Fall foliage: the science behind fall's changing leaf colours

Photo: iStockphoto.com/Wlad74

As adapted from Pure Green Magazine

As winter looms on the horizon yet again, some may dream of whiling the days away in a lush, green, tropical paradise. But don't pack your bags just yet, for every fall, those who are lucky enough to live in temperate climates witness an amazing transformation of the forest into a vivid palette of magnificent reds, oranges, yellows, and golds. While many Canadians may take fall colours for granted, only 14 percent of the world’s forests put on a fall colour display. Forests famous for their colours are found in many places in Canada—Southern Ontario and Quebec, Alberta, Nova Scotia, and New Brunswick—as well as abroad in the Northeastern United States, Japan, and Korea.

Changes in Leaf Colour 

Throughout most of the year, the leaves of deciduous tree species appear to us as green. This is because the energy-capturing pigment at the center of the photosynthetic machinery in leaves— chlorophyll—is dominant in the leaf and does not absorb much light radiation in the green part of the light spectrum. Thus, the relative proportion of green light reflected back from the leaf is greater, and green is the colour we see. Other important pigments—carotenoids and anthocyanins—are also present in the leaf during the growing season. As summer fades into fall, changes in the balance of the types of pigments lead to leaf colour change. Due to differences in the concentrations of leaf pigments, tree species characteristically exhibit particular fall colours. For example, sugar maple leaves turn orange-red, red maple leaves turn a vibrant scarlet, and aspen and poplar leaves turn golden yellow.

Timing of Colour Change

Different species of trees undergo the colour change at different times in the fall. Whereas the leaves of the ash tree will change colour and drop relatively early, for oaks the change comes later, and leaves will not fall until well into the fall season (and often even stay on until the spring). It is thought that differences in the timing of leaf fall among species is genetically inherited (as opposed to being determined by external cues) because trees of a particular species found at the same latitude will change colour at the same time, despite growing anywhere from cool, high-elevation sites to warmer, lowland sites.  

Chemical Changes and Leaf Fall

As the days grow shorter in the fall and the intensity of the sunlight declines, deciduous trees begin to prepare to close up shop for the winter (whereas conifers prefer to stay open all year). The veins that allow the leaf to exchange fluids are gradually sealed off from the branch. As this happens, the products of photosynthesis—sugars—are trapped in the leaf, and promote an increase in anthocyanins. Meanwhile, the production of new chlorophyll progressively declines, and we begin to see the diverse palette of fall colours that results from the influence of anthocyanins and carotenoids.

The fall season is an optimal time to go out and explore the diverse community of trees in a forest near you. Nature has made it a bit easier for the budding ecologist to identify leaves; besides being colour coded and therefore easier to identify, they are also in abundance at our feet. Using the Leaf Identification Guide accompanying this article (or buy it here), you should have no difficulty in matching the silhouettes and colours of the leaves you collect on your walk through the forest with those illustrated in the guide.

Pure Green Magazine is an independent print publication for a stylish green lifestyle. Lead by founder and editor-in-chief Celine MacKay, the magazine introduces readers to the diversity in the green design and lifestyle market.