Those sturdy conifer and deciduous trees gracing our Campus and Natural Area prove each winter, by standing and taking the full brunt of whatever winter throws at them, that they are tough cookies. Some of how they do this is understood, but the adaptive mechanisms described below will safeguard trees only so far into the realm of freezing temperatures. A reason exists for the “tree line” across northern Canada.

Because significant differences in both internal structure and physiology exist between conifers and deciduous trees, and because of my limited writing space, I am going to make just a few general comments in an attempt to sketch the basic bag of botanical survival tricks. First, many cells in living trees are dead, and thus, very temperature hardy. So, we are concerned with the living cells.

In the “cell-damage-from-freezing" business, formation of ice crystals is the big danger. If water can be kept from crystallizing (freezing), it can be super-cooled to several degrees below normal freezing. We did this often in both my teaching and research labs. The trick is to keep the water free from "crystallization nuclei,” e.g., a small particle of dust. Also, greatly increasing the number of chemical compounds in solution in a cell’s cytoplasm can lower the freezing temperature of water. Seawater freezes more slowly than most freshwaters.  

In the fall, when leaves and more conifer needles start to fall, the membranes of living cells in tree trunks, limbs, and roots become more pliable, allowing some water to leave the cells to reside outside and around the living cells. Meanwhile, the living cells are busy breaking down certain large molecules into smaller units, e.g., in some trees, starches (large molecular compounds) are enzymatically broken down into much smaller individual molecules of certain sugars. The combined loss of water and increase in cytoplasm density often result in what Dr. Paul Schaberg, USDA plant physiologist in Vermont, describes as a “glass phase.” This very dense, super-cooled state allows the living cell cytoplasm to escape crystallization, and cell damage—during normal winters. The water outside of cells does routinely crystallize and expand, but the increased cell membrane pliability prevents membrane damage—unless the temperature falls too low.

Although others processes are part of the picture, the ones described above seem to represent the core tree mechanisms for surviving normal winters. The more we look, the more we find things that amaze, the more we appreciate the mystery and majesty around us—however hidden, small, or silent. The more we look, the more we are thankful for being allowed to be a part of it all—even though our actions often suggest that we do not deserve the privilege.