Do Trees Need Mitts?
All this cold weather and time spent in front of the wood stove has got me thinking about those creatures who have no way of escaping the frigid temperatures. Deer, Moose, Coyotes, and even Chickadees each have their own way of managing and adapting to the often wild temperature fluctuations we deal with in North-Central Alberta. My way of managing the cold is to throw another log in the stove and turn the kettle on for a hot cup of tea. The horses out in the pasture have a winter coat of hair that can stand up at different angles, providing a number of different levels of insulation. My old dog, Levi, has grown a thick coat of black hair and has no problem lying in the sun and having a nap in -20 C. But, as I hold my cup of tea and gaze out the window onto a frozen landscape, the Maple trees along my driveway catch my eye as the samaras (seeds) blow around in the icy breeze. How do trees adapt to this cold weather?
We join the more than 40 percent of the world's land area that can achieve winter temperatures of less than -26 C (a great accomplishment, I know). If trees did not react to cold and freezing temperatures, they would die. Trees manage their cold tolerance on a much less visible scale than most living beings, they manage it largely on a cellular level. Cold damage is avoided by converting large materials such as starches into smaller components such as sugars, thus reducing the formation of ice crystals. They also prepare for a dehydration phase by moving water out of their cells. This process is called 'Hardening'. To put it simply, ice crystals can damage the cell walls so the tree does everything it can to avoid their formation.
Trees prepare for cold at around 5C, their 'antifreeze' stage is around -2C, 'dehydration' stage is around -6C to -15C, 'supercooling' is -15C to -30C, and 'freeze dried' is -30C to -40C. Trees only harden themselves as much as is needed based on average temperature conditions. this is part of what compromises 'Zone Hardiness' (Vegreville sits right on the edge of Zone 3a and 3b). Different tree species have different cold level tolerances. American Elm (very common in Vegreville) can survive to -40C (Zone 2a), Black Willow can survive even further, down to -73C. Most Poplars, as well as Tamaracks, and some species of Spruce are considered Zone 1 hardy, and will survive right up to the tree line in Northern Canada.
This little Burr Oak is considered Zone 2a and will hopefully thrive on our little acreage!
The temperature fluctuations that we have been seeing in recent years, especially the ones that occur in late fall and early spring, can be fairly damaging to a tree, as one day of warm temperatures can reverse one weeks worth of cold temperature hardening. Not only does this deplete a tree's resources, but can leave it unprotected from rapid temperature drops. The cell walls can literally explode. This is what we refer to as "Winter Kill'. Most Cedars (Thuja) are Hardiness zone 3a or 3b, right on the cusp of being able to survive in Vegreville and Minburn county. Winter kill is very common in Cedars, especially if the weather has been colder than average, or less than steady.
What can be done to help trees out with cold weather? Number one is proper site selection. Areas of increased frost should be avoided, as well as areas prone to strong air currents. And Number 2 and the most important thing to consider is proper tree selection. Know your zone and the hardiness rating of the tree you are planning to plant!