Maple Sap Runs On Gas

[Paul J Hetzler]

Some foods give us gas, but gas is what ultimately gives us maple syrup. If not for gas bubbles in the wood, sap wouldn’t flow. Who knew maple trees were CO₂ powered?

A mere two decades ago, arborists were at a loss to explain what caused maple sap run. We'd typically mumble something about transpirational vacuum in the canopy before changing the subject. Everyone knows sap runs when warm days follow freezing nights. But it wasn’t until recently that the mechanism behind sap flow was better – although still not perfectly – understood.

Aside from maples, few species have a spring sap run. Birch and butternut do, but their roots generate pressure that forces sap upward. In contrast, maple sap flow is due to how its wood reacts to freeze-thaw cycles. Wood, or xylem, has vessel cells to carry liquids, and fiber cells for strength. Unlike most trees, maple fiber cells are partially filled with carbon dioxide and other gases, which can dissolve in sap.

At night, gases in fibers shrink as they cool, eventually dissolving into sap in the vessels. This drops the tree’s internal pressure, creating suction that draws sap from the roots. As daytime temperatures rise, gases bubble out of solution and expand, increasing internal pressure and forcing sap out tap holes at between 103 and 276 kPa.

Rather than flowing up from the roots and out the tap as was once thought, sap flows down from the crown (plus some lateral flow) toward a tap. When a warm day follows a sub-freezing night, sap may run from a few hours to several days, depending on the tree and barometric pressure. If it’s warm all night or freezing all day, sap won’t run.

Although sugar and black maples are most commonly tapped, producers also use silver and red maples when available. Even the humble Manitoba maple can be tapped. Maple sap is 2-3% sucrose on average, though this can range from below 1 to over 10%. In addition, sap contains organic acids, amino acids and minerals which contribute to maple’s flavour.

If great taste isn’t enough reason to use syrup, consider its health benefits. A 100-gram serving of maple syrup provides over 100% of the recommended daily amount of manganese and vitamin B-2, and it’s a significant source of magnesium, zinc and calcium. In 2016, Toronto-based researchers announced they’d identified a compound in maple syrup that inhibits clumping of beta-amyloid brain proteins, which may help prevent Alzheimer’s disease.

The freeze-thaw, dissolved-gas, pressure-differential explanation of sap flow has some holes, though. While the mechanism should work with pure water, sap only flows if it has a minimum level of sucrose. Flow should also happen in all xylem tissue, not just the living sapwood, but that’s not the case either. So the mystery of sap flow continues.

Recently, maple producers have been branching out into value-added products like maple candy. Another item is bottled, pasteurized and, of course, carbonated maple sap. What goes around, comes around, it seems.

ISA-Certified Arborist Paul Hetzler never shakes pop cans or maple trees before opening.