When liquid water freezes, it expands as the water molecules spread out to form a solid crystalline lattice. If the freezing water is contained in a closed vessel, such as a metal pipe, it can press hard on the sides as it expands, causing the container to burst.
Trees contain water that is transported throughout the tree by the phloem and xylem, which we can think of as pipes.
Trees are cold hardy because of many factors. In preparation for winter, many trees reduce the amount of water held in their tissue. They usually enter a dormant state to survive cold winters. Some trees, such as maple trees, produce more sugar. When this sugar dissolves in the water, it lowers the freezing point of the fluid. Unlike plumbing pipes, tree tissue is somewhat flexible; as water in the channels freezes and expands, the tree’s tissue can stretch somewhat.
Trees can suffer damage during periods of cold winter nights followed by warm sunny days. The bark alternately contracts with freezing temperatures and expands on warm days. The alternating freezing and thawing temperatures can result in a vertical crack called a “frost crack.” This often happens during late winter or early spring. The formation of a frost crack can be accompanied by a loud noise, like a gunshot sound. But the trees don’t “explode” in the in subzero weather.
Dangerous winter weather for trees are storms that lead to heavy snow and ice accumulating on the trees. The weight of frozen precipitation can put enormous stress on tree limbs, causing them to break and fall.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
