Posted: October 7, 2020

Written by: Lilly Jensen, Education and Outreach Coordinator

Why do Leaves Change Color in the Fall?

The beauty of autumn is hard to miss, whether on a walk through the neighborhood, in a local park, or while driving along gravel roads. On a basic level, we recognize that the shift in leaf color signals the start of fall, but we rarely dig into what exactly leads to the showstopping autumn displays. There are three primary factors that influence the fall leaf-viewing season: leaf pigment, night length, and weather.

Factor #1: Leaf Pigment

Leaf color is determined by pigment. Chlorophyll is the green pigment that enables all plant life to conduct photosynthesis, the magical process through which plants take in carbon dioxide, water, and sunlight and transform it into sugar (their food) and oxygen (a byproduct).

All spring and summer, trees have green leaves full of chlorophyll. In the fall, trees stop producing chlorophyll and the existing chlorophyll in leaves begins breaking down. As the leaves empty of chlorophyll, their green color fades, new pigments are revealed or produced, and the leaves change color.

Some pigments, like yellow xanthophylls and orange carotenoids are present in green leaves, but overshadowed by chlorophyll during spring and summer. Anthocyanins—red and purple pigments—are only produced in the fall because of changes in the chemistry happening inside the leaf. The combination and abundance of different pigments—which can be widely variable but are generally similar across trees of the same species or type—determine a leaf’s fall color. Oaks can be red, brown, and russet. Hickories are bronze; aspens and poplars yellow; and dogwoods plum-colored. Maples vary by species and variety, ranging from scarlet (red maples), to orange-red (sugar maples), to yellow (black maples).

Factor #2: Night Length

This should have been factor #1 since the seasonal transition to longer nights and shorter days is what triggers the whole process of leaf change in the first place, signaling to the tree that it’s time to stop producing chlorophyll and seal off leaves so they can fall.

But why do trees drop their leaves in the first place? Leaf drop is a survival strategy for making it through the winter. Leaves would leak moisture from the tree in the winter, and their thin tissue structure would burst in winter’s freeze/thaw cycle, potentially creating pathways for disease into the tree. Too much snow or ice building up on branches full of leaves could weigh them down to the point of breaking. So, the best thing for deciduous trees is to seal off the layers connecting leaves to twigs, let the leaves fall, and start growing new leaves in spring. This is also why trees stop producing chlorophyll and instead begin storing nutrients in the fall, so they can draw on that storage when it’s time to produce new leaves in the spring.

“Evergreen” or coniferous trees like pines, cedars, and spruces stay green all year because their needles (modified leaves) are covered with waxy coatings and filled with anti-freeze chemicals that protect them from winter’s extremes, so they don’t have to drop them.

Fun fact: there are a few species of deciduous coniferous trees, trees that lose their needles each fall and regrow new ones each year. You can see one type, American larches, in their golden fall glory in the upper meadow areas at Lake Meyer Park.

Factor #3: Weather

Weather conditions—mostly temperature and moisture—help determine the brilliance of leaf change. Warm, sunny days and crisp (but not freezing) nights often result in the best color. A late spring or severe summer drought can delay the start of leaf change, and warm fall nights can lower the intensity of leaf color. Strong winds or rains can make leaves fall early.

Lake Meyer Park, located outside of Calmar, is a fantastic place to view fall leaf color on the 3+ miles of hiking trails that wind through hardwood forests, prairies, and along the lake shore.

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