Fall foliage explained: The science behind leaves changing colour

By Ansharah Shakil, September 27 2024—

For as long as it lasts, autumn in Calgary, known as Alberta’s golden season, is defined by the brilliant colours of the city’s trees. It’s all the easier to appreciate when the weather is at a perfect balance between summer’s warmth and autumn’s chill. Not only is the changing of the leaves a beautiful sight, it’s an essential and interesting part of our ecosystem, a way for plants to survive the changing climate. 

The leaves of deciduous trees — the kinds of trees which change colour in fall, like aspens and oaks — turn from green to autumn colours when the chlorophyll inside them breaks down due to changing temperatures and less direct sunlight. With a lack of chlorophyll, pre-existing yellow (xanthophylls) and orange (carotenoids) pigments that help protect leaf tissue from UV damage are revealed. Red and purple colours are caused by the creation of pigments called anthocyanins. When you see multiple colours in one leaf, you’re noticing the numerous pigments within it.

In Alberta, red foliage is uncommon — likely because our fall season is much shorter than that of other Canadian regions and our trees prepare for winter far quicker. Even once winter has set in, shedding leaves — a phenomenon known as leaf abscission, beginning with the wearing down of chlorophyll — assists the environment by providing nutrients in soil. 

During the city’s transitionary period, before sudden snow but after heat waves, we have the perfect weather for leaves changing colour: dry, sunny and cool. The timing of leaves changing colour is because of the environment, but also partially down to genetics. For example, sourwood trees can change colour as quickly as late summer, while aspens won’t change colour until September or October, and aspens are only yellow because they only have cartenoids. The genetic code of deciduous trees is what informs the colours they change, and when they change.

If you’ve noticed the leaves changing colour later than usual, it’s a pattern observed by scientists as part of climate change. Warm, long summers indicate a longer wait for leaves to change colour — along with increasing the number of wasps in the city, something we observed at the beginning of September. Pollution levels impact the timing of the leaves changing colour as well. It’s a reminder of the many ways in which climate change has influenced our expected environment.