To: North American Fruit Explorers <nafex@lists.ibiblio.org>
Subject: [NAFEX] Here is the dope on maple Sap flow
Date: Sun, 16 Feb 2003 08:32:33 -0800
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