Sugar Maple
(hard maple, rock maple)

Sugar maple (Acer saccharum) is the most abundant of the seven maple species found in New York State, and is common throughout New England, the Lake States, Mid-Atlantic states, and several Canadian provinces.  Its historical and economical importance, both in the production of maple syrup and as a timber species, has earned sugar maple its status as the official state tree of New York.  The sugar maple leaf on the Canadian flag is evidence of this species' importance in Canada.

Description
Under optimal growing conditions, sugar maple can attain heights in excess of 100 feet. The largest reported individual was found near Bethany, West Virginia; it had a diameter of 5.6 feet, a crown spread of 75 feet, and a height of 110 feet. Most mature trees, however, range from 70 to 90 feet in height and have diameters at breast height (4.5 feet above the ground) commonly measuring from 2 to 3 feet. Trees grown in the open have trunks that branch near the ground, forming crowns that spread 60 to 80 feet. In contrast, those found in shaded forest conditions normally develop clear, straight boles and narrow crowns.

The leaves of sugar maple are simple (single) and like the buds are in an opposite arrangement on the twigs (Figure 1). They are usually five-lobed although certain trees may possess leaves with three, four, or five lobes. Leaves are dark green on the top surface and paler underneath. They are generally smooth on both sides, although the veins underneath may be slightly hairy. Leaves typically measure from 3 to 5 inches long. The margins between lobes are shallow and smooth, which distinguishes them from leaves of the similar-looking red maple (Acer rubrum), which has serrated lobe margins.   Another difference in the leaves is the "U-shaped" connections between lobes of sugar maple leaves versus the "V-shaped" connections of red maple.

Twigs are fairly slender, a shiny reddish-brown color, and covered with lenticels (small openings in the bark, Figure 1d). The pith (inside core) of the twig is white. Buds are narrow, sharply pointed, and brown. The terminal bud may be larger (0.25 to 0.5 inch) than the lateral buds (0.10 to 0.25 inch).

The bark on young trees is dark gray, but as the tree ages the bark develops rough vertical grooves and ridges (fissures) and may appear dark brown.  On mature trees, the bark typically appears to have long plates that peel along the side edge.  The bark on mature red maple trees typically has more narrow plates that peel from the top or bottom edge.

The flowers of sugar maple are greenish yellow with long stalks (pedicels) appearing in drooping clusters 1 to 2.5 inches long. Sugar maple is monoecious; that is, it has female (staminate) and male (pistillate) flowers on the same plant (Figure 1b and Figure lc). The fruit, a double samara, has a characteristic winged shape (Figure le).

Distribution and Habitat
Sugar maple is one of 148 maple species found in the Northern Hemisphere, which includes about 90 native and introduced species in the United States. The range of sugar maple in North America extends from Nova Scotia and Quebec at its northern edge, west to Ontario, southeastern Manitoba, and western Minnesota, south to southern Missouri, and east to Tennessee and northern Georgia (Figure 2). Sugar maple is most common in New England and the Great Lakes states as well as Ohio, Pennsylvania, and New York.

Figure 2. Natural range of sugar maple

Elevational limits vary throughout sugar maple's extensive range. In northern New York and New England sugar maple occurs at elevations up to 2,500 feet. In the Great Lakes states, 1,600 feet is usually the upper limit. However, in the southern portions of its range, where the climate is typically warmer, sugar maple has a lower elevational limit of 3,000 feet and an upper limit of 5,500.

Naturally occurring combinations of different tree species form associations called cover types. Sugar maple is part of six forest cover types within its range. These are listed in Table 1.

Climate
Average temperatures within the geographic range of sugar maple have average January temperatures from 0 to 50° F and average July temperatures from 60 to 80° F. Maximum temperatures in the summer months range from 90 to 100° F, while winter minima vary from -40 to +20° F. Annual precipitation throughout the geographical range averages 20 to 50 inches of rain, plus from 1 to 150 inches of snow. In unusually wet years in the southern reaches of this broad range total annual rain in excess of 80 inches has been recorded.

The first killing frost usually occurs between September 1 and November 10 and the last from March 20 to June 15, depending on latitude and elevation. Thus average growing seasons are from 80 to 260 days.

