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Feature Article

Where does water in trees go during the winter?

By Mike Albers DNT-Forest Health Unit

We are all familiar with what happens when we leave a bottle of water in our car and the temperature hits -40 or what happens when we forget to put antifreeze in our car radiator. Ice forms, it ruptures the bottle or radiator and the liquid squirts all over and then freezes where it landed. When water freezes, it expands creating a tremendous amount of pressure, more than enough to rupture glass, plastic and even metal containers.

Why doesn't the same thing happen to trees? Trees contain a great deal of water. And we know it can get very cold here.

The water in a tree may weigh two to three times more then the weight of the wood. The moisture content of the sapwood of most trees is about 69 to 89%. Why don't cells rupture or tree trunks split when temperatures drop below freezing? The answer is that these things would happen if trees did not "harden off "for winter. Trees harden off in response to autumn conditions of cooler temperatures and shorter days to protect themselves from freezing water once winter sets in. And winter does set in, here in Minnesota.

Plants have developed a number of ways to avoid freezing injury. The first thing most plants do in hardening off is to increase the soluble solute (sugars and salts, etc.) content of the water in the cells. This lowers the freezing point of water just like antifreeze does in your car radiator so even though it is freezing outside the water inside the plant cells does not freeze.

Another survival mechanism involves a process that allows ice to form in plant tissues without killing cells. This process involves the formation of ice crystals outside and between cells (intercellular ice) rather than inside the cells. Plants are generally killed if ice crystals form within cells (intracellular ice) because the membranes are ruptured. Many temperate zone plants tolerate intercellular ice formation without injury. When trees that are hardened off cool slowly, ice initially forms in the intercellular spaces. As the temperature continues to decrease, more water is drawn to these ice crystals. As the water is drawn out of the cell, the concentration of the cell sap increases further lowering the freezing point of water inside the cell. Plants that tolerate very low temperatures must be able to accommodate large volumes of ice in the intercellular spaces without causing physical damage to cells. These plants must also be able to tolerate the dehydration of tissues caused by water being drawn out of the cells and into the intercellular spaces where it freezes.

Rapid cooling of plants that are hardened off may still cause injury if the water does not move out of the cells fast enough and freezing inside the cell membrane occurs. Much of the cold injury of hardened plants probably is the result of dehydration of tissues, as often happens with winter burning of conifer needles.

So, where does the water go? It stays inside the tree but it moves out of the living cells and into spaces and cell walls where it can freeze and not damage the delicate cell membranes.