Trees avoid damage from freezing temperatures: Part 2

The science behind how trees prepare for the cold winter months with chemical changes.

After trees lose their leaves, they begin to go through several chemical changes in preparation of the cold winter months. The process is triggered by a gradual decrease in temperature, rather than a sudden drop. Trees that experience a sudden drop in temperature may sustain damage to its leaves, needles or fruit. In contrast, the gradual cooling of temperatures allows the tree to prepare for cold temperatures by initiating several processes that will limit injury due to freezing temperatures.

Trees produce leaves for the following year 

Throughout the summer, every tree is preparing for the following years growth by producing buds that contain all the required components including miniature leaves and twig materials. Some trees also produce buds for flowering structures, ensuring that flowers may open at nearly the same time as the buds containing leaves. It is important for trees to produce these structures when the rate of photosynthesis is at its maximum, as the production of buds requires an increased amount of energy. Once the buds are complete, the tree begins to store extra carbohydrates and sugars in the root cells to ensure that the tree has the energy necessary to open the buds and support leaf elongation the following spring.

Trees put their “coats” on 

Trees have insulating tissue just under the bark in the outer cambium layer. The cambium layer is the living layer of tissue under the bark that produces new wood, contains specialized cells and acts as a transport system of water, sugars and carbohydrates for the tree. The cork cambium contains specialized cells that are non-living at maturity and contain a fatty substance, called suberin that acts as an effective insulator. 

Protecting cells from freezing 

Trees initiate a change in the living, conducting cells in twigs, branches and the trunk to prepare for below freezing temperatures. The liquids that occur in the cells of these portions of the tree, called protoplasm, develop a more concentrated mix of sugars and salts to lower its freezing temperature. The resulting freezing temperature of the protoplasm helps determine the tree’s USDA Hardiness Rating, when compared with the minimum freezing temperature of a region.

A tree’s version of hibernation 

While the term hibernation refers to animals, trees undergo a similar process. When the ground freezes, the tree goes into a state of dormancy, where nearly all growth processes and functions slow to a stop. This dormancy allows the tree to conserve the energy it produced during its summer photosynthesizing period for the opening of the buds the following spring. The only function a tree continues in the winter months is respiration, as trees require oxygen year round. Respiration occurs very slowly and infrequently, reminiscent of the slow snore of a hibernating animal. 

To learn more about changes your tree may be undergoing as it prepares for winter contact Michigan State University Extension.

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