Why Leaves Change Colour in Autumn

Have you ever wondered why certain trees change leaf colour in autumn, and why some leaves are more red or yellow than others? Let’s explore this interesting topic.

As we know, trees can be divided into two categories: evergreen and deciduous.

Evergreens

Evergreen trees lose and replace their leaves gradually all year round, as each one ages and falls off. Evergreens in the tropics have broader leaves than their cousins in very cold climates, because they don’t need to cope with cold. Most of the evergreens in colder areas have long and thin needles rather than leaves, which enables them to tough it out in strong winds, heavy rain and snow. Some needles are infused with a resin that resists cold and rain, and some of these trees are able to move water out of the cells in the needles if a big freeze comes along. This allows the cells to remain undamaged from the freezing conditions, and after a thaw the plant can continue to grow.

Deciduous trees

Trees that lose their leaves in winter (which is done to preserve energy and protect the tree from the cold) are usually the ones that give us the iconic autumn display of colours. The leaves of trees in this category contain certain chemicals that are responsible for colour. The chemical responsible for green is the easy one – chlorophyll. Trees rely mainly on chlorophyll in the leaves to convert sunlight into energy (through photosynthesis) that the tree needs to grow. By the time the temperatures start to fall, the trees have already stored enough energy to get through the coming colder season. The production of chlorophyll is now no longer needed and production drops. When this happens, the other chemicals in the leaves emerge and perform specific functions. Specific carotenoids and flavonoids are responsible for yellow to orange colours in leaves, and anthocyanins are the pigments that produce spectacular pinks and reds. The more anthocyanins present in the leaves, the redder the leaves will be. The process of chemicals changing over is done slowly over autumn, so different parts of a tree may have a mix of greens, yellows and reds at different times, which changes as the season moves on. Each tree variety has a different quantity of each of these chemicals, which is why each variety has a different way of showing off its colours. This is why one tree will be more yellow and another startling red, or a combination of all colours, before the leaves drop off.

The colour-making chemicals in leaves are always present, just hidden by the chlorophyll, and some play important roles in repelling pests and protecting the tree against cold. When the temperatures reduce enough and the days are shorter, their job is over. The production of hormones in the tree reduces, which causes the union between the leaf and the stem to weaken and eventually break, which is when the leaves drop to the ground. Once all the leaves have dropped, all of the tree’s energy and moisture are stored in the trunk, ready for any harsh winds or cold snaps, and it can settle in until the spring when it will once again explode with fresh leaves.

Fallen leaves are free garden food. Watch our video on Garden Tube for tips on how to use these leaves in the garden.