Sugar Maple (Acer saccharum)

3/2/2019

By: Dan Petters

 

Sugar maple (Acer saccharum) is a large, commonly known member of the eastern and midwestern deciduous hardwood forests. Their leaves are the classic maple leaf shape featured on the flag of Canada, with their distinct, beautiful yellow to red fall color. Buds and branching are opposite, with true terminal buds. They are extremely shade tolerant when young, often living for decades as a sapling, putting on almost no growth until an opening in the canopy can release them from suppression. They compete very well on rich, well drained soils and often dominate on these sites. They can be a successful shade and street tree in areas where road salts and soil compaction are not significant issues. 

Sugar maple foliage

Sugar maples are also known for their high sap and maple syrup production, with a high sugar content and especially fast rate of flow. Other maples may also be tapped but do not have as high a sugar content as A. saccharum. 1880s nature writer John Burroughs once wrote that “A sap-run is the sweet goodbye of winter. It is the fruit of the equal marriage of sun and frost.” That statement is true in several ways. That marriage of sun and frost, below freezing nights followed by warm days when the sugar maple’s sapwood gets above 32° F, are the conditions required for sap to flow. These warming temperatures are a sign of spring’s approaching arrival. Finally, maple syrup, the product of the combined labor of trees and people, is certainly a sweet goodbye.

Sugar maple with metal sap buckets

While some smaller scale operations still use traditional spiles and buckets or bags for sap collection, larger commercial syrup farms mostly use vaccum or gravity powered tubes to transport sap for processing.

 Sugar Maple sap tubes

The understanding of the process of sap flow is complicated and not fully understood. Contrary to a common misconception, the sap that drips out of holes drilled into the trees is flowing down from the branches and canopy, not up from the roots at that moment. Also, the sap has a positive pressure inside of the tree, pushing it out of the holes actively, rather than the negative pressure that water in xylem has when it is being pulled up through the tree during the growing season. 

The best hypotheses state that cooling temperatures cause gasses in the xylem and surrounding wood fiber cells to dissolve and decrease pressure, pulling sap up from the roots. As the temperature dips below freezing, frost begins to form on the insides of cells from the gaseous water inside them, pulling up even more sap. As temperatures then warm the next day, sap is pulled down the tree by gravity and pushed by expanding gases trapped in last night’s ice. This process is dependent on the gas-filled wood fiber cells that surround the vessles in maples. In most other trees, these cells are water filled and instead push sap out during the freezing process. What remains unexplained is why this process seems to require sugars in the sap and living cells.


References

Acer saccharum. The Morton Arboretum. http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=h240

Saupe, S. Biology of Maple Sap Flow. College of St. Benedict/St. John's University. Accessed March 1, 2020. http://employees.csbsju.edu/ssaupe/biol327/lab/maple/maple-sap.htm

Tyree, Melvin. Maple Sap Exudation:
How it Happens
. University of Vermont Maple Syrup Journal. January 1, 1984. Accessed March 1, 2020. https://www.uvm.edu/~uvmaple/maplesapexudation.pdf

Images

John Ruter, University of Georgia, Bugwood.org

T. Davis Sydnor, The Ohio State University, Bugwood.org

Geoff Manaugh, Crown Maple. https://gastropod.com/the-maple-boom/