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 (82 feet). It is aptly named as its flowers, petioles, twigs and seeds are all red to varying degrees. Among these features, however, it is most well 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 feet). 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.
Though A. rubrum is generally easy to identify, it is highly variable in morphological characteristics. It is a medium to large sized tree, reaching heights of 18 to 27 metres (60 to 90 feet) and exceptionally over 35 metres (115 feet). The diameter can range from 46 to 76 cm (18 to 30 inches), depending on the growing conditions. In forests, the bark will remain free of branches until some distance up the tree. Individuals grown in the open are shorter and thicker with a more rounded crown. Generally speaking, however, the crown is irregularly ovoid with ascending whip-like curved shoots. The bark is a pale grey and smooth when the individual is young. As the tree grows the bark becomes darker and cracks into slightly raised long plates. The largest known living red maple is located near Armada, Michigan, at a height of 38.1 metres (125 feet) and a bole circumference, at breast height, of 4.95 metres (16.25 feet).
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 coulour, 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 polygamo-diecious, 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 milimeter (.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.
Red maple seldom lives longer than 150 years, making it short to medium lived. It reaches maturity in 70 to 80 years. Its ability to thrive in a large number of habitats is largely due to its ability to produce roots to suit its site from a young age. In wet locations, red maple seedlings produce short taproots with long and developed lateral roots, while on dry sites, they develop long taproots with significantly shorter laterals. The roots are primarily horizontal, however, forming in the upper 25 cm (10 in) of the ground. Mature trees have woody roots up to 25 meters (80 ft) long. They are very tolerant of flooding, with one study showing that 60 days of flooding caused no leaf damage. At the same time, they are tolerant of drought due to their ability to stop growing under dry conditions by then producing a second growth flush when conditions later improve, even if growth has stopped for 2 weeks.
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 foot) 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 feet). 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 w:List of Lepidoptera which 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 frequently hybridizes with Silver Maple; the hybrid, known as Freeman's Maple Acer x freemanii, is intermediate between the parents.
Numerous cultivars have been selected, often for intensity of fall color, with 'October Glory' and 'Red Sunset' among the most popular. Toward its southern limit, 'Fireburst', 'Florida Flame', and 'Gulf Ember' are preferred. Many cultivars of Freeman's Maple are also grown widely. Below is a partial list of cultivars:
- 'Armstrong' - Columnar to fastigate in shape with silvery bark and modest orange to red fall foliage
- 'Autumn Blaze' - Rounded oval form with leaves that resemble the silver maple. The fall colour is orange red and persists longer than usual
- 'Autumn Flame' - A fast grower with exceptional bright red fall color developing early. The leaves are also smaller than the species.
- 'Autumn Radiance' - Dense oval crown with an orange-red fall colour
- 'Autumn Spire' - Broad columnar crown; red fall colour; very hardy
- 'Bowhall' - Conical to upright in form with a yellow-red fall colour
- 'Burgundy Bell' - Compact rounded uniform shape with long lasting, burgundy fall leaves
- 'Columnare' - An old cultivar growing to 20 metres (70 feet) with a narrow columnar to pyramidal form with dark green leaves turning orange and deep red in fall
- 'Gerling' - A compact, slow growing selection, this individual only reaches 10 metres (30 feet) and has orange-red fall foliage
- 'Northwood' - Branches are at a 45 degree angle to the trunk, forming a rounded oval crown. Though the foliage is deep green in summer, its orange-red fall colour is not as impressive as other cultivars.
- 'October Brilliance' - This selection is slow to leaf in spring, but has a tight crown and deep red fall colour
- 'October Glory' - Has a rounded oval crown with late developing intense red fall foliage. Along with 'Red Sunset', it is the most popular selection due to the dependable fall colour and vigorous growth.
- 'Red Sunset' - The other very popular choice, this selection does well in heat due to its drought tolerance and has an upright habit. It has very attractive orange-red fall colour and is also a rapid and vigorous grower.
- 'Scarlet Sentinel' - A columnar to oval selection with 5-lobed leaves resembling the silver maple. The fall colour is yellow-orange to orange-red and the tree is a fast grower.
- 'Schlesingeri' - A tree with a broad crown and early, long lasting fall colour that a deep red to reddish purple. Growth is also quite rapid.
- 'Shade King' - This fast growing cultivar has an upright-oval form with deep green summer leaves that turn red to orange in fall.
- 'V.J. Drake' - This selection is notable due to the fact that the edges of the leaves first turn a deep red before the colour progresses into the center.
Red Maple is widely grown as an ornamental tree in parks and large gardens, except where soils are too alkaline or salty. In parts of the Pacific Northwest, it is one of the most common introduced trees. Its popularity in cultivation stems from its vigorous habit, its attractive and early red flowers, and most importantly, its flaming red fall foliage. The tree was introduced into the United Kingdom in 1656 and shortly thereafter entered cultivation. There it is frequently found in many parks and gardens, as well as occasionally in churchyards.
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.
The leaves of red maple, especially when dead or wilted, are highly toxic to horses. The toxin is currently unknown, but it is believed to be an oxidant due to its destruction of red blood cells and thus causing the blood to be unable to properly transport oxygen, a condition otherwise known as acute hemolysis. The ingestion of 700 grams (1.5 pounds) of leaves is considered toxic and 1.4 kilograms (3 pounds) is lethal. Symptoms occur within one or two days after ingestion and can include depression, lethargy, increased rate and depth of breathing, increased heart rate, jaundice, dark brown urine, coma, and death. Treatment is limited and can include the use of methylene blue or mineral oil and activated carbon in order to stop further absorption of the toxin into the stomach. About 50% to 75% of affected horses die or are euthanized as a result. 
Pests and Diseases
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