Soils
Sugar maple can survive in a wide variety of soil types, but for maximum tree growth and sap production, soils should be deep, moist, and well drained with medium or fine textures. In the Northeast, such soils are common along glacial tills and benches. Areas generally not favorable to sugar maple establishment include swamps, dry sandy ridges, and thin rocky soils. The pH of soils supporting sugar maple ranges from 3.7 (strongly acidic) to 7.3 (slightly alkaline), but the species is most commonly found on soils with a pH of 5.5 to 7.3.

Forests dominated by sugar maple often produce a heavy leaf litter.  Through the nutrients present in sugar maple leaves, the decaying litter can gradually have an influence on soil pH and nutrient status.  Sugar maple leaves contain about 1.8% calcium, 0.8% potassium, 0.1% phosphorus, and 0.7% nitrogen based on their dry weight.

Over the full range of soils where sugar maple occurs, it may be expected to reach a height at age 50 of between 40 and 80 feet.  This relationship between height and a standard base age is known by foresters as site index.  However, site index has limits in utility for shade tolerant species such as sugar maple that can remain suppressed as juveniles in the understory.  In these cases, site index values would underestimate the potential productivity of the site.

Life History
The leaves of sugar maple are usually fully expanded three to four weeks after the leaf buds begin to swell in the spring. The flowers emerge soon after the leaves and are in full bloom within a week. Normally, a single tree will produce both perfect (those containing both male and female parts) and imperfect (those containing either male or female parts) flowers. The ratio of male to female flowers is rarely less than ten to one and is usually about fifty to one. There is a noticeable localization of female flowers on certain areas of a tree crown.

The flowers are pollinated by bees. Fruits that result from flower pollination usually mature in about 10 to 12 weeks and become ripe in September or October. The double samara fruit is characteristic of sugar maple, but usually only one seed is viable. The samaras fall about two weeks after ripening, which is approximately the same time the tree suspends its annual height growth. Birds, squirrels, and other rodents usually consume only a negligible amount of the seeds.

The minimum seed-bearing age for sugar maple is about thirty years. After this age some seed is produced every year, but massive quantities of viable seed are produced cyclically, usually at two to five-year intervals depending on climatic conditions. During these good seed years, trees are loaded with flowers, which gives them a yellowish cast when seen from a distance. The seeds are relatively heavy (average clean weight of 6,100 seeds per pound), but the winged shape of the samaras provides for wide dispersal by wind and blowing over snow.

The large quantities of seed produced in good years normally result in many seedlings. Seeds usually germinate in the spring following their autumn dispersal. In addition, sugar maple has a strong tendency to sprout in response to fire, cutting, disease, or physiological disorders. Stump sprouts are an important means of vegetative (asexual) reproduction in many hardwood forest stands.  For example, the Ice Storm of January 1998 broke crowns from many sugar maple trees throughout the northeastern United States and Canada.  Some of these sugar maples will be able to resprout and ultimately regain their original vigor, but others will have been so badly damaged they will ultimately die.

Reproduction or regeneration of a forest should be a major concern for any woodland owner who is planning to harvest trees. In the case of sugar maple, natural regeneration through seed establishment and prolific sprouting is generally successful in replenishing the amount of growing stock in a stand even after a fairly heavy cutting. Eastern forests can usually be regenerated naturally, without the need to plant seedlings.  However, this involves careful planning, consideration of factors such as interfering vegetation and deer browsing, and is best accomplished in consultation with a professional forester.  In New York, contact professional foresters through the NYS Department of Environmental Conservation, the New York Institute of Consulting Foresters, or Certified Foresters in the Society of American Foresters.

Once the seedlings are established, frequent browsing by white-tailed deer (Odocoileus virginianus) can be a problem. Sugar maple seedlings can survive heavy browsing for many years, but they will most likely become stunted and deformed. Such selective removal through browsing by deer and other wildlife can cause significant changes in the composition of hardwood forests.

The annual initiation of height and radial growth in sugar maple usually corresponds with leaf emergence. Depending upon location and local weather patterns, height growth is usually completed in about 15 weeks; radial growth normally ceases in 14 to 17 weeks following initiation.

