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Trees and Cold: Where Are Their Long Johns?

Posted: January 15, 2015

When temperatures drop, trees have to have a plan to handle freezing water. They just can’t add a coat or sweater.

It’s winter in the northern climes. For many people, it’s time to hibernate, curl up in front of a fire, drape yourself over the nearest working radiator, layer if you have to go outside, stay inside if you can, hope that the heat is on at work so you can justify a low thermostat setting at home. As mobile denizens of the region, people and animals find ways to hide from or tolerate the weather. What about those rooted in the ground? How do they survive cold weather?

Logically, layers of bark on trees and woody shrubs do provide some protection, but not much. When temperatures drop, trees have to have a plan to handle freezing water. They just can’t add a coat or sweater. They don’t have to deal with wind (wind chill is human-construct), which makes us feel colder as water evaporates from our skin; nonetheless, cold is cold. How do trees adapt?

We all know that the deciduous trees lose their leaves, which are not designed to take cold temperatures. They enter a phase of not quite hibernation called dormancy brought on by a combination of shorter days and falling temperatures. They’re not growing. They’re not moving water and nutrients up and down. But they still have challenges. And the evergreen trees, that have leaves all year round, photosynthesizing, when temperatures allow, requires water and moving nutrients around.

There are two big risks to trees in frigid weather. Water can freeze within living cells and rupture them, and air bubbles can form in the xylem (the part of the tree that moves water from the roots up) which can cause a break in the water column (embolism). When there is a break in the water column, trees can no longer pull water up to their photosynthesis factories, the leaves.

Remember from your elementary science class, water expands when it freezes. And ice does tend to have sharp edges. Imagine the water in a living cell freezing. Ice crystals would take up more space and rupture the cell membrane, causing the cell to die. Trees have to have some ways to prevent this. The prevention process for all trees starts in the fall when they change the water content of their cells. They start by taking in more sugars and creating a highly saturated solution within the cell, shrinking the cell somewhat -- the tree now has an antifreeze to prevent ice crystal formation. It takes time for trees to complete this process, a period of acclimation. Even trees that are good at creating antifreeze experience cell death with an early cold snap or late spring frost. As the cells create this antifreeze, water moves out of the cell and into the spaces between cells, where it will not cause damage if it freezes. Evergreen trees use this technique to keep their leaves year round, along with small openings in the needles (stomata) that allow gases in (and sometimes water out), and waxy coverings that help to protect the needles from frost damage. Deciduous trees use this technique to a lesser degree because they lose their leaves in the fall thereby preventing winter damage to the leaves.

Even as conifer trees have adapted ways to retain their leaves and to reduce ice crystals throughout, they may still need to move water on those warm days when some photosynthesis takes place. How do they do this with their roots in frozen soils? In Pennsylvania, most of the time forest soils don’t freeze, at least not deeply. So there is some access to water; however, they still need a strategy as freezing of the water column can cause air bubbles to form from gases dissolved in the water. When the water thaws, these air bubbles remain and may rupture the water column. In the process of moving water, trees are dependent upon water’s cohesive nature (water molecules bond to each other) to aid movement against gravity. Trees living in cold regions, especially conifers, have evolved water conducting cells that are very narrow hollow tubes (narrower than those in deciduous trees) that enable the water to move at lower pressure and thus reduce the potential for embolisms.

It turns out that many of the evolutionary adaptations that allowed trees to respond to drought, allowed them to adapt to colder regions of our world. It’s all about water and its presence or absence. So, even if it is cold outside, know that your trees are doing well and waiting to share their glory with you come the warmer temperatures of longer days in the spring.

MinuteEarth has a great video about this process.

The Pennsylvania Forest Stewardship Program provides publications on a variety of topics related to woodland management. For a list of free publications, call 800 235 9473 (toll free), send an email to RNRext@psu.edu, or write to Forest Stewardship Program, Natural Resources Extension, The Pennsylvania State University, 416 Forest Resources Building, University Park, PA 16802. The Pennsylvania DCNR Bureau of Forestry and USDA Forest Service, in Partnership with Penn State’s Department of Ecosystem Science and Management, sponsor the Forest Stewardship Program in Pennsylvania.

Contact Information

Allyson Brownlee Muth, Ed.D.
  • Forest Stewardship Program Associate
Phone: 814-865-3208