How trees keep their sap from freezing in winter

HostWe see trees standing out in the snow, looking totally still and stiff. It's easy to think they're just resting or even dead until spring comes back. But if you think about it, a tree is full of liquid, and we all know that when water freezes, it grows in size and breaks things. How do they get through those freezing nights without just popping open?

GuestWell, it's a huge problem for them. If the water inside the living parts of a tree turned to ice, it would be like putting a glass bottle of water in the freezer. It would just shatter. Ice grows as it forms, and those little ice crystals have very sharp edges. They would shred the skin of the cells from the inside out. So, trees have to become master chemists the moment the days start getting shorter. They have to change the way their insides work before the first big frost hits, or they simply won't make it.

HostSo they're basically making their own kind of antifreeze, like what we put in a car?

GuestHmm, it's a bit like that, but maybe more like making a very thick syrup. When the tree feels the air get cold, it starts taking the starch it stored up during the summer and breaking it down into sugar. It floods its cells with this sugar. Now, we know from making candy or even just sweet tea that sugary water doesn't freeze as fast as plain water. It lowers the point where things turn to ice. By making their sap very sweet and thick, they can stay liquid even when the temperature drops below the point where a puddle on the ground would be solid.

HostBut sugar only does so much. If it gets down to twenty below or lower, a little bit of sugar in the water isn't going to stop it from freezing solid. There has to be more to it than just being sweet.

GuestYou're right. Sugar is only the first line of defense. When the deep cold really sets in, the tree does something that sounds a bit scary. It actually pushes most of the water out of its living cells. It moves that water into the tiny gaps between the cells. These gaps are sort of like the hallways of the tree, and they're not alive. If ice forms in those hallways, it doesn't really hurt the tree. It's like the tree is letting its hallways fill up with ice so that its bedrooms can stay safe and dry.

HostWait, so there actually is ice inside the tree? If those gaps fill up with ice and that ice grows, why doesn't the whole trunk just split apart?

GuestOh, it can happen. If a tree isn't ready, or if the cold comes on too fast, the trunk can actually crack with a sound like a gunshot. People in the woods on very cold nights hear those pops all the time. But usually, the tree has enough give in its wood to handle a little bit of ice in those gaps. The real trick is what happens to the cell that stayed behind. Because it pushed out so much water, the stuff left inside the cell gets so thick and gooey that it can't form ice crystals. It becomes what we call a glass.

HostA glass? Like the stuff in a window?

GuestWell, not exactly like a window, but the same idea. In a normal liquid, the bits are all moving around. When it freezes into ice, those bits snap into a very stiff, sharp pattern. But if you make a liquid thick enough, fast enough, the bits just stop moving before they can form that sharp pattern. They get stuck in a messy, gooey state. It's still a solid, but it doesn't have those sharp edges that shred things. The cell becomes a little bead of glass. It's a very weird state of being. It's not really liquid, but it's not a crystal like ice either. It just sits there, frozen in time, until the sun warms things up.

HostThat sounds like the tree is basically turning itself into a statue to wait out the winter.

GuestIt really is. It stops all the work of being a living thing. It doesn't grow, it doesn't breathe much, and it doesn't move nutrients around. It just holds its breath in that glassy state. If it tried to keep its normal liquid self during a freeze, it would be gone in a single night.

HostAnd I guess that's why some trees can live for hundreds of years in places where it stays below zero for months at a time.

GuestEven in the deepest part of a sub-zero night, the heart of the cell stays as a soft, amber-colored glass, waiting for the first hint of a thaw to turn back into a liquid.

HostThose quiet trees in the backyard aren't just standing there in the cold; they're busy turning themselves into glass so they can wake up when the snow finally melts.