The root system of sugar maple in good soils is deep and branching. Natural root grafts are common. The seasonal periodicity of root growth is independent of aerial growth and often continues into the winter months if the soil remains unfrozen. Over half of the root growth for any given year, however, occurs during the same period as height and radial growth.

The growth rate and crown form of sugar maple are largely dependent on the origin of the saplings, as well as environmental factors. Saplings originating from sprouts tend to grow faster than those from seeds or even planted seedlings. The latter two, however, usually produce a healthier and better-formed mature tree. Sprouts from stumps with diameters of less than 2 inches are less likely to develop decay than those from larger stumps or multiple sprouts.

Sugar maple trees average about 1 foot of height growth and 0.2 inch of diameter growth annually for the first 30 to 40 years. Hence a 30-year-old tree might be 6 to 8 inches in diameter and 30 to 35 feet in height. After about 140 to 150 years, height growth ceases and radial growth slows greatly. Although rare, old-growth sugar maple stands can average 300 to 400 years in age; individual trees range from 70 to 110 feet tall with diameters at breast height of 20 to 36 inches.

Common Diseases, Insects, and Injuries
All trees, whether in a forest or urban setting, are constantly exposed to many damaging agents during their lifetime. Various diseases, numerous crawling and flying insects, and physical injury can all cause the decline of sugar maple.

Diseases
Sugar maple is susceptible to various disease organisms at all stages of development. Rhixoctonia solani and Sclerotium bataticola attack nursery seedlings; the latter damages the pith. As the trees age, they are subject to various foliage and stem diseases. Some of the more common foliage diseases include anthracnose (Gloeosporium apocryptum), which forms necrotic (dead) areas on the leaves. These are sometimes circular in shape and may be of various sizes; they turn brown, purple, or black as the disease progresses. Tar spot (caused by Rhytisma acerinum) is the name commonly given to a foliage disturbance in sugar maple that starts with small black dots on the leaves, which eventually combine to form large, black, thickened areas.

Most stem diseases exhibit cankers or galls. Cankers are sunken portions of the trunk or woody branches that often have a callus-like buildup of bark around the edges. Two common cankers of sugar maple are caused by the fungi Nectria and Eutypella.   Galls are obvious out growths on the main trunk of a tree, usually within 8 feet of the ground.

Vascular (stemwood) diseases that can injure sugar maple are usually caused by fungi entering the stem through wounds in the trunk or branches. Most vascular diseases are not fatal, but they can produce toxic substances that cause decay. For example, trunk rots are caused by fungi that enter the tree through frost cracks, dead branches, and stubs and less frequently through the roots.

Sugar maple is relatively free from diseases of the root system, although they sometimes attack otherwise weakened or diseased trees. For example, verticillium wilt (V. alboatrum) is a common killer of shade trees, but it is less damaging to sugar maple than to Norway maple (Acer platanoides). This disease infects the roots and causes green streaks in the wood. Stains are another broad group of diseases affecting both hardwood (broadleaf) and softwood (conifer) trees. They discolor the wood, reducing its commercial value.

Insects
Insect predation of sugar maple is usually not a serious problem, although in some years under certain conditions insects can initiate significant growth decline or mortality. In periodic cases of unusually large insect population buildups, such as with gypsy moth (Lymantria dispar) in the Northeast, however, insect damage may be serious. Other insect species that commonly attack sugar maple include the fall webworm (Hyphantria cunea), saddled prominent (Heterocampa guttivitta), sugar maple borer (Synanthedon acerni) pear thrips (Taeniothrips inconsequens), forest tent caterpillar (Malacosoma disstria), maple trumpet skeletonizer (Epinotia aceriella), maple leaf cutter (Paraclemensia acerifoliella), and maple petiole borer (Caulocampus acericaulis). As implied by the names of these pests, most insect-related problems in sugar maple involve the leaves.

