Photo by Bob Taylor

Why leaves change colour

Share This Story!

When fall arrives and the leaves begin to turn, it could make anyone—even cottagers—wonder what all the fuss around summer is about. In 2009, Cottage Life featured an in-depth piece on the season’s grand finale, boldly labeling it “The greatest show on earth.” But when you consider just how awe-inspiring it is, even for those who witness the transformation year after year, it doesn’t seem like an overstatement.

In fact, only northern forests are able to pull off this spectacular show. While forests in warmer climates may contain a more diverse set of species, their trees cannot match ours when it comes to colour. So while you may be feeling a little depressed about the shorter days and crisper temperatures, when the trees’ quiet green is transformed into a vivid palette of reds, oranges, yellows and golds, you’ll no doubt feel differently. So how do they do it? Although scientists haven’t been able to answer all of the “whys” involved in the process, here are a few they’ve managed to figure out.

The science behind the scenery:
Chlorophyll, the pigment that gives leaves their green colour, is necessary for photosynthesis. In this chemical process, chlorophyll absorbs energy from sunlight, which is then used to transform oxygen into carbohydrates such as sugars and starch. These sugars are what drive trees to fruit and flower.

Fast-forward to fall: As the days grow shorter and temperatures drop, chlorophyll production slows down, and eventually, it stops altogether. In a podcast for the Ontario Science Centre, scientist Donna Francis puts it into terms a cottager can relate to: “It’s sort of like closing up the cottage for winter time.”

But the degradation of chlorophyll isn’t the only factor that contributes to fall’s splendor. “As the chlorophyll stops being produced,” Francis says, “there are helper molecules that have been there all along,” which work with the chlorophyll during spring and summer as a secondary absorber of sunlight. While there’s too much chlorophyll for these “helper molecules,” known as carotenoids, to be revealed in summer, when the chlorophyll begins to break down, the carotenoids take centre stage.

Anthocyanins create the radient red leaves found among fall landscapes

Cartenoids, which also produce colour in things such as corn, carrots and daffodils, are the molecules responsible for leaves turning various hues of yellow, brown, and orange. What about those shades of red and purple that really stand out? They’re created by a set of molecules known as anthocyanins, which are formed in leaves in autumn. Anthocyanins also produce colour in cranberries, red apples, concord grapes, strawberries, and plums.

How weather affects the process:
The intensity of these colours depends on the weather conditions both before and during the breakdown of chlorophyll. There are a number of factors that influence how brilliant our forests appear, but temperature and moisture are the main ones.

Chlorophyll production may not dominate in fall, but it is still present—at least enough for trees to continue absorbing sunlight and producing sugars. These sugars hook up with proteins in the lead to create the anthocynin pigments mentioned earlier, which tint the leaves the most radiant reds, scarletts and crimsons. This process is spurred by a succession of warm, sunny days in conjunction with cool, crisp (but not freezing) nights, so it’s under those conditions that you can expect to see the most eye-popping autumn displays.

You can always count on seeing shades of yellow in autumn

While an extended drought like the one we experienced this summer can cause colours to come early and weakly, too much rain is equally detrimental. Heavy rain not only means less sunlight, it also cuts down on the tree’s oxygen supply. This leads to fewer sugars and—you guessed it—more muted reds. You can however, always count on the strength of yellows and golds. Because cartenoids are always present in leaves, these colours are quite consistent from year to year, regardless of weather conditions.

What determines colour:
Tree colour is generally determined by species, but there’s a bit more to it than that. While you can expect sugar maples to turn an orange-red, dogwoods a purply red, white elms yellow, and so on, within each species’ characteristic colour, a tree’s individual genetics will establish its exact hue.

As for the leaves that are green, yellow and red all at once? That’s merely evidence that the process of chlorophyll disappearing and carotenoids and anthocyanins appearing isn’t always synchronized.

The best places to see leaves in Ontario:
While anywhere in cottage country is sure to yield some pretty spectacular views, there are certain destinations that are definitely worth mentioning. Whether a train ride through the province’s wilderness region or a walk along tree canopies peaks your interest, our list of fall tours covers the best Ontario has to offer.