We all appreciate the shade trees give us in the summer and their beautiful leaves in the fall, but what about during the winter season? What are trees doing when it’s cold and they are covered in a blanket of snow?

When the days start getting shorter and the nights a little cooler, trees start a very important change. For a brief moment we get to observe their beautiful transition as the leaves change from hues of green to deep oranges, reds, and yellows, before they fall, leaving the tree barren for the winter months.

Why do trees lose their leaves in the fall?

One of the reasons deciduous trees lose their leaves in the fall is to protect them from the weight of snow and ice. If hardwood trees like, oaks or maples, kept their leaves all year long, there would be increased surface area available to accumulate snow and ice buildup during the winter. This added accumulation of snow and ice could burden the trees with the extra weight and cause their branches, and even their trunks in some cases, to break.

However, coniferous trees, like spruce and pine, don’t drop their needles in the fall. Their thin needles offer significantly less surface area for snow and ice to rest on, preventing them from having to withstand the weight of a significant snow cover.

Another physical difference that impacts a trees likelihood to hold the weight of snow is their shape. While deciduous trees have the bulk of their canopy at the top, coniferous trees are the opposite. Their cone shape prevents large quantities of snow from collecting on the branches. Instead, the small amount of snow that a coniferous tree does collect will act as a blanket and protect the needles from harmful rays of sun, while offering shelter to small animals like birds.

Acclimation

Beyond leaf loss in the fall, trees will also undergo an internal transformation to survive the freezing temperatures experienced during the winter. Trees that spend part of the year in sub-zero temperatures go through a two-stage process called acclimation.

The first stage happens in late summer, when the days get shorter and thus a tree’s exposure to light decreases. During this stage, light-sensitive photopigments respond by sending out signals to initiate dormancy.

The second stage gets activated by cold weather, typically temperatures of 10ºC or less. During this stage, saturated fatty acids found in the cell membranes get replaced with unsaturated fatty acids. These unsaturated fatty acids have a lower freezing temperature than their saturated counterparts, helping the tree to continue the acclimation process as the temperatures drop.

Cold Hardiness

Once the acclimation process is complete, the tree is ready to initiate cold hardiness. Cold hardiness is a plant’s ability to survive through sub-zero temperatures. While there are many different transitions happening inside the tree in sub-zero temperatures, the main goal is to protect the tree from internal freezing. Freezing can cause the water in the tree to expand, cause damage to its cells and result in the death of the tree. In an effort to avoid this, trees pull out water from inside their cells and store it in the apoplastic pathways between them. Although this water will still be prone to freezing, it will not cause harm to the tree.

Some northern species, like the black spruce and trembling aspen use this amazing adaptation process to survive temperatures as low as -80ºC!

Resources

Runtz, Michael. “Beating the Odds.” Natural History, Kendall Hunt Publishing Company, Dubuque, IA, 2012, pp. 201–214.