By PAUL HETZLER
While it doesn’t feel the least bit spring-like, it’s only a matter of time before maple sap begins to run. We know the weather conditions that lead to a run, but until just a few years ago, science was at a loss to explain maple sap flow.
(I always like it when people a lot smarter and better-paid than I am don’t know the answer, either.)
But because of recent discoveries, the mechanism behind sap flow is now mostly understood. Probably.
Throughout sugar maple’s range, maple production has been economically important ever since Native Americans first taught European settlers how to gather maple sap to make sugar. Back then it involved placing red-hot stones into sap to boil it down. Fortunately, technology has improved. Today’s maple producers have their reverse-osmosis units and vacuum pumps ready to go, anticipating the first sap run. The forecast will tell us when that will likely be, but how does it happen?
Aside from maple, very few tree species have a spring sap run. Birch and butternut are exceptions, but their roots generate pressure that forces sap upward, which is not the case in maples. It turns out maple sap flow is due to the way its wood interacts with freeze-thaw cycles. In biology we learn that wood, or xylem, is responsible for upward transport of water, while sugars move down through the phloem, the outermost layer of cells. Actually, xylem “misbehaves” during the spring sap run, ferrying sugars up for a while before nonchalantly returning to the textbook model as if nothing happened.
Xylem is composed of several types of cells, including vessels to transport liquid, and fiber cells to provide strength. Unlike most trees, maples have gas-filled fiber cells. Carbon dioxide and other gases in those fibers are critical to generating flow because they dissolve in sap. The geyser that results when we open a seltzer bottle (especially a warm one) too fast is a reminder that plenty of carbon dioxide can dissolve in water. If that bottle is icy cold, the risk of a gusher is low because cold water holds more dissolved gas.
During the night, gases in fibers shrink as they cool, eventually dissolving into sap contained in the vessels. This contraction of gases causes the tree’s internal pressure to drop, creating a suction that draws sap up from the roots. As the temperature warms in the morning, gases bubble out of solution and expand, increasing the tree’s internal pressure and forcing sap out the tap hole at about 15 pounds per square inch (psi) on average, but occasionally as much as 40 psi.
Rather than flowing up from the roots and out the tap during the day as was once commonly thought, sap actually flows down from the crown in addition to flowing laterally toward the tap hole. When a warm day follows a sub-freezing night, sap may run for just a few hours or for as long as several days, depending on the tree and on factors like barometric pressure change. If temperatures stay warm at night or below freezing during the day, sap will stop running.
All native maples yield sweet sap. Even the much-maligned boxelder belongs to the genus Acer and can be tapped. Unfortunately, the imported Norway maple, including its red-leaf cultivars, exude a bitter, milky sap. Sugar maple sap is two to three percent sucrose on average, although it can range from one up to ten or more percent. In addition to sugar, sap contains organic acids, amino acids, minerals and other compounds. Some of these contribute to maple’s flavor, and others form insoluble sediment called “sugar sand” that must be filtered out.
The freeze-thaw, dissolved-gas, pressure differential hypothesis has some holes. Although the mechanism should work with pure water, it turns out that sap only flows if it contains a minimum level of sucrose. Sap flow should also happen in any xylem tissue, not just the living sapwood, but that’s not the case either. So the mystery of sap flow continues. Sometimes it’s a relief not to have all the answers.
Cornell Cooperative Extension of St. Lawrence County provides equal program and employment opportunities.
Paul Hetzler is a forester and Cornell Cooperative Extension of St. Lawrence County horticulture and natural resources educator.