The act of tapping a tree can be a mystical experience. The tree, so often just a drab giant in the landscape, a taken-for-granted piece of architecture, little different than a house or a fence, is pierced. And then things get spiritual because the tree bleeds. It becomes apparent immediately that the hole is, in fact, a wound, and so in this regard the tree is more akin to you, a human being, than it is to a fence. This speaks to the poet in all of us. Read the Aeneid, the Lord of the Rings series, Shel Silverstein’s The Giving Tree, and you will see authors projecting human characteristics onto trees in their stories.
Now the science part of what’s going on is equally fascinating. You may think that tapping a tree to collect the sap is just like sticking a needle into a human vein and taking blood (and in fact this can be a useful analogy, especially as we stress to people that you can overtap a tree). But contrary to conventional thought, sugarmakers aren’t tapping into a tree’s circulatory system and collecting sap as it’s pulled up from the roots to the branches; rather, the sap they collect is free falling down the trunk of the tree, from top to bottom.
Here’s my understanding of the science:
Maple sapwood contains vertical vessels that move sap (picture them as a bundle of straws). Surrounding these vessels are gas-filled fiber cells.
As a maple tree freezes, ice crystals form in the fiber cells and grow by pulling sap out of the vessels. Since there’s billions of cells in a tree, this process creates considerable energy, which results in suction throughout the tree. Researchers say that during the freezing process, negative pressure at a taphole can be as low as -7.5 PSI. A tree will continue to suck up water from its roots until the sap transportation system becomes frozen solid.
The next day, when the warm spring sun melts the ice crystals in the tree’s fiber cells, the sap re-enters the sapwood vessels and the negative pressure becomes positive pressure (as high as 30 PSI – the equivalent pressure in a car tire – on a good sap day). Gravity and gas bubbles play a big role in creating this pressure (you can see these gas bubbles, like train cars, in a tubing system). The sap that was frozen solid in the maple’s branches free-falls down the trunk and out the taphole. When the pressure diminishes to zero, the sap stops flowing and a sugarmaker must wait for another freeze (as, unfortunately, many are doing as you read this).
The greater the difference between the barometric pressure outside and the internal pressure of the tree, the better the sap run. This explains the effectiveness of tubing vacuum systems. In natural conditions, high barometric pressure can restrict sap flow; vacuum, on the other hand, creates a permanent negative pressure environment.
If you’re interested in learning more about maple science, visit a sugarhouse in the next few weeks and ask questions in between samples.
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