Sugar maple (Acer saccharum)

ACERACEAE, Maple Family

People in the northeastern part of the United States and adjacent Canada must endure cold, snowy winters; but in late winter, they also get to enjoy the "running of the sap." To some, this event is analogous to grunion runs on southern California beaches, and it usually is accompanied by festivals and good times for all.

The running of the sap is not, of course, a foot race between weaklings, but rather the release of sugary liquid from sugar maple trees (Acer saccharum). Long ago Indian tribes of eastern North America discoverer--one supposes by accident--that sugar maple trees in late winter (February to April, depending on location) bleed sweet sap when they are cut. A person would make a diagonal cut into the trunk of a tree and drive a reed or concave piece of bark under the cut to convey the sap into a receptacle. Then the sap was boiled in clay or bark vessels by repeatedly dropping hot rocks into the liquid, driving off the excess water and thereby concentrating the sugar. Alternatively, some tribes permitted the liquid to freeze and then removed the ice, leaving behind a concentrated sugar solution.

Gradually this home brew was refined into an industry. European settlers used iron or copper kettles instead of clay and bark. They learned to tap the trees with a biter (auger) at a height of about one meter and to use wooden or metal spouts (one or two per tree); they learned to use metal pails with covers to keep out impurities and to stop dilution by water or snow. Sap from buckets was carried by hand or by ox- or horse-drawn gathering sleds (even narrow-gauge railroad) to a sugarhouse, where the liquid (1-6% sugar) was boiled over a wood fire in large, open kettles to evaporate off the water until it became a thick syrup (85% sugar). Impurities had to be skimmed off the surface of the syrup, and niter (sugar crystals) was filtered from the syrup. Further evaporation of the syrup was also done to crystallize incredibly sweet maple sugar for candy ("sugaring off").

The process has developed to become more efficient and sanitary. Just as iron and copper harvesting vessels were replaced with galvanized pails or even strong plastic bags, so also wood-fired kettles were replaced with long, wide, shallow galvanized vats (evaporators) heated by steam or gas and fitted with overhead hoods to remove water vapor and automatic drawoffs for the finished syrup. Spouts were standardized for internal diameter (7/16" bore) and inserted slanted upward to maximize flow. Nowadays sap is often efficiently pulled from trees by a pump into a holding tank through flexible plastic tubing. Where wild collections are made, caterpillars or snowmobiles can be used instead of animal sleds. Finally, syrup is canned in airtight, sterilized containers or glass in which the syrup can be stored indefinitely.

Most people associate plant sugar with phloem and assume that sugar maple sap comes from the phloem. Not so! Sugar here comes from the wood, and the spout is driven into the outermost sapwood. In late summer and before it loses its colorful leaves in the fall, this tree stores large quantities of starch in the wood parenchyma. Then when temperatures rise in late winter, the starch is broken down and converted into sucrose, which is released into the wood vessels. The high concentration of sugar in the vessels causes soil water to be brought (diffuse) into the roots, building up pressure in the root and forcing the sugary sap upwards toward the unopened, dormant buds. Alternating freezing or cold nights and warm days causes sharp changes in xylem water pressures; flow is greatest in the warm part of the day and stops at night. Seasonal flow lasts for two to six weeks, and it stops when buds begin to open and when all of the initial liquid with the sucrose has ascended the plant. The amount of sap removed from a tree, typically about one gallon, is not detrimental to the growth of the tree.

In March, 1949, Governor E. W. Gibson (no relation) of Vermont signed a bill making sugar maple the official state tree, even though maple syrup is made in more than 20 states. Vermont also has strict laws for grading and labeling maple syrup. Grade AA (Fancy Grade) maple syrup is clear and light amber, and has the finest maple flavor.

Acer saccharum is also a prized timber tree for furniture--if you can spare it--and is one of the very important, dominant trees throughout the eastern deciduous forests of North America. In fact, the leaf of this species is the widely recognized emblem of Canada.

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