Autumn has arrived and the colours are at their peak…what a wonderful time of year to get outside and take in the amazing show Mother Nature is putting on!

So where do these colours come from? There’s actually a complex chemical process happening inside each tree during the fall season, and with a bit more understanding, you might even have a greater appreciation for the stunning sight!

Let’s start with the inside of a leaf – leaves get their green colour from a chemical called chlorophyll. Chlorophyll helps deciduous trees (trees that lose their leaves each year) take in sunlight. This process is known as photosynthesis which is how the tree produces energy (think of it like food!). Trees absorb carbon dioxide (CO2) and water (H2O) and use sunlight to break these compounds down into oxygen which the tree expels, and glucose which the tree consumes for energy. This process occurs throughout the growing season when sunlight and warm temperatures are abundant! As the sunlight days shorten and the temperatures grow cooler in the fall, trees no longer want to waste energy on leaves and they begin to enter their more dormant stage of the year. The process of photosynthesis stops and the green pigment in the leaves caused by chlorophyll actually fades creating fall foliage.

As the seasons change, and the nights get longer, another chemical process occurs in the tree to start to block off the connection between the tree and the individual leaves by creating a corky layer of cells known as the abscission layer. This layer exists to protect the tree branch when it becomes exposed to the open air once the leaf has fallen off the tree. The abscission layer protects the tree, but also disrupts the flow of nutrients and chemicals that move from the branch to the leaf and back. Chlorophyll needs to be constantly replaced and the abscission layer interrupts this renewal process, so once the chlorophyll starts to fade, other colours start to emerge.

SO! Leaves don’t actually “turn” red, orange or yellow in the fall season, these colours have been in the leaves of deciduous trees all along!

Two chemicals are responsible for the fall colouration of leaves. Carotenoids, the same chemical that make carrots orange, create the orange and yellow pigments in leaves, and anthocyanins that create shades of red and purple. As soon as the chlorophyll renewal is halted in the leaves, the green begins to fade and the vibrant fall colours appear. Carotenoids are present in the leaf all summer long, but they are masked by the green of the chlorophyll. Anthocyanin forms as a result of glucose that is trapped in the leaf by the abscission layer. These chemicals are the same ones that give colour to red apples, purple grapes, and blueberries.

The colours of a particular tree are a result of the carotenoids and the anthocyanins reacting to each other in different amounts, in combination with any chlorophyll that might be left in the leaf. The formation of these chemicals and the amount of each of them are dependent on temperature, moisture and sunlight, so every foliage season is unique, because every season the chemical balance found inside the leaf changes!

Carotenoids and anthocyanins also break down after being exposed to sunlight. If a leaf manages to stay on the branch after the chemical processes have broken down, you would see the bright colours fade until it would eventually turn brown, a result of a final chemical, called tannin. Tannins are found in the membranes of the cells that make up the leaves, so they never fade, which is why brown is the final colour present in late autumn.

So, next time you’re wow-ed by the stunning colours of the autumn season, remember there’s a whole lot of chemistry behind the view! Get outside and enjoy!