than the sugar maple, and after sprouting chemical makeup of the sap changes, imparting an undesirable flavor to the syrup. This being the case, red maple can only be tapped for syrup before the buds emerge, making the season very short.