I love autumn. Cool nights and sunny days. The forest is alive with colour—until the leaves fall to the ground, of course.

So, what makes leaves change colour and eventually drop from the tree?photoKevin Callan

Deciduous trees lose their leaves. Coniferous (cone-bearing) trees have needles that don’t change colour or drop off in the fall (except for the Larch/Tamarack tree).

Shorter days and colder nights are what trigger the deciduous tree to kick into its dormant stage. Winter is dry, cold and dark. The tree would be wasting precious energy to try and keep leaves healthy on the branches. To survive, the tree must shut down.

The sap of the tree begins to make its way down to the roots of the tree. Sap of a deciduous tree is over 80 per cent water, and when water freezes it expands. If it remained in the trunk and branches, the tree would split open and be vulnerable to disease. The roots keep a constant temperature, so the sap doesn’t freeze.

Without sap, the tree then loses its ability to go through the cycle of photosynthesis, which creates the green pigment called chlorophyll. The pigment you then see—what makes up the incredible fall colours—are the leaves “true” colours. Pigments in leaves called carotenoids create yellow and orange. Anthocyanins are pigments that cause red, pink and purple colours.

Once the leaves fall, decompose and create perfect potting soil for the tree to flourish, the tree then seals the spots on its branches where the leaves were attached.

In spring, the days grow longer, the temperature warms up and the sap makes its way up the trunk to give energy to the new foliage (unless it’s captured and boiled down into yummy syrup).photoKevin Callan

So, why don’t confers do the same (except for Larch/Tamarack)? First off, you’ll be glad they don’t. Trees give off an amazing amount of oxygen and soak up a lot of carbon dioxide. We need some type of plant to remain green throughout the winter. Conifers do just that. Their sap is thick with resin and is resistant to freezing. The sap remains in the trunk and branches, providing the needles chlorophyll. That's why they’re labelled “evergreens.” It’s also why the more north you go (longer nights and colder temperatures) the more coniferous trees you’ll see.

Take note, however, that if you’re camped out one winter night and you hear the spruce and pine crack around you, it’s likely minus 30 to 40 degrees Celsius. The sap of a coniferous tree may be thick and gooey, but it will still freeze under extreme cold temperatures.photoKevin Callan

There is nothing more unsettling than being curled up in your sleeping bag and hearing the forest around you snapping and cracking due to the frigid cold. That’s when you know it’s going to be a long night…



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