Why do leaves turn orange in the autumn?

Orange is just one of the many colours we see emerge amongst the leaves of deciduous trees and shrubs throughout the autumn months. But why does this happen? We're here to fill you in!

The month of September marks the beginning of autumn, and with this seasonal change we begin to see a delightful transformation in all of the plant life around us. Autumn rather wonderfully provides us with one final burst of glorious colour before the desolate winter months set in. Throughout autumn many of the leaves around us quickly transform into a rich tapestry of diverse colours, ranging from burnt orange to vivid red.

Though the arrival of nature’s magnificent autumnal colour pallete is frequently celebrated and romanticised, unfortunately the real reason behind this sudden change is not quite so charming. In essence, leaves begin to change colour as they starve themselves of vital nutrients, and suddenly begin to die.

Autumnal Brockholes woodland by Dave Bennion

Autumn leaves start with photosynthesis

Throughout the spring and summer months leaves are commonly a luscious green shade, as a result of photosynthesis taking place. Photosynthesis is the process whereby plants convert light energy into chemical energy. Simply put, plants need three natural elements to keep them alive; water, carbon dioxide and sunlight.

During the process of photosynthesis sunlight is absorbed by a chemical within leaves known as chlorophyll. Chlorophyll absorbs red and blue light, which is why leaves appear green when this process is successfully taking place. Once adequate sunlight has been absorbed it reacts with the water and carbon dioxide previously collected to produce natural sugars, which in turn adequately feeds the plant, and allows it to retain its vibrant green colouration.

Why do leaves then turn orange in autumn?

As the weather begins to change and summer turns into autumn, the production of chlorophyll diminishes. Chlorophyll production is dependent upon warm and sunny weather, so once autumn takes hold, the existing levels of chlorophyll within a leaf begin to break down. As a direct result the green colour rapidly begins to fade and reveals the natural carotenoids left behind, which are commonly red and orange in colour.

Carotenoids behave much in the same way as chlorophyll, but contrastingly, they absorb green and blue light and reflect red and yellow light strongly. This is why they create the delightful orange colouration which we see in leaves throughout the autumn months. Similarly to chlorophyll, carotenoids begin to degrade at a similar time, but do so at a much slower rate. This is why they are the dominant factor in determining the colour of a plants leaves throughout the autumn.

Although plants of all varieties change dramatically during autumn, the difference year upon year in the colours present is markedly distinct. This is because, although the production of new chlorophyll effectively stops during the autumn months, photosynthesis can still take place on sunny autumn days. In turn this produces more vibrant and eye-catching orange and yellow tones.

Why do trees lose their leaves altogether?

The final stage in the life cycle of a leaf is known as abscission. Abscission takes place in order to preserve a plants resources throughout the winter months. The dropping of leaves allows plants to preserve vital nutrients which effectively enable them to survive the colder weather.

Though the scientific process involved in why leaves turn orange may seem rather complex, it results in a simply wondrous spectacle of nature that absolutely everyone can revel in.