Winters in Dayton can be frigid, with temperatures well below freezing (although we sometimes have warm spells that feel almost like spring!). We all know that trees contain water; it’s transported throughout the tree by “pipes” (the phloem and xylem). And we know that water expands as it freezes. So why don’t trees freeze solid in winter? And why don’t they burst like a frozen pipe in your home when the furnace breaks down?
The fact is that many trees do partially freeze in winter, and some do burst. Those loud cracking or gun-shot sounds you hear in winter, especially on very cold nights, are the sound produced when frozen water-carrying channels in the tree (phloem and xylem) explode.
Thankfully, trees have thousands of these fluid channels. If one bursts, there are plenty more to take on the water-carrying task. Plus, because the channels are small, a rupture doesn’t do much damage to the tree.
On the other hand, if the water-filled cells in the tree freeze and rupture, it can be fatal to the tree. That’s why trees have evolved to protect the internal cells from long periods of freezing temperatures. They do that by using several sophisticated internal methods to either:
- Remove water from the tree,
- Change the consistency of water in the tree, or
- Put water molecules under pressure so they don’t freeze.
Depending on species, trees may use one or more of these internal methods.
Freeze-Protection Method #1 – Freeze Drying
Many trees remove or reduce the amount of water held in their tissue during winter.
During the growing season, leaves pull water up from the ground to keep the tree hydrated and then expel excess water vapor into the air. But because trees are not actively growing above ground in winter, moving water through their vascular system is not as vital as it is in spring and summer.
Drier tree tissue is less susceptible to the expansion of water as it freezes. Internal water freezing around tree cells is ok, but water inside cells will expand as it freezes, rupturing and killing the cells. To prevent this, the tree pushes water out of its cells in fall as it prepares for dormancy.
This natural dehydration is also why it is so important to make sure trees have enough water in late winter and early spring when they break dormancy. Once trees start making buds and leaves, they need water to support their growth. If the weather isn’t cooperating by making rain, you’ll need to make sure there’s sufficient irrigation water for tree roots to take up so the tree can be rehydrated.
Freeze-Protection Method #2 – Add Extra Sugar
We know sugar is delicious but it can also be good, especially in trees. When preparing for dormancy, trees will produce more sugar in their sap. This sugar acts as an anti-freeze, diluting pure water and lowering the temperature at which the sap will freeze.
Maple trees are the best-known example of this; the high sugar level in maple sap is what lets us make maple syrup. When temperatures rise in spring, it marks the end of dormancy for the tree. Sap begins moving through the tree to aid the development of buds and leaves. During daylight hours, when ambient temperatures are high, sap runs easily. This is when maples are tapped for maple syrup and the sap is boiled down to the concentrated sweetness we love. At night, when temperatures drop, there’s a resulting drop in sap flow.
Freeze-Protection Method #3 – Put It Under Pressure
Supercooling is another method trees use to protect themselves in winter. Instead of fighting the expansion of water molecules as they turn to ice, the tree creates increased pressure inside its cells to prevent water from freezing in the first place.
Freeze-Protection Method #4– Stretch It Out
Unlike plumbing pipes, tree tissue is somewhat flexible. As water in the tree’s “pipes” and between internal cells freezes, there’s enough flexibility to let the tissue stretch or expand a little. This can prevent it from bursting.
Does Tree Bark Offer Winter Protection?
Bark does help somewhat to keep trees from freezing, but its real purpose is to protect trees from sun and heat. In winter, warm daytime temperatures and sunlight raise the temperature of bark and it expands. Heat in the bark is absorbed by tree tissue, helping sap to move and preventing it from freezing solid.
The color and texture of tree bark are related to a tree’s native range. In cold northern locations like Dayton, we often see trees with light-colored bark, such as birches and sycamores. This light-colored bark reflects heat and keeps winter temperature fluctuations in the tree to a minimum. In contrast, dark-colored bark absorbs more heat, but northern trees have evolved with furrowed and ridged bark that act as heat diffusers, releasing heat from the many facets and edges of the bark plates.
Trees are amazing. They have adapted to survive the huge changes in temperature that occur around them and protect their internal vascular system. But even these sophisticated methods have limits; not all trees can survive Dayton’s cold winters, partially because they don’t have the capability to protect their internal cells from long periods of freezing weather.
That’s why we recommend checking our Dayton Tree Catalog before choosing a tree for your property – we only list trees that can withstand a long winter freeze.