What happens to trees in winter
5th Dec 2019
With their branches naked, creating a stark outline against the winter sky, deciduous trees turn into sort-of ghosts in winter. At this time of year, they're in the tree version of hibernation: dormancy. As the word suggests, it's a kind of sleep, during which the tree protects itself from freezing weather and readies itself for waking up in spring.
In common with hibernating animals, trees begin preparing for winter in autumn. Some produce lots of fruit and seeds, which animals gather and spread, offering good odds that the tree will have some offspring somewhere, even if it doesn't make it through winter itself. They also work on producing the buds for next year's leaves, which have a protective coating to see them through the season.
In the early stages of dormancy, deciduous trees drop their leaves. There are several advantages to this strategy. Firstly, the tree won't be making food in winter, so it doesn't need leaves to photosynthesise. Keeping them would be a drain on energy in days short on sunlight. For example, leaves need to be constantly pumped with water, which then evaporates into the air in a constant cycle. By dropping leaves, the tree can take a break from this task.
Then there's the fact that if water in leaves freezes, it damages the leaf by bursting cell walls (like a burst pipe), so leaves on some trees wouldn't survive periods of freezing temperatures and would be lost anyway. So, from late autumn, the tree produces a chemical called abscisic acid to tell leaves to fall off. This chemical is also responsible for stopping growth, in both deciduous and evergreen trees, allowing the tree to conserve energy in the months of limited sunlight and cold.
Evergreen trees have a number of strategies for keeping their leaves alive in freezing weather. Thick, waxy surfaces are less susceptible to frost damage, and some pines even produce their own natural antifreeze from sugars that lower the freezing point of water. Other coping strategies employed by conifers include pliable cell walls that stretch to accommodate freezing water, and prevention of the formation of ice crystals in leaves through dehydration, which renders the liquid in leaves like glass - supercooled but not technically solid.
Fine needles also lose less water than broad leaves, so they are less of a burden on the tree. Conifer leaves have a disadvantage, though, in that they aren't generally as efficient at photosynthesis as broad leaves. By continuing to photosynthesise in winter, though, they make up for this (albeit at a slower rate).
This doesn't mean evergreens never lose their leaves. They do get old and drop off, especially in winter and early spring. The scientific term for this is senescence. Old leaves on evergreens often get spotty or start to look less than fresh, before they fall. Read more about evergreens losing leaves.
When the tree detects lengthening daylight hours and a pattern of rising temperatures, it knows it's ready to leave dormancy and burst those overwintered leaves from their buds. How exactly it works out that spring has come, scientists are still trying to work out. What this means for tree planters, however, is that it's no longer safe to plant bare root trees and hedging, as the plants are in active growth again and so exposed roots run the risk of the plant drying out. Container-grown is the way to go, from about mid-March.
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