Tree Profiles: Sugar Maple (Acer saccharum), Part 1
This is the first half of a two part series on sugar maple. The following examines the tree’s distribution and habitat, developmental traits, longevity, height, bark, leaves, flowers, fruit, seed production, and root development.
Sugar maple, also called hard maple or rock maple, is one of the largest and most recognizable hardwoods in North America. It is most notable for its hardiness, vibrant fall foliage, and for being the primary source of maple syrup.
Distribution & Habitat
Sugar maple is generally restricted to regions with cool, moist climates. Its northern limit extends from southeast Manitoba, through Central Ontario to southern Quebec, and across all of New Brunswick and Nova Scotia. Within the United States, sugar maple is found throughout New England, New York, Pennsylvania, and the middle Atlantic states, extending southwest through central New Jersey to the Appalachian mountains, then south through North Carolina and Tennessee. The western limit ranges from Missouri to Kansas, eastern Iowa, and eastern Minnesota. Outliers have been discovered in Georgia, and the Carolinas.
Sugar maple has a round to ovate growth habit. Height and radial growth begins around budbreak. Each year, growth is completed within fifteen weeks. Radial growth concludes in fourteen to seventeen weeks, depending on the season, and the geographical location. Sugar maples may experience height increases of one to two feet per growing season. Growth of young sugar maples is slow. As sugar maples mature, their growth increases. Sugar maple has a high shade tolerance, which is exceeded by few other hardwoods.
Mature trees and stands of sugar maple can live for over four hundred years. Select varieties of sugar maple may only live for up to one hundred years.
Sugar maple typically reaches heights of 80 to 120 feet, with a 40 to 50 foot spread. Height growth generally ceases once the tree is around 140 to 150 years of age.
On mature trees, the bark is dark brown, with vertical grooves and ridges.
Sugar maple leaves are deciduous. They typically grow three to five inches in length and width. Occasionally, they may reach up to eight inches in length and width. Leaf buds are pointy and brown. The current year’s growth twigs are green, before turning dark brown. Sugar maple leaves feature three to five distinct lobes that are coarsely toothed. The basal lobes are relatively small, while the upper lobes are larger and deeply notched. Notches tend to be rounded at the interior. Sugar maple leaves have a resplendent fall color, ranging from bright yellow and orange to fluorescent scarlet and orange. This fall display is particularly striking in the northern part of the tree’s range.
Sugar maples seldom bloom until they have reached 10 to 15 years of age. Flowering intensifies as sugar maples mature. Flower buds begin to swell at or just prior to leaf expansion, achieving full bloom one to two weeks before the leaves fully emerge. Flowers appear from late March to mid May, depending on the geographic location. Flowering is polygamous, occurring across the entire crown. Flowers are greenish-yellow, and lack petals. They develop in groups of five to ten, forming loose clusters. Flowers reach around 2.5 inches long. Generally, only one sex is functional in each flower. Both sexes are typically produced in the upper part of the crown. Only males form in the lower portion. On some sugar maples, certain limbs may produce only male or female flowers. Pollination occurs naturally in sugar maple. Its does not require assistance from insects.
Sugar maple fruits are a double samara. Samaras are globose in shape. They average between 0.3 and 0.4 inches in length. Samaras ripen in about four months, turning a yellowish-green when mature. Samaras reach maturity in September or October. They begin falling about two weeks after ripening, just before the tree’s leaves are shed. Of the paired samaras, only one is filled with a single seedling. Occasionally, both samaras may contain seeds, or be empty. Some sugar maples produce triple samaras, while others form samaras with double wings. Seeds are produced annually, with heavy crops occurring every two to five years.
Each sugar maple seed has its own stratification period threshold. Germination requires moist seed stratification at temperatures slightly above freezing for 35 to 90 days. The optimum temperature for germination is around 34 degrees Fahrenheit. This is the lowest required temperature of any known hardwoods. Under the proper conditions, sugar maple seeds have a high germination rate, exceeding 95%.
The moisture content and temperature affect how long the seeds must be stored for. Seeds may be stored for at least five years without a loss of viability. In natural stands, few seeds remain viable beyond the first year. Seedlings in the understory of dense stands may not survive for more than five years. In drier regions, seedlings require overstory shade for survival. Once they have reached two to four feet in height, their root systems will have developed, and they become capable of sustaining themselves. Plantings made in open fields have a high survival rate. However, they may grow poorly due to their inability to compete with herbaceous plants. Generally, open field plantings require several years of site maintenance to achieve success. Regardless of the setting, plantings should be made in early spring. This will allow for greater root regeneration, and increase the potential for survival and growth.
Sugar maple establishes a shallow, fibrous root system, with strong, oblique laterals, and extensive branching. Roots on the upper side of the laterals grow upwards, while those on the lower side grow downwards. Intraspecific root grafting occurs frequently. Sugar maple roots are shade tolerant. Gradual exposure to sunlight will cause the roots to penetrate deeper into the soil, promoting growth. Sugar maple roots form endotrophic and ectotrophic mycorrhizae, which increase water and nutrient absorption. The root system is susceptible to flooding during the growing season. Sugar maple roots release an exudate that can inhibit the growth of nearby yellow birches. Other tree species may be similarly affected.
Photo courtesy of James St. John/CC-by-A2.0