Why do leaves change color in the fall?
September and October are the peak months for admiring fall foliage, the orange, yellow and red leaves.
To understand why leaves change color, you have to start with the process of photosynthesis. All plants, including trees, have green leaves because of a compound called chlorophyll. In photosynthesis, chlorophyll captures energy from sunlight and uses it to convert carbon dioxide and water into sugar and oxygen.
Perennial plants—for example, trees in northern climates like Wisconsin—are seasonal in terms of their productivity. The leaves age and deteriorate, known as senescence, in the fall as an adaptation to dealing with cold winter climates. The leaves would freeze in the winter, so the trees need to adapt. They preserve food produced by transporting it out of the leaves at the end of the summer and losing those leaves.
Part of senescence involves the breakdown of chlorophyll, what we think of as green leaves. Chlorophyll isn’t the only plant pigment that occurs, and the green colors we see mask the yellows and oranges and reds produced by other plant pigments. As the chlorophyll breaks down, it reveals the underlying yellow and orange pigments, the carotenoids and xanthophylls, that are also involved in photosynthesis.
Sunlight can be damaging to leaves in the same way it’s damaging to human skin, and these other pigments are involved in protecting plants from very high levels of light.
The trigger for senescence is day length, particularly the length of nights. As the days get shorter and the nights too long—from the standpoint of the plant—it sets in motion the set of processes leading to leaf senescence, the breakdown of chlorophyll, and ultimately the revelation of those other plant pigments.