By Jennifer Meikle, Environmental Educator

​

Winter is a time for rest and hibernation. The shorter daylight hours and cold temperatures tend to make humans want to cozy up inside and bide our time until spring. This time of year, most animals are doing the same. While trees are rooted in the ground and can’t take refuge until the cold winter months are over like humans and animals do, they have adapted their own protections against the harsh winter elements.  
For any tree, the first line of defense against the cold is their bark. Bark helps prevent them from freezing and cracking in cold temperatures. The different textures, densities, and colors play various roles in reflecting light and dispersing wind and heat. The bark also holds air pockets that work as insulation for the tree. 

As the hours of daylight get shorter and shorter, the trees prepare. As the temperatures drop and the days grow shorter, trees produce a stress hormone called abscisic acid. Abscisic acid is responsible for causing big chemical changes in the trees that help them prepare for the oncoming winter season. 
There are two main types of trees that exist in cold climates: deciduous (broad leaves) and coniferous (needle leaves). Throughout most of the year, the broad leaves on a deciduous tree use sunlight to make sugars (energy) for the tree through photosynthesis. Another important process going on within the leaves is called transpiration. During transpiration, the leaf cells open and allow oxygen and water vapor to leave the cells, and as the water vapor evaporates, more water is pulled up from the roots into the tree. Water is also needed for photosynthesis. However, in winter when temperatures are low, the roots cannot pull frozen water from the ground. As late summer goes and autumn comes, the abscisic acid closes the connection between the leaves and the tree, cutting off their resources and causing them to drop to prevent water loss. Dropping their leaves also helps prevent damage from winter storms, as snow collecting on leaves is too heavy and can cause branches and trees to break.

The stress hormone is also what triggers dormancy, which is the slowing of the tree’s internal processes. Deciduous trees stop growing during this time, and conserve energy to survive through the winter. Dormant trees must rely on the sugar that they produce during the spring and summer. Due to chemical changes happening within the tree, during winter the sap inside of the tree loses a lot of its water content and the sugar content becomes more concentrated. This is why winter is a great time to tap trees for sap to make syrup. Not only does the tree use its stash of sugar to stay alive through the winter, but the high concentration also acts as an antifreeze. 

Coniferous trees have different adaptations that help them survive through the winter months. Most coniferous trees have needle-like leaves that they keep throughout the winter season. Their leaves are adapted for the winter weather both physically and chemically. Conifers keep their leaves because the long, thin shape makes them have a smaller surface area and the waxy outer coating helps retain moisture. The needles also contain a high concentration of resin, which is the conifers version of antifreeze which prevents the needles from freezing. This means they can retain water in their leaves through the winter, thus allowing them to continue photosynthesis through the winter as well.

Even with all these amazing adaptations, if the temperatures get too extreme, or the trees didn’t have enough time to prepare for the change, it can get so cold that the trees actually explode. Their sap contains water that can freeze and expand, putting enough pressure on the bark to cause an explosion. Younger trees are more vulnerable to this because they have thinner bark that is less resistant to temperature changes. If you have ever come upon a tree with a giant split or crack in the trunk- this may have been the cause. 

While some people might think that in winter trees die, this is so far from the truth. On a cold day in the middle of January, the trees are demonstrating their own form of hibernation and taking a cozy winter nap in their own way.