In much of the northern United States, temperatures are now regularly falling below freezing. For organisms made mostly of water — like you and me and all other animals and plants — this poses some problems. Many mobile creatures like humans get away from the cold by finding shelter where temperatures aren’t as cold as they are outside. But immobile creatures can’t do that, so they have to come up with different strategies to make it through a freezing winter.
Trees depend on water transport to live. The water is drawn up through their roots, through the trunk, and into the leaves, where it eventually evaporates into the air. When the weather gets cold and water freezes, trees face two dangers: if water inside cells freezes and then thaws, it can break apart cell walls, causing the cells to die. The other danger trees face is that when its internal water freezes, air that was dissolved in the water turns into air bubbles. If they’re large enough, these air bubbles can destroy the water tension inside the tree when the ice thaws, causing the collapse of the internal water column.
Trees have evolved several strategies to deal with these dangers. One is very obvious: many trees shed their leaves in the fall. Among other things, this slows water flow, saves on water, and reduces the likelihood that air bubbles will form in the water column. Some trees also produce a form of antifreeze that prevents ice from forming inside the cells even when temperatures fall below freezing. The antifreeze can work all the way down to about -40 degrees Fahrenheit! Finally, some trees in very cold places can pull the water out of their cells in the fall, so that ice only forms outside the cells where it can’t hurt the cells themselves.
In the spring, trees remove the antifreeze, refill cells with water, and grow new leaves, ready to repeat the cycle anew.