Why leaves change colors when summer turns to fall

The First of September is the official beginning of fall, at least, as far as meteorologists are concerned, and the vegetation at Mammoth Lakes has received the memo.

Photos shared by Mammoth Lakes Tourism showed leaves beginning to turn yellow in a preview of what’s to come.

Convict Lake, one of the popular destinations for Mammoth Lakes visitors, already has some yellow on the tops of trees and a few fallen leaves scattered on the ground. Rabbitbrush, shrubs that dot the landscape, are also starting to shift toward fall colors.

Blake Engelhardt, a botanist for the United States Forest Service, said he’s optimistic that the lingering snowpack and moist soils from spring rains could translate into vibrant fall colors.

When will the fall colors peak in California?

During the summer growing season, leaves are actively producing chlorophyll, which gives them their green color, Engelhardt said. When the seasons shift to fall and days get shorter, things begin to change.

“Longer nights trigger the formation of the abscission layer. This is essentially a thickened layer of cells at the base of the leaf where it attaches to the stem that blocks the transport of water and sugar into the leaf,” Engelhardt said. “Thin deciduous leaves won’t survive the freezing temperatures of winter, so there’s no reason to keep feeding them.”

Once leaves stop producing chlorophyll, the green quickly disappears and the leaves’ other pigments become visible, including the famous yellows, reds and purples we associate with fall.

If the spring and summer growing season is particularly dry, that abscission layer can form sooner and the leaves may turn earlier or fall from trees before they have a chance to create a “a very brilliant display,” Engelhardt said.

Because California had a wet growing season, we might be in store for one of those brilliant displays of fall colors. But the Forest Service botanist stresses that foliage seasons can be hard to predict in terms of length, because “unexpected frosts can cause leaves to drop quickly.”

If you’re looking to find a vibrant display of autumn’s colors, Mammoth Lakes is a great place to do so. The official tourism site for Mammoth Lakes has a guide of when and where to find the fall colors.

If that’s too far out of the way, the California Department of Parks and Recreation also has a list of state parks where the changing leaves will be on full display.

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