Why Do Leaves Change Color in Fall?

What Triggers the Color Change?

The primary trigger for fall color change is decreasing daylight and dropping temperatures. As days get shorter in autumn, trees receive signals that winter is coming. Trees need to prepare for the cold months ahead when water will be scarce.

To survive winter, trees begin shutting down their food-making machinery. This is where the color change comes in. Trees stop producing new chlorophyll and begin breaking down the chlorophyll that exists in the leaves. As the green pigment disappears, other colors that were always present in the leaves but hidden beneath the green become visible.

What Are These Other Colors?

Carotenoids are yellow and orange pigments present in all leaves. As chlorophyll breaks down and disappears, carotenoids become visible, giving leaves their golden and orange hues. 

Anthocyanins are red and purple pigments that are a bit different. Unlike carotenoids, anthocyanins aren't always present in leaves. Trees actually produce them in the fall as a response to bright sunlight and cool nights. The brighter the fall sunlight and the cooler the nights, the more anthocyanins are produced, resulting in more vibrant reds and purples.

Xanthophyll is a yellow pigment similar to carotenoids, also masked by chlorophyll during the growing season.

The specific colors you see depend on which pigments are most abundant in each tree species and the environmental conditions that year.

The Purpose Behind the Change

Scientists have several theories. One hypothesis is that the bright red color is actually a warning signal to insects. This might discourage insects from infesting the tree. Another theory suggests that anthocyanins protect the leaf while the tree extracts valuable nutrients from it before it falls.

When Do Leaves Actually Fall?

The color change is just the prelude. After the pigments shift and the leaves display their autumn colors, another process begins: the formation of the abscission layer. This is a specialized layer of cells that forms between the leaf stem and the branch. Over time, these cells weaken the connection until the leaf eventually falls with the slightest breeze. Before the leaf falls, the tree seals off the connection point with a corky substance that prevents water loss and protects the tree. The timing of leaf fall varies by species and weather conditions, but it usually happens weeks after the color change begins.