My favourite season tends to depend on my mood, but most often, my answer is autumn. It’s refreshing, a cool breeze washing away the heavy haze of summer. Paradoxically, it also feels warm, like shrugging into your favourite coat as you catch a whiff of someone’s wood stove in the crisp morning air.
I think it’s the colours of fall that give the days their warmth. The cool greens slowly fade into yellows, golds, russets and umbers. The forests are suddenly ablaze with a riot of hues.
In the boreal mixedwood forest where I live, the dominant colour is yellow. The poplars and birches sparkle with it against the sapphire September sky. Still, if you look closer to the ground, you can find a little more variety. The dogwoods (Corylus stolonifera) go purple, their leaves a lovely compliment to their reddish branches. The mountain maple (Acer spicatum), like the one pictured above, show quite a bit of variation, ranging from a pale yellow in individuals that are growing in the shade to brilliant orange and deep red for those lucky shrubs that are exposed to full sun.
But, where do these colours come from?
To a certain degree, they’re always there, hiding just below the surface, waiting for their curtain call. New, functioning leaves are full of chlorophyll, a brilliant green pigment that is packed into structures within the cell appropriately known as chloroplasts. These are the food factories for the tree, working throughout the growing season to transform carbon dioxide and sunlight into nourishing sugars via photosynthesis that are then funnelled into the rest of the tree. During this period, chlorophyll is constantly being degraded and replaced, keeping the leaves a brilliant green, overshadowing any other colours lurking within.
However, as the days become shorter and the sun’s intensity begins to wane, these factories shut down, using up their last stores of chlorophyll until there’s nothing left. Once the green is gone, the veil is pulled back giving other the hues a chance to shine. Carotenoids, a pigment that also plays a role in photosynthesis, remains, painting the trees with bright yellows and oranges. Some leaves also contain pigments known as anthocyanins, a watery dye that stains leaves with intense washes of reds and purples.
Just how bright and varied the fall palette is depends a lot of the weather. Warm, sunny days, followed by cool, but not frosty nights gives the leaves a chance to build up a lot of sugars and trap them within their cells. High sugar levels often results in greater amounts of anthocyanin, yielding more reds and purples, adding to the variety in the forest.
This year’s fall in the north woods has been just the kind we need for a spectacular display and the trees have not disappointed. Every day for the last few weeks, I’ve watched in awe as more and more of the canopy sparkles with colour, filling in the autumn landscape, a spectacular display against the clear blue skies.
Still, all good things must come to an end. Eventually, the nights get too cold and the days too short, signalling to the tree that it’s time to lock down for winter. The veins bringing moisture to the leaves close up and the branches seal over, cutting off the leaf’s lifelife. The late October winds howling off the lake will tear the foliage from their bases, sending them fluttering to the forest floor and returning their nutrients back into the soil to feed next year’s crop. However, those days are a little ways away, and in the meantime I plan enjoy nature’s yearly blaze of glory for as long as I can.