Why do leaves change colour in autumn?

Green is temporary

During the growing season, chlorophyll absorbs light for photosynthesis and masks many other pigments. As autumn approaches, a tree reduces chlorophyll production and a layer forms between the leaf and its branch. The green colour then fades rather than being replaced by one universal autumn pigment.

Carotenoids, which help capture light and protect leaf tissues, remain visible as yellow or orange. Their presence explains why a leaf can reveal warm colours without the tree producing a new pigment that week.

Where red comes from

Some trees make anthocyanins in autumn. These red and purple pigments can form when sugars remain in a leaf while the exit to the branch is being restricted. Bright days and cool nights can favour colour development, but the exact pattern depends on species and weather.

Red colour is not simply a sign that a leaf contains more sugar. It is a regulated response whose benefit may include protection from light stress or delayed nutrient loss, and researchers still study its evolutionary role.

Why the timing varies

Drought, heat, frost and cloud cover all influence the sequence. A warm autumn can delay colour, while an early freeze can damage leaves before a vivid display develops. Latitude and elevation also change the date because daylight and temperature shift at different rates.

Autumn colour is therefore a seasonal process, not a chemical switch shared by every tree. The same forest can contain several timelines at once.

The leaf is also moving nutrients

Before a leaf falls, the tree can reclaim nitrogen, minerals and sugars from it. The forming separation layer helps limit future exchange with the branch. Colour is therefore part of a larger seasonal process: photosynthesis declines, valuable materials are transported away and the remaining tissue becomes easier to detach.