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Freezing Trees

Tina Hartell

How do tree wells form?  Why does the snow directly around the tree melt faster than the snow in the surrounding area? I’ve always imagined that trees must emit heat because they, too, are alive and warm and have juices flowing through them. And while their juices (sap) aren’t exactly 98F like ours, they’re above freezing, right? Maybe. 

These yellow birches do emit heat but not through an internal furnace like mammals and birds have. Trees have a lower albedo than the surrounding snowpack. Or to it put another way, they’re absorbing more radiant energy from the sun. This absorbed heat then melts the snow. Some plants do have an internal furnace so to speak and can generate energy (aka heat). Bulbs, in particular, demonstrate their capacity to generate energy as they pop up through the snowpack in early spring. 

At our house two nights ago it was -7F. If trees don't possess an internal furnace, what is it that keeps them from freezing? They do absorb some sunlight like ectothermic animals (reptiles, amphibians, insects), but a sunny, -10 degree day won't keep cells from freezing. The snakes and frogs know this. They spend their winter buried in mud or locked down in a cave in some strange dormant torpor. Alas, trees do not have horizontal mobility, so those evolved to live in northern climes, must ride out the bitter cold. 

Like any living thing, if a tree's cells die, the tree dies. Water is plentiful inside the tree's cambium - the thin layer of xylem and pholem just under the bark. Their job is to move water and nutrients (aka SAP!) up and down the tree's stem. Xylem carries water and on cold nights it does freeze! As the ice crystals expand in the xylem, the surrounding wood cracks and makes a sharp "pop" noise that I often hear when I'm outside trying to warm up the truck on cold mornings.

But xylem and the surrounding wood is made out of dead cells. So if they freeze, the living cells of the tree aren't affected. The living cells are in the phloem, the tissue carrying nutrients. So how does the phloem not freeze? Supercooling. 

Supercooling is a strange phenomenon in where liquids can sink below their designated freezing point by not allowing any nucleation (growth) sites for ice crystals to form. This can occur up to -36.6F for water but because there are dissolved solids in phloem, it lowers the supercooled temperature to around -41F without freezing. What if the temperature goes BELOW -41F? Then trees employ another strategy that involves the dehydration of the phloem cells. The trees (somehow) force water out of the living cells into the extracellular space (the space BETWEEN the cells). There, the phloem freezes. But the cells survive, completely dehydrated and shriveled, waiting for it to warm back up so the phloem can reenter.

Of course, forest composition changes as you move away from the equator and eventually the great boreal forests of the far north peter out to tundra. It just becomes too cold for trees. But they survive some intense conditions with just a few pops.  (Adapted from a previous post.)