Why the Eastern Sierra Turns Gold Every Fall
Every October, aspens across the Eastern Sierra turn gold, orange and red. Here’s the chemistry behind California’s best fall color, and why the timing is starting to shift.
Updated
Every October, the Eastern Sierra goes off like a match. The aspens along the canyons turn gold, orange and sometimes red against the gray granite, and people drive up from Los Angeles and over from the Bay Area just to look at them. In 2023, the color started showing up in early October.
The Eastern Sierra Fall Color Report tracks the show every year, grove by grove, so you can time a trip. What it doesn’t tell you is why the leaves change in the first place. The answer is a nice bit of chemistry.
(Photo: Erik Olsen)
Start with chlorophyll. It’s the molecule that lets plants photosynthesize, turning sunlight, water and carbon dioxide into sugar and oxygen. Chlorophyll soaks up red and blue light and reflects green, which is why leaves are green all summer.
It’s also fragile. Each chlorophyll molecule is built around a ring with a magnesium atom at its center, and it breaks down easily from strong light and from the wear and tear of its own work. So all summer long, a tree is constantly making more of it just to keep up.
Seen through a microscope, chlorophyll is concentrated within organisms in structures called chloroplasts – shown here grouped inside plant cells. By Kristian Peters — Fabelfroh - Self-photographed, CC BY-SA 3.0
When the days get shorter and the nights get cold, the tree stops bothering. It slows down photosynthesis on purpose, stops replacing its chlorophyll and lets the green fade. That’s when the other pigments show through.
I flew my drone over the aspens in the Eastern Sierra to shoot the video below.
The yellows and oranges come from carotenoids. They were in the leaf all along, just hidden under all that green. (Carotenoids are also what make flamingos pink.)
Drone view of changing Fall colors in the Eastern Sierras
The reds and purples are different. Those come from anthocyanins, which the leaf only makes in the fall, and only when conditions are right. How much a leaf makes depends on how much sugar is in it and how acidic its cells are. Warm, sunny fall days followed by cool nights trap sugar in the leaves, and that’s when you get the deepest reds. Anthocyanins are also what give boysenberries their dark purple color.
The high Sierra is a good place for all of this. Aspens, black oaks and dogwoods grow there, and the mix of bright sun, cold nights and changing elevation brings out every color in the range. You’ll even see different colors on trees standing right next to each other, or on different branches of the same tree, depending on the soil, water and light each one gets.
Changing aspens in the Eastern Sierra (Erik Olsen)
It doesn’t last. Peak color usually runs only a few weeks before the leaves drop. That isn’t just leaves getting old and falling off. The tree grows a layer of cells at the base of each leaf stem, a process called abscission, that cuts off its water and nutrients. The leaf dies and falls, and the tree seals the wound and waits for spring.
Climate change is starting to mess with the schedule. Warmer falls can push the color change later and shorten the peak, and changes in rain and snow can affect how bright the colors get. Because the chemistry depends so much on temperature and moisture, even small shifts show up in the leaves.
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So if you’re planning a trip, check the color report, go early in the season and bring a jacket. The show is short, and it may not keep the same schedule forever.
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