Winter would be that much sadder without the green that remains on some trees – like our beloved
Christmas trees conifers. They resist the peer pressure of other trees that drop their foliage as soon as it gets a little bit colder.
Here in the north, most trees drop their leaves to protect themselves and their investment in the sunlight capturing devices called leaves. Below the freezing point there is a strong risk for the leaf cells to be pierced by ice crystals. If you’ve ever put delicate herbs in the freezer and then thaw them you know what happens- the inner structure is destroyed, the leaves become limp (and in the case of living plants, dead!).
When the leaf colour changes, it is a visible sign of an internal breakdown of anything that’s valuable: nitrogen, carbon, minerals are recycled and moved inside the tree where they’re stored for spring. Chlorophyll is particularly expensive to make and is therefore quickly recycled – that’s why the green fades away and only the less important molecules are left behind.
Most conifers (firs, pines, spruce,…) do it differently. They keep their foliage all through winter and are therefore known as evergreens. That doesn’t mean however that they never drop their needles, it just doesn’t happen all at once. Just like human hair, their needles have a lifespan at the end of which they drop to the ground. Because those needles grow asynchronously, there are constantly newly growing needles and others that fall to the ground.
During winter, the growth slows down as the tree, like some animals, effectively hibernates. A frozen ground means less available water means less possibility to run photosynthesis. During the shutdown phase, water is moved from within the cells to the intercellular spaces. This increases the relative concentration of salts, sugars and other molecules within the cell, which in turn lowers the freezing point. On top of that, anti-freeze molecules like sugars and special proteins protect the cell from freezing over.
Finally, the needles of evergreen conifers are covered in a thick layer of wax. This stops the evaporation of water and prevents a drying out of the tree. Good in summer when things get too hot, but also good in winter, when all the groundwater is frozen.
Taken together – asynchronous foliage growth, anti-freeze measures and a wax coating – evergreen conifers are well prepared for winter. If it only wasn’t for us humans who come and cut them down simply because we’d, too, like some green in our homes when it gets cold outside.
This is our second advent on trees. The first one, on the history of the Christmas tree, is here.
(By the way, we’ve written previously about how some evergreens prepare for winter and how they know when to do it over here).
This article is part of our Christmas Advent’s Calendar. To check it out, go here.