Incidentally, if a tree freezes quickly, the sap could freeze solidly rather than as a frost-like etching on the walls of the fiber cells. In that case there can be a surprisingly meager sap run when it thaws.
The optimal situation is a slow gradual freeze that allows the cells to absorb the highest volume of sap, all of which will be released at the thaw. Tyree suggests a small backyard experiment. During sugaring season, after a freezing night and while the tree is still frozen, cut down a sugar maple sapling close to the ground. I have two questions? This year the winter seems to be mild. Here in Mid Michigan the extended forecast temperature is mid 30 during the day and mid 20s and night for the next days.
I am thinking there is no reason to tap as long as frost is in the ground. I have been reading a lot of articles on maple sap. I understand the process of how the tree system produces maple sap. There seems to be some question as what create the flow movement. I cannot find a straight answer that I believe that the change in pressure in the tree and gravity pushes it out, as the tree heats up and then at negative temps a vacuum is caused by cooling of empty wood fibers and draws liquid back into the tree.
Notice the drop of water at the base of the cut. The sap was noticeably sweet to the taste. This is not always the case as the sugar concentration varies quite a bit. This branch has been bleeding sap for about 5 days. It is harmless to the tree, since the volume of sap involved is quite small. This bleeding is the basis of the maple syrup industry. There have been several online discussions lately about making syrup from other species, such as hickory, walnut and birch.
Not all trees develop a high enough sap pressure to bleed. For the maple syrup industry, this is make-or-break time. The window of opportunity for gathering maple sap is very small, but the reward is sweet. In most trees, the cells in the rays are filled with water, but in maple trees these cells are filled with gas. When most trees freeze at night, the water filled cells, freeze, pushing the sap out of the tree. When a maple tree freezes at night, the sap filled conduits freeze and expand.
During the freezing process, the gas filled cells in the rays are condensed, or made smaller, and a frost, that contains sugar , forms on the sides of the ray cells. This freezing and condensing of gas cells in the tree creates a negative pressure in the tree, which then pulls more sap up the tree, from the roots.
When the tree thaws, the frozen sugars attached to the ray cells then reenter the conduit cells , while the gas filled cells expand. This expansion causes a positive pressure in the tree, pushing the sap around and out of the tree. The maple tree has several ways in which it gets rid of this pressure, including evaporation off of branches and through any cracks or weak points in the tree, including our tap holes.
This process then continues, for several days or weeks in the early spring, freezing or negative pressure at night and thawing or positive pressure during the day; pulling of sap into the tree at night and pushing of sap around and out of the tree during the day. The maple tree makes and stores more sugar than it will need for the following spring due to the loss of sap through this process.
So, when the maple tree loses a bit of sap during the positive pressure times, it still has plenty to be healthy and grow. So why does the sap stop flowing then? When this freeze thaw cycle stops, the positive pressure in the tree stops, and so the sap stops running out of your tap hole. Many maple syrup producers will pull their taps and stop collecting sap when the trees start to bud, even if there is still sap flowing.
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