How Do Conifers Survive the Cold?

Snow on Noble Fir

December is the month of the fir, when it stands resolute, defying the wind and cold.  May is the month of the oak, when it springs to life in an outburst of vivid green.  October is the month of the Maple, when it blazes with the glory of the sun.

But in December, when the fickle oak and failing maple stand barren and leafless in the deepening cold, the fir flourishes, spreading its verdant needles in a steadfast display.  Even when winter covers its branches with snow, underneath the fir endures, a faithful testament to the promise of life.

Ken Denniston - December 1991

The recent cold weather in Oregon got me thinking about just how conifers are able to survive freezing temperatures. Even in the mountains where temperatures drop well below 0° Fahrenheit, conifers endure the cold, often showing no ill effects. When temperatures warm in spring, they, in turn, spring to life and put on new growth.

Ice on Douglas Fir

Freezing is not good news for most living things. It kills many plants and most animals. When living cells turn to ice, they are usually destroyed. Frost kills many crops and other plants. How is it, then, that some trees can survive the freezing cold temperatures of winter?

If you search the Internet and have the patience to wade through some very technical scientific papers, you will find a mountain of information about conifer cold hardiness. Here is a short, summary of what I was able to find and unscramble.

Conifers have developed two mechanisms to deal with winter cold: Freezing tolerance and freezing avoidance. 

Freezing tolerance: Plant cells can tolerate freezing by moving water out of the cell where it freezes without damaging the cell. The concentrated sap in the cell has a lower freezing point and remains in liquid form. This process also leaves the cells dehydrated. To survive, they must also be tolerant of dehydration.

Freezing avoidance: Plant cells can avoid freezing using a process called supercooling. This is a process in which water drops below its freezing point of 32° F. without turning to ice. How is this possible? Normally, when water freezes at 32°, it requires a seed particle. The ice crystals can then begin to form around the seed. This seed could be a tiny spec of some impurity. Water with no impurities can cool to about -40° and still remain liquid. Plants survive by maintaining pure water inside their cells.

Of course, there are limits to how well these strategies can protect conifers from freezing cold. This is why you won’t find any conifers growing on the north coast of Alaska.  

Not all conifers are equally cold hardy. You will see the cold hardy ones growing at the timberline in the mountains. Most conifers that you see growing at lower elevations cannot survive the cold temperatures near the timberline in the mountains. The US Department of Agriculture has developed a hardiness zone system and mapped these zones for the US, shown on the map below.

Even cold-hardy conifers can be damaged by extreme cold. The trees acclimate to the cold in the winter, but rapid temperature changes can still cause damage. New growth on trees and seedlings are more vulnerable to freezing damage. If a cold snap comes in the spring after new buds begin to grow, they can be killed. This happened to the new growth on many subalpine firs around Mt. Hood in the spring of 2016.

Note that there is considerable genetic variability in cold hardiness within a species. Trees adapt to the conditions where they grow. For example, Douglas firs growing at 5000 feet on Mt. Hood are more cold hardy than those growing in the valleys of the Coast Range. If you gather seeds from the low-elevation trees and plant them up at 5000 feet in the Cascades, they will not be able to thrive in that cold environment.

Hoyt Arboretum - January 12, 2017

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More info.
“Mechanisms in Frost Survival and Freeze-Damage in Nature” by Marja-Liisa Sutinen, et al.  in Conifer Cold Hardiness, Ed. by Francine J. Bigras and Stephen Colombo, P. 90ff. Available here.