But what do trees do in winter ?13 February 2018
It’s strange, a tree, in winter. Motionless, dark, emaciated. Alone, against a background of white and dull sky. Subjected by the hostile assaults of wind, cold, rain and snow too. What is it doing ? How does it spend time ? Is it waiting ? But what exactly ?
It’s strange, a tree, in winter. Motionless, dark, emaciated. Alone, against a background of white and dull sky. Subjected by the hostile assaults of wind, cold, rain and snow too. Its branches like those of an unfortunate beanpole. In spring, it exults. Its buds are bursting, adorned with a thousand leaves, the green is announced, it is the vegetable celebration. But in winter ? What is it doing ? How does it spend time ? Is it waiting ? But what exactly ?
1. It stops its growth – Plants are poikilothermic organisms, whose temperature varies with that of their environment. Against the harshness of winter, trees have developed survival strategies. The first is to gradually enter dormancy, encouraged by the increasingly cool temperatures (between 8 and 12° C), even cold. How ? By interrupting the division of their cells ! Those are then unable to use the nutrients and other growth hormones brought by the roots via the sap, because some molecules essential to their functionning (the enzymes) are then inhibited by the temperatures. A a result, when winter comes, trees, whether they are deciduous (those that lose their leaves) or persistent, stop all growth activity. They give the impression of being dead ; as if their life was frozen during the bad season…
2 . It makes antifreeze – To face the two coldest months of the year in France, January and February, trees harden. Not in the literal sense, but by developing processes of resistance to cold and frost. These are put in place in autumn, with the gradual fall in temperatures, and allow trees to gradually lower the freezing point of their cells, so that they do not burst under the freezing effect, even at very low temperatures. « It is a very complex process. It consists of several progressive molecular and cellular changes that results from biochemical reactions that activate, inhibit and reactivate constantly according to the fluctuations of the outside temperature » enthuses Thierry Améglio, INRA researcher. Among them, the creation of « antifreeze ». Thus, as soon as the temperature drops below 5°C, the tree synthesizes enzymes that will degrade the starch (large sugar molecules) – made by photosynthesis and stored in summer in the bark and the wood – in more smaller soluble sugars with high « anti-freeze » potency. But as soon as the temperature rises above 5°C, (in the afternoon, even in winter), the starch is reconstituted because the antifreeze proteins fuse together to reform it. Then, when the temperature relapsed in the evening, it is hydrolysed again, and so on. Listed in the genes of the trees, this ability of hardening varies among species : if the walnut tree can withstand to -20°C maximum, needles of Pinus sylvestris pine still survive by… -80°C !
3 . It repairs the damage if necessary – From end February, the tree is preparing for the arrival of beautiful days. There, under the bark, begins a process of repair, or even production of new transporter vessels (the xylem) of raw sap (water and mineral salts). An essential step, because in the leaves, photosynthesis transforms this raw sap into elaborate sap, then redistributed from roots to the whole tree. But, under the effect of freezing, the air dissolved in the water and circulating in the vessels forms bubbles which, if they are large enough, remain stuck and obstruct the passage of the sap. We talk then about winter embolism. In order to fix them, the plant appeals to water and sugars in the vessels, which genrerates a pressure chasing air bubbles.
4 . It prepares for the arrival of good weather – from January to the end of February, sometimes even until March, the growth of trees, at their buds level, is revived. But it is so slow that it remains invisible. And paradoxically, it is the cold that lifts this phase of dormancy. Indeed, to get out of its lethargy, the tree must have accumulated cold hours (temperatures below 7°C) for three to four weeks. As soon as its quota is reached, often at the end of December, it enters then a phase of growth, limited by environmental conditions : it is called the « eco-dormancy ». This phase is prolonged as long as the tree has not accumulated, this time, enough hot hours, more than 7°C. It is mainly determined at the cellular level, the sap not circulating in the tree yet. In bud tissus, the multiplication of undifferentiated cells has started again. They divide, grow, differentiate and imperceptibly ensure the growth of the bud.
Source : Science&Vie – December 2015
Picture : © Christophe Deschanel – Bertranges forest from the sky – November 2017