A tree in wintertime

A tree in wintertime
Credit: Jardin botanique de Montréal (Gilles Murray)
A tree in wintertime
A tree in wintertime

What do trees do to get through the cold season? They start preparing in the month of August!

It’s a fair question: how do trees manage to be reborn after going through so many days – and nights! – at temperatures well below zero? The issue also involves other plants like perennials, bulbs and biennials, but let’s confine ourselves to trees here. And trees from temperate zones, since the story in the tropics would be quite different.

The first warning signs of change become evident in August. If you find that winter often arrives too early, see what the trees have to say about it! In August, the days are getting shorter and signaling the tree that it’s time to prepare for the cold. Two essential things are going to happen: that year’s fragile branches will lignify (become woody), and the buds of future leaves and flowers will form. These phenomena combined constitute what’s called the hardening off. Thus, unlike what you might imagine, trees are starting on a certain form of rest in midsummer. They gave it all they had in terms of growth during the first weeks of warmer weather. That rapid growth (spring-summer), its slowing down (summer) and its stopping (fall-winter) form the rings that allow us to calculate how old a tree is.

Ready for winter

Ideally, the drop in temperature will be gradual throughout the autumn to give the tree the time it needs to complete its winter preparations. It goes to sleep, it slumbers, its growth slows down seriously: it goes into dormancy. But under the bark, things are happening! Complex processes are set off. Since the temperature of a tree varies from that of its environment1, it has to protect itself from frost; otherwise, the water still present in its cells will freeze, breaking the cell walls and leading to death. Hence, once the leaves have fallen, the tree manufactures antifreeze by synthesizing enzymes that break down starch – those long chains of simple sugars produced by photosynthesis – into smaller and more soluble sugars that lower the freezing point of the water in the cells and allow the tree to resist very cold temperatures. Not sure? Put a container of sugar water in the freezer and observe the result. However, if a mild spell occurs, the sugar molecules group together and re-form starch. This cycle repeats as often as is necessary: from day to night and from one day to the next according to weather fluctuations.

Antifreeze for trees

The presence of antifreeze is not a guarantee for a damage-free winter. Towards the end of the cold season, the tree repairs the areas that might have been injured during the freeze-thaw cycles. It may happen that under the effect of the freeze, the air dissolved in water forms bubbles that prevent sap from circulating freely: it’s a winter embolism! This repair process begins with the formation of new vessels – the xylem – that will transport the raw sap. To clear its vessels, the tree makes use of water and sugars: the pressure thus created drives out the air bubbles.

Time spent in complete lethargy is essential for resumption of growth. It goes into effect quietly, gently, without any obvious sign, once there have been sufficiently frequent warmer temperatures (above 7 ⁰ C). When the bud tissues start to divide again, the tree will open and allow us to see the new leaves – or the new flowers, depending on the species.

1. They’re poikilothermic organisms, as opposed to homeothermic organisms, which generate their own heat.

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