Leafy chemistry: What happens when a leaf changes colour in autumn?

For a moment, close your eyes and imagine yourself in the fall season. The cold, chilly wind kisses your face, and the leaves begin to dance gracefully as they fall off the tree. What’s the first thing you notice about the leaves? That they change colour before they fall off. But how? Asked and answered.

The science underneath

Usually the trees that are deciduous (plants or trees that shed leaves annually) have leaves that change colour. They have large and broad leaves.

Most of the year, these trees are green throughout the year because of chlorophyll content. And if you have learnt in school, chlorophyll is important to absorb sunlight during photosynthesis, and the leaves convert sunlight into energy and energy into sugars to feed the tree. This pigment is constantly produced throughout the spring and summer. Other pigments are present, but in much lesser quantities.

As seasons change, the weather gets colder towards the last few months of the year. Days also get shorter with time. Trees get less direct sunlight, and hence, the chlorophyll in the leaves begins to break down. This in turn leads to other pigments being produced in higher quantities, marking the trees’ preparation for winter.

The leaves eventually fall off when nutrition is cut off, and they decompose, enriching the soil for the next life cycle.

Visitors walk inside a Mughal garden as they enjoy during the autumn season, in Srinagar, Kashmir.
| Photo Credit:
IMRAN NISSAR/The Hindu

Chemicals and colours

So, why are all the leaves coloured differently? Well, it is because of the various other pigments in the leaf and their reaction to the sunlight. The yellow and orange shades come from pigments called carotenoids, which are present year-round, but masked by the higher amounts of chlorophyll. Red and purplish-brown colours arrive from anthocyanins. Healthy trees have an enhanced red colour due to increasing sugar production for anthocyanins.

Trees stressed by drought, disease, and poor soil conditions may lose leaves prematurely, leaving them bare in the autumn season.

Fallen chinar tree leaves float on the waters of Tchunte Kul, a tributary of Dal Lake, during an autumn day in Srinagar, Wednesday, 19, November 2025. Fall in Kashmir, locally known as “Harud”, transforms the valley into a vibrant canvas of red, orange, and yellow hues.
| Photo Credit:
IMRAN NISSAR/The Hindu

Hotspots of change

These changes are mostly seen in areas in and around the temperate zone (middle part of the Earth which is characterised by moderate climate, and a distinct four-cycle season). Mount Mansfield in Vermont, U.S., is home to prominent colours of red and gold. Lake Tahoe, in the U.S. and Agawa Canyon in Canada showcase golden aspens and maples from mid-October. Other hotspots include Douro Valley in Portugal; the red maples in Nara and Kyoto, in Japan; and the Chinar trees around Dal Lake in Kashmir.

Leaves changing colour is not just a pretty sight to see, but a natural response to the changing seasons, and a unique survival strategy. Autumn’s colours are nature’s way of preparing for renewal.