Have you ever wondered how trees survive the bitter cold? Trees are rooted and cannot move, of course—which means they have nowhere to hide from the wind, snow, and below-freezing temperature. While I struggle every year to fight off the cold, I can’t help but wonder: how do trees survive and adapt to the wintery condition and come back year after year?
The two main challenges trees face in the winter are lack of liquid water and cold. To protect themselves from these conditions, trees have two main strategies: dropping leaves or having needles to limit water loss, and changing their cellular structure to prevent ice from forming inside the cells.
The Water Challenge: Dropping Leaves and Growing Needles
During winter, the ground freezes, so trees can’t access the frozen water through their roots. They must conserve the water they already have. Every drop counts!
Photosynthesis occurs mainly through the tree's leaves. During this process, they absorb sunlight and carbon dioxide (CO2) to create sugar. This is done through small pores on the leaves called stomata. Opening stomata allows trees to absorb CO2. However, this comes at a cost—opening the stomata also lets out water vapor from the inside, which leads to water loss, also known as transpiration. Trees face the dilemma between intaking energy and conserving water.
Photo credit: ScienceFacts.net
Broad-leaved trees such as maples and birches have big leaves and many stomata, which means they’re better at photosynthesis, but also lose water faster. During winter, many broad-leaved trees shed their leaves to prevent water loss, even if it means they cannot get energy from photosynthesis during this period. (This also explains why trees in tropical rainforests are mostly broad-leaved—they have plenty of water to go around from the moist climate, but not enough sun; each must compete for sunlight in the dense jungles and maximize photosynthesis!)
For needle-bearing trees such as pines and spruces, their tiny needle leaves have less surface area, which means fewer stomata. They’re excellent at minimizing water loss, but are also less efficient in photosynthesis and CO2 absorption. That’s why needle-bearing trees get to keep their needles in the winter. The waxy layer on needles also serves to prevent water loss.
I understand it like this: broad-leaved trees are productive in the summer, but this advantage turns against them in the colder months, so they temporarily shed their leaves in winter. Needle-bearing trees are year-round slow workers, so they can continue business-as-usual even during the harsh winter.
Against the Cold: Changing Cellular Structure
Just like humans, trees have “blood”—a liquid that flows through its body to transport nutrients, called sap. Without adaptation, when temperatures drop below freezing, the sap inside tree cells would freeze. This would lead to internal punctuation of tree cells, killing them.
The good news is, trees adapt to this condition by filling their cells with sugary sap, which acts as antifreeze. They do so by converting the energy they get from sunlight into sugar, and storing some of it in their leaves before winter. (This is why maple trees, which are native to the northeastern part of North America, have sweet sap that can be tapped to make maple syrup!)
I hope you found these mechanisms fascinating! Trees are well-adapted to their geography and climate, and have developed their own ways of dealing with the cold. When winter comes around this year, maybe on your morning walk in the woods you will, too, marvel at the intricacy of tree’s cold-adapting strategies.
For additional information on how trees adapt to a cold climate, check out these sources:
Video: How do Trees Survive Winter? By MinuteEarth - A fun, illustrated, 2-minute video that explains everything you need to know on this topic!
Video: Plant Structure and Adaptations by Amoeba Sisters - A slightly longer video that dives into plant structures, photosynthesis, and how they conserve water.
River Falls School Forest Visit in January 2022
Photo credit: Nikki Henger