Why leaves fall and change colour


Ever wonder why leaves change colour and fall? We’ve all thought about it at some point in our lives, and yet most of us don’t have the answer.

A pigment called chlorophyll, stored in membrane-bound organelles called chloroplasts, is what gives leaves their green colour. The reason it’s green is that it cannot absorb the green wavelengths of the white light, which are reflected off instead.

The leaves appear green throughout the seasons when they use sunlight for photosynthesis — the process through which they make their food and release oxygen. By the time autumn comes, plants usually have enough energy stored, and chlorophyll production stops, which leads to an abundance of carotenes. Carotenes are also pigment molecules, giving a yellowish-orange colour to the leaves as chlorophyll continues to break down. If the temperature stays above freezing, anthocyanins are also produced, which in turn produce a reddish pink colour. They keep presenting their vibrant colours as long as the temperature remains relatively warm. These remaining pigments create the palette of appealing colours seen throughout Ontario during the fall.

As colder temperatures hit the trees during autumn, the leaves eventually fall due to a layer of cells called the abscission layer, found at the point where the stem of the leaf is attached to the tree. This layer is fairly intact during spring and also serves as a passageway for the energy produced in the leaves to travel to the branch and the trunk.

As temperatures lower, the tree reduces the production of a hormone called auxin, a plant molecule that affects almost every metabolic process. With a decrease in the hormone, the strength of the linking point weakens enough that the leaf becomes susceptible to falling off due to environmental stress like the wind.

There are advantages to trees losing their leaves, most notably less energy spent throughout winter, allowing trees and plants to survive harsh winters and snowfalls. Moisture is also conserved within the trunk, which prevents the tree from drying out. Lastly, less strain is applied on the tree due to strong winds as they blow through the branches instead of getting caught in between clusters of leaves.

This, however, does bring us to another question — why do some leaves not shed at all? Pine trees, spruce trees, hemlock trees and some others actually don’t lose their leaves during the cold months, which is why they are known as evergreens. These trees have needle or scale-like leaves that are heavily covered in resin, which resists the cold and dry weather. The leaves do eventually fall, but it can take quite some time for the tree to reach that point.

The Waterloo region has multiple tree species such as hackberry, red maple, black maple, sugar maple, horse chestnut and tulip tree — all blooming with autumn foliage pigments. With the winter season quickly approaching, it’s a great time to go out and explore these species.