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Why don't trees freeze and burst in the winter like cold pipes?
Category: Biology
Published: January 15, 2014
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
In many cases, trees do partially freeze in the cold of winter and burst like plumbing pipes in an unheated home. When liquid water freezes to ice, it expands in volume because of the way the water molecules spread out to form a solid crystalline lattice. If the water is contained in a closed vessel, it can press so hard as it freezes and expands that it bursts the container. This explosive effect is common in insufficiently heated houses, where the cold leads the plumbing pipes to burst. The fluid transport tissue in trees (xylem and phloem) can be seen as little pipes carrying water and nutrients throughout the tree. They too can freeze and burst, causing the tree to crack and/or explode. The crackling sound or gun-shot pop you hear in the forest in the winter is the sound of trees freezing and bursting. The bursting is usually not as violent or as deadly to the tree as you may first expect for a few reasons. A tree has hundreds to tens of thousands of these fluid channels. If one bursts, the tree has plenty of other ones to rely on. Furthermore, each channel is small, so that an individual channel bursting does not do much damage.
In addition to the fluid channels, each cell of the tree is itself a little bag of water that can pop upon being frozen. If the water inside the cells freeze, it is instantly fatal to the tree. While many trees can withstand freezing of the fluid transport channels outside the cells, none can survive intracellular freezing. Trees that survive in cold climates must therefore protect their cell interiors from freezing. The cold hardiness of trees is caused by many factors:
- Trees are softer and more flexible than metal. As the water in the channels and between cells freezes and expands, the tree's tissue can stretch somewhat instead of bursting.
- In preparation for winter, the cells of many trees get rid of a lot of their water content. Less water means less expansion upon freezing. The tree enters a dehydrated, dormant state to survive the winter.
- In preparation for winter, some trees produce more sugar. When this sugar dissolves in the water, it lowers the water's freezing point and thus acts like anti-freeze. In maple trees, this sugar-producing winterization can ultimately end up as maple syrup on your pancakes.
- Some trees take advantage of supercooling. In supercooling, water can be kept liquid well below its traditional freezing point if there are no nucleation centers for ice crystals to start forming on.