Why do leaves turn in the fall?

One of the benefits of going to college in the Berkshires is being surrounded each fall by colorful foliage. Why do trees and shrubs turn such vibrant colors? And how do they do it?

Some trees, like sumac and dogwood, use color to advertise their ripe fruits to birds. Some healthy trees may warn insects that they are strong enough to defend against invasion. For other trees bright color is simply a necessary step between being green in summer and falling off in winter.

Chlorophyll is the pigment in leaves that absorbs blue and red light to turn sunlight into sugar. It reflects green light, making summer leaves so green. As a plant gets ready for winter, it stops producing chlorophyll and pulls nutrients out of the leaves to store them away. The lack of chlorophyll exposes other photosynthetic pigments like carotenoids, which leave foliage a vibrant yellow. Some trees, like ginkgo, actually produce a brightening chemical to make their golden color especially intense.

As trees prepare to drop leaves they form a plug between the stem and the twig. The leaf is still photosynthesizing a little bit so the plug traps these last sugars in the leaf. The sugars of some species are then turned into pigments called anthocyanins, which are red. The shade of crimson will depend on the pH of the soil that the tree lives in – if the soil is acidic then leaves will be a bright orange-red color, but if it is basic, the leaves will be more purple. Red fall species include maple, sumac and sweetgum.

The more late season photosynthesis goes on, the more color we see. Leaves exposed to direct sun will be more colorful and sunnier fall weather will mean a better show in general. A late summer drought can start all the processes earlier, making for a great leaf-peeping season.

The combination of all this chemistry and ecology makes this a great time to go for a walk in our Berkshire woods!