Winter is a tough time for trees, that have to cope with the double challenge of little sunlight and low temperatures. Animals either keep moving and eat more food than usual to survive, or they hibernate to avoid the cold season altogether. But what do trees do? They certainly can’t move around to keep warm. Sure, the roots are kept insulated underground by a layer of snow, and that is important to winter survival, but the exposed parts of a tree are not so protected. So they go through a process similar to hibernation called dormancy, and that's what keeps them alive during the winter.
During dormancy, a tree’s metabolism, energy consumption, and growth all slow down significantly in order to endure the harsh season, when water and sunlight are more scarce. The rate of growth is brought nearly to a halt. The stored energy is utilised to maintain the tree’s health, instead of being used for growth. Without cell division and growth, or the task of keeping its leaves alive, trees are able to survive through winter by maintaining only the “essential systems” until the spring.
Dormancy is closely associated with environmental conditions, and trees can enter a dormant phase before the onset of adverse conditions. For example, the length of days or night and the decreasing temperatures are used by many plants to predict the oncoming winter.
The preparation for this process begins in late summer, when trees start putting sugar aside and store it to use during winter. They fill their tissues with food until they can’t hold anymore, and they store it under the bark. Once they are full, they can start shutting down for winter.
And this is when deciduous trees start to loose their leaves. But before they can do this, they have to pump the chlorophyll out of the leaves and store it somewhere else, so that they can send it back to the new leaves the following spring. This process is what takes the green pigment away from the leaves, and make them turn yellow and red.
And while autumn colour seems to get all the attention, it’s what trees do later in autumn that is the most stunning, if harder to see.
Trees, like other living things, contain a lot of water within their cells. But what happens to this water in winter? Won’t it freeze and damage the trees like bursting water pipes in a house? Trees have some seemingly miraculous methods of coping with the extreme cold, and one of these is preventing living cells from freezing. There are three basic ways in which this happens.
The first one is to allow water to migrate out of the cells and into the spaces between the cells, so if this water freezes the cells don’t freeze with it.
The second way is to sweeten the fluids within the living cells. Come autumn, a tree converts starch to sugars, which act as something of an antifreeze. The cellular fluid within the living cells becomes concentrated with these natural sugars, which lowers the freezing point inside the cells, while the sugar-free water between the cells is allowed to freeze.
The third coping mechanism is altogether different. The liquid cell contents become so viscous that they appear to be solid. This mechanism is triggered by the cellular dehydration that results from the first two mechanisms and allows the contents of the tree’s cells to avoid crystallising.
All three cellular mechanisms are intended to keep living cells from freezing and to avoid damage, and that’s the key to survival. However, this is only aimed at living cells. A large portion of the trunk of trees, in fact, consists of dead cells. These cells can and do freeze in winter, but that is not an issue as long as the living cells stay warm.
Deciduous and evergreens
The two major tree types, deciduous and evergreen, adopt very different strategies to cope with winter.
Deciduous trees shed all their leaves. A chemical called abscisic acid is released, which signals the leaves to detach so trees do not expend energy in keeping them alive during the winter.
This can seem absurd, given that by doing so they can’t photosynthesise and therefore generate fresh energy for five to six months of the year. However, there’s an upside to losing foliage, because it ends the loss of moisture that occurs through leaves.
Another reason to drop them is that leaves rely on water from the tree roots. But, to prepare for the dry winter ahead, trees need to save as much water as possible. Without leaves, that’s one less thing to hydrate and they can save more water.
The dropping of leaves also means deciduous trees can cope better with high winds and heavy snow fall. Without leaves to act as a large sail in wind, branches can bend and flex, hopefully without too much damage, and the snow has nowhere to land.
Lastly, together with their leaves trees also get rid of waste.
To ensure they don’t loose water to transpiration over winter, conifers cover the exterior of their needles with a thick layer of wax. The fact that their leaves have a small surface area also helps in this purpose, and allows conifers to keep their leaves over winter. Loosing water would in fact be tragic, as the tree wouldn’t be able to replenish supplies from the frozen ground, it would dry out and die of thirst.
While needles are not as efficient as deciduous leaves, the fact that conifers can continue to photosynthesise year-round compensates for this.
Another advantage of keeping the needles is that, come the spring, they can start photosynthesising quicker than the deciduous trees.
Conifers also get rid of waste material before winter by dropping those needles that are damaged and don’t work well anymore. They always keep a few years’ worth of needles though, and grow some new ones once spring comes.
Sometimes it’s just too cold
While trees have evolved amazing strategies for withstanding the winter cold, sometimes it gets so cold that they just can’t cope and can explode. During spells of extreme cold or when trees haven’t had time to acclimatise, the sap inside a tree can begin to freeze. Sap contains water so it expands when frozen, putting pressure on the bark, which can break and create an explosion (similar to rocks breaking off after water has frozen in cracks).
At the beginning of spring, when the cold season comes to an end, trees are “reactivated” by warmth and sunshine, and start growing and producing energy again.