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Title: Here is the dope on maple Sap flow

Maple Sap Flow; by David Brynn, Addison Cty Forester

The maple sap has started to flow and it is time for Vermonters to get ready for one of Spring's most commonly asked questions: "Why does maple sap flow?" Most folks can bluff their way through at least part of the answer by saying "there must be cold nights and warm days to set up a pressure differential." This explanation will ordinarily suffice but, in case it doesn't, an article by Melvin Tyree in the Maple Syrup Journal gives more details.

Many people assume that maple sap flows up from the tree's roots on warm days.  Actually-on warm spring days which follow cold nights- sap flows down from the maple tree's branches and then out the spout, broken branches, or wound.  To illustrate this, Tyree suggests a simple maple sap flow experiment which involves cutting down a maple tree when it is thoroughly frozen.  (Hopefully you can find one that is badly deformed, growing in the wrong place, or competing with a maple which has a superior future prospects!)

When the temperature rises above freezing you will find that the stump has little or no sap flow while the sap will readily flow from the severed stem of the tree.  Clearly, the sap is not coming up from the roots but down from the stem and branches of the tree.

Tyree explains that as the maple trees begin to freeze, sap is actually sucked up into the tree branches through the large wood pores which connect with the trees roots.  As the tree freezes from the outside in, the rising sap freezes and turns into ice crystals.  This process continues as long as there are freezing temperatures and rising sap.  If the freezing process is a long and slow one, the tree can be expected to suck up a great deal of sap.  The next sap run will be a good one as long as the warming temperatures cooperate as well.

Unfortunately, even this explanation does not make much sense when one considers that water expands as it freezes.  If water expands as it freezes, then trees ought to push out the sap when they freeze and to suck it up when they thaw.  And this is the case in most hardwood species.  However maples suck sap as they freeze and drip sap when they thaw.  Why?

There are four longitudinal cell types in hardwood trees: vessel elements, tracheids, dead wood fibers, and parenchyma.  Vessel elements are by far the largest cells.  The vessel elements appear as dark, open pores and they act as pipes which transport sap through the living sapwood much like pipes transport water through a house.

In most hardwood trees, the dead wood fibers which surround the vessel elements are filled with water.  However, in maples, these dead wood fiber cells are filled with gas.  When these dead wood cells start to freeze frost forms on the inside of the cells much like the frost which forma on the inside of a single glazed window on a cold winter night.  The water for the frost comes from the vessel elements using the same forces which move water through a sponge.

After a long, slow, freezing periods charge the branches with a fresh slug of sap, the maple tree is ready to yield sap the next time the temperature warms up enough to thaw the frozen crystals stored in the vessel elements and the wood fibers.  The sap is then released back into the vessel elements and gravity allows it to move down the tree, out of the taphole and into the bucket or tubing.

So the next time you are asked how it is that maple sap flows, you might be able to get away with saying it has something to do with a pressure differential set up by cold night s and warm days.  If that doesn't suffice, you'll now be equipped to explain the story of maple sap flow in much more detail.

-- 

Henry W. (Hank) Parker
113 Peter Coutts Circle
Stanford, CA 94305-2518
email: hwparker@pacbell.net