When winter hits, we stay bundled up indoors, birds fly south, and many other animals hibernate. Trees have their own way of surviving the winter, and their processes are more complex than you might imagine.
Deciduous trees rely on a process called dormancy, similar to hibernation, in order to survive the winter. During dormancy, all tree processes — including growth, energy consumption, metabolism, and the like — slow down. Because trees don’t produce food during the winter, they don’t have the energy that it takes to maintain their leaves. This is where leaf shedding comes in.
When the time comes, deciduous trees produce a chemical called abscisic acid (ABA) in their terminal buds, where their stems connect to their leaves. This chemical triggers the tree to shed its leaves and to halt all growth to save energy. Think of an animal that has stored up food as fat, and is now living off of that reserve during hibernation.
Unlike deciduous trees, coniferous (or evergreen) trees don’t lose their leaves during the winter. In fact, that provides one of the easiest ways to tell the difference between coniferous trees and deciduous trees. Another difference lies in tree appearance: Whereas deciduous trees have wide leaves, coniferous trees have small, thin needles
The small surface area of these needles prevents snow from accumulating on them, weighing them down, and eventually causing them to break off. The needles’ characteristics also reduce the amount of water evaporation, which is necessary for coniferous trees to retain enough water to continue to conduct photosynthesis when bad weather hits. Less surface area means fewer pores through which water can escape; and the waxy polymer called cutin that coats the needles further prevents water loss and freezing.
Conifers are also aided in their winter survival by their narrow tracheids, or tubes that carry water throughout a tree. Because conifers’ tracheids are narrower than those of deciduous trees, the problem of air bubbles forming, bursting, puncturing the tracheid, and ultimately blocking the waterway is reduced. In conifers’ narrow tracheids, only small air bubbles can form, which are more likely to be reabsorbed than to burst.
A few other strategies exist to help both deciduous and coniferous trees keep their cells from freezing:
- Trees adapt their membranes to make them more elastic, allowing water to migrate from within the cells to the spaces between the cells. The cells then compensate for any pressure that this water puts on them by shrinking and taking up less space.
- Trees transform starch into sugars that act as a sort of antifreeze. The fluid inside the tree’s living cells becomes concentrated with these sugars, and this in turn lowers the freezing point of the cells’ contents and prevents them from freezing. However, the liquid between the cells does not contain any sugar, meaning that it will freeze. The pliable nature of the membranes keep any ice crystals that form from damaging the cells, as they can bend to accommodate the frozen external liquid rather than being punctured. When this external water freezes, it also releases small quantities of heat energy that further prevents the internal liquid from freezing.
- The first two strategies cause dehydration within the cells, which triggers the tree’s third tactic. The cells’ contents thicken so much that they enter a “glass phase” in which they appear almost solid, thus preventing ice crystals from forming.
How to Help Your Trees Survive Winter
As you can see, trees are incredibly self-reliant when it comes to adapting to winter weather. However, taking a few steps to make their work easier never hurts. To get started, read about how to prepare your trees for winter; and, as always, give Washington Tree Experts a call if you have questions or need any help!
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