Other Problems
Sugar maple is very sensitive to high sodium levels in the soil. Hence it can sustain major injury from salt added to road surfaces to melt winter ice and snow. The use of salt in the winter is a common practice throughout much of New York State and New England. Affected trees are found primarily along roads, although in extreme cases water drainage from roads into maple stands has caused extensive damage. Salt enters the roots and accumulates throughout the tree. When soil moisture levels decrease, the high salt concentration in the tissues causes the death of many twigs. Therefore, salt damage becomes obvious following long dry periods during the summer.

For more information on Cornell Extension research on sugar maple diseases, click here.

Management
The successful management of any forest must begin with specific goals. Stands of sugar maple can be managed to produce sap for syrup and wood for a variety of products. Both of these alternatives can be beneficial to wildlife by providing cover, browse, and nesting sites.

A sugar maple stand managed for the production of maple sap is commonly referred to as a sugar bush or sugar grove. The ideal tree in such a stand has special genetically and environmentally controlled characteristics that provide for a large amount of sweet sap to be produced annually; it can then be gathered and evaporated efficiently. Most important of these characteristics is a large crown in which many leaves are exposed to direct sunlight. Sap flow is also enhanced by large stem diameters, which develop from wide, deep crowns. Hence open-grown trees with wide crowns favor high sap production rates. Therefore, in a closed stand heavy thinning is normally recommended to simulate an open-grown environment.  Before completing abrupt changes in your forest through timber or firewood harvests, it's best to consult with a professional forester.

Growing trees for high-quality timber requires a different type of stand than that desired for maximum syrup production. A good sawtimber stand has trees with tall, straight stems and no branching below the growing crown. Historically, forest management in hardwood forests in the Northeast has favored uneven-aged stands and natural regeneration. As markets for fuelwood and other uses for pole-size (4 to 12 inches in diameter at breast height) trees develop, thinning of young stands becomes increasingly attractive to the forest landowner. Thinning a stand, practiced at various stages of development, involves removing poorly formed trees, as well as dead and diseased trees, to promote the growth of healthy, well formed members of the stand. Hence the goal for dense sugar maple stands is to remove as many inferior trees as possible without promoting open-grown characteristics in the remaining trees.

Sugar maple is a very prolific seed producer, and most stands regenerate naturally. Nursery seedlings are sometimes planted to establish a stand for future maple syrup production or to establish individual trees for ornamental use. Normal transplanting procedures should be followed, and water, fertilization, and weed control should be provided as needed. Transplanting seedlings from the wild, however, is much more involved because it requires locating good seedlings (open-grown trees 4 to 7 feet in height) and takes about a year of preparation time (trenching is required 9 to 12 months before transplanting to develop an adequate root ball). In any case, the root ball should be planted intact slightly deeper than in its original location. Extra watering and fertilizing may also be necessary. For optimal production, trees should be spaced at 35-foot intervals in rows placed 35 feet apart.

This site also contains information about management and production for experienced syrup producers, beginners or hobbyists, and the general public.

 Products and Uses
Products derived from sugar maple trees are common in house holds throughout the country. The maple syrup and sugar industry is an important part of many agricultural economies in the Northeast. The earliest written accounts of maple sugaring were made in the early 1600s by European explorers who observed American Indians gathering maple sap. Today, sugar maple stands and roadside trees provide private landowners with an annual cash crop as well as a rewarding hobby (see the Cornell Maple Tour).

Sugar maple has long been valued as a hardwood timber species because of the wood's hardness and resistance to shock. In early America, the wood was used for a variety of household items, including rolling pins, scoops, apple grinders, and cheese presses. Today its uses include lumber for general construction, flooring, furniture, cabinet work, and woodenware. The high density of sugar maple wood makes it a popular fuel for home heating.

Sugar maple is a popular ornamental tree because of its tolerance to shade, spreading form, and brilliant autumn foliage. Of the large number of maples available for residential plantings, however, sugar maple is chosen less often than Norway maple, red maple, and silver maple (Acer saccharinum). Nonetheless, sugar maple has been widely planted in the eastern United States, especially as a roadside tree.

Taken from: Luzadis, V.A. and E.R. Gossett. 1996. Sugar Maple. Pages 157-166. Forest Trees of the Northeast, edited by James P. Lassoie, Valerie A. Luzadis, and Deborah W. Grover. Cooperative Extension Bulletin 235. Cornell Media Services.

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