Eli5: How do trees survive temperatures well below freezing that would cause frostbite/cell death in animals?



I was curious given this week’s extremely low temperatures, and how yearly, trees seem to avoid death of their cells and structures in what would be otherwise destructive temperatures. How do they do this?

In: Biology

As tree grows older it generates new cells from inside out. The outer layer of cells then becomes it’s bark. The nutrients (sap)flow through the walls (or layers). Over a period of time the outer bark stops receiving the nutrients. It dies and forms the basis for a woody structure. It has dried fiber and miniscule air dots. Many such layers protects the inside of a tree.

However, depending upon the size of the tree and bark thickness, a tree can get freez and die too.

In tropical landscapes, such things happen to even older trees because the fiber of the bark is not strong enough to withstand cold.

They cycle out water in the fall and replace it with pitch and antifreeze.

This happens at a cellular level, not just in the trunk and branches.

Since there’s no water in the cells that can crystallize, the cells are safe until much lower temperatures.

The sugars and other chemicals in the sap lowers the freezing point so they can survive. If the sap were to freeze, ice crystals would probably destroy the cell walls like they do for some vegetables in a freezer. Trees don’t grow above a certain altitude where the temperature would drop too low in that location, called the “tree line”.

So, a couple of things, plant cells are not like animal cells. Our cells are soft and squishy, essentially like a thin-walled water balloon. Plant cells are more firm walled, more like a juicebox. That helps to make them more resistant to freeze damage right off as the membrane is tougher. Plants can tolerate a greater range of internal conditions than we do, as well as being able to survive and recover from serious physical damage better than animals, so even when they do get freezing damage it may only kill a part of them, not the entire tree.

They have some specific tricks they use, which are actually pretty much the same tricks many animal species that have ‘antifreeze’ use. Below is a summary of them copied from the article *[How do Trees Survive Winter Cold?](https://northernwoodlands.org/outside_story/article/trees-survive-winter-cold)*

>Schaberg’s work suggests three basic ways in which living tree cells prevent freezing. One is to change their membranes during cold acclimation so that the membranes become more pliable; this allows water to migrate out of the cells and into the spaces between the cells. The relocated water exerts pressure against the cell walls, but this pressure is offset as cells shrink and occupy less space.

>The second way a tree staves off freezing is to sweeten the fluids within the living cells. Come autumn, a tree converts starch to sugars, which act as something of an antifreeze. The cellular fluid within the living cells becomes concentrated with these natural sugars, which lowers the freezing point inside the cells, while the sugar-free water between the cells is allowed to freeze. Because the cell membranes are more pliable in winter, they’re squeezed but not punctured by the expanding ice crystals.

>The third coping mechanism is altogether different. It involves what Schaberg describes as a “glass phase,” where the liquid cell contents become so viscous that they appear to be solid, a kind of “molecular suspended animation” that mimics the way silica remains liquid as it is supercooled into glass. This third mechanism is triggered by the progressive cellular dehydration that results from the first two mechanisms and allows the supercooled contents of the tree’s cells to avoid crystallizing.

Here’s [another longer multi-part article that goes into more detail](https://www.canr.msu.edu/news/trees_avoid_damage_from_freezing_temperatures_part_1).

Depends on what you call “freezing” I lived in Northern Wisconsin, and around 1976 or 7, at my girlfriend’s parents house, their back yard thermometer said 52 below zero, the local TV said 48 below zero, but lower in the low lying areas. My car would not start, i had to stay the night, later that night, we would hear these very loud Caaaararrrrcccks! out in the woods, all around the house, next morning, just walking out the front door, i saw the first evidence of what the loud crack noises were, as I looked over at a poplar tree, similar to an Aspen tree, they could get pretty big, this tree had a huge split, a crack, running from the ground up to the branches, and as i scanned the woods, i saw many poplar trees with big splits running up one side, my girlfriend’s father came out with the dog, I showed him the cracks in all the trees, he said, “i’ve heard of that from many years ago”, it can sometimes get so cold that the fluid, and sap, in the trees freezes solid, just like an ice cube, the damp, then frozen solid wood expands and then violently splits the wood and bark in one quick crack running up the side of the tree. In the spring, they cut down as many of the trees by the house as they could, stacked them up for year’s worth of firewood, because the soft poplar wood rots quickly and they didn’t want the dying trees to fall on their house.