When water freezes it expands, so ice formation within the confines of a tree trunk is potentially lethal.
Tree heartwood, formed from annual rings of water-transporting xylem, is made of dead cells, and water can freeze within these without fatal damage (though in extreme conditions ice expansion has been known to split trunks).
The real hazard comes from water freezing in the narrow zone of living cells that lie just below the bark, and are essential for the tree’s survival and regrowth in the spring.
Each of these cells has a rigid, dead wall enclosing a bag of living sap that’s confined within a delicate membrane.
If the sap freezes, spiky ice crystals burst the membrane and the cell dies, so as winter approaches the cell will accumulate sugars that protect it by lowering the sap’s freezing point.
At the same time, water moves out of the shrinking bag of sap through the membrane, into the space between it and the cell wall, where ice crystals can form harmlessly.
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