These are the weeks the broadleaved trees of Yosemite's middle elevations typically turn. On the site's fall color calendar by elevation, the Pacific dogwoods (Cornus nuttallii) at 5,000 to 6,000 feet on Highways 41 and 120 and in the Tuolumne Grove go first, typically in early to mid October, in reds the Valley rarely shows. The bigleaf maples (Acer macrophyllum) along the Valley's south-wall creeks typically become showy from about mid-October. The maple grows to about 6,000 feet in most of California, by the Forest Service's account.

A summer leaf is green because chlorophyll, the pigment that turns sunlight into food, is the most common one in it, and it masks the rest. As daylight shortens and the nights cool, the tree sends an enzyme into each leaf to take the chlorophyll apart, the Park Service explains, and what was hidden shows: the yellow, orange and brown carotenoids, joined in some trees by the red, purple and crimson anthocyanins, in the U.S. Geological Survey's list. The maple goes yellow and the dogwood red, and the site's field guide to the park's trees separates them by the leaf as well: the maple's is five-lobed.

In November 1935 Junior Park Naturalist C. A. Wagner set out the why and wherefore of fall coloring against a belief he meant to retire, that frost paints the leaves. "The frost does not cause or enhance it, but kills the leaves and thus prevents it," he wrote. Red, he explained, "is the one color which is not already present" in the summer leaf; the maples and cottonwoods never go beyond yellow, while poison oak, azalea and dogwood go on to orange and red. He left readers a test: trees in sunny, warm places color brighter than those in cool shade.

The National Weather Service forecast for Yosemite Valley, issued at 8:56 am Saturday, calls for a chance of rain from late Saturday night, rain likely on Monday and a low around 34 Monday night, then sun from Tuesday, with highs climbing from 64 to 82 by Friday and lows from 38 to 46. After Monday that is the pattern the USGS names as the best for color: warm, sunny days and cool, crisp nights above freezing. The Valley floor sits at 4,000 feet and colors last, usually late October into mid-November, which is the trade a Yosemite fall asks for: the best Valley color comes after the high country has closed.

Autumn color on the Yosemite Valley floor. Photo: Bernard Spragg / Wikimedia Commons (CC0)

From the naturalists' notebooks

A year before Wagner, Charles W. Michael wrote of the September Valley: "The Maples on the far talus slope are changing from green to gold." The Valley as a whole, he wrote, was "taking on the supreme peace that comes with October days." (November 1934)

In January 1942 the park's self-guiding auto tour stopped drivers at Happy Isles for the dogwood, best known for its white spring blossom. "This same tree is glorious in reds during the season of fall coloring," it told them. (January 1942)

The places have held for ninety years: the maples still line the south wall's creeks, and Happy Isles still ends the maple strip on the site's fall color map.

Also in season

The dogwood's fruit. The fruit, small drupes held in a tight cluster, is often ripe by September or October and turns red with the leaves, and the Forest Service lists band-tailed pigeons and pileated woodpeckers among the birds that eat it. The band-tailed pigeon (Patagioenas fasciata) wanders in flocks after acorns and berries, Audubon notes, feeding up in the trees and even hanging upside down to reach berries. In the poor acorn year of 1934, Michael found the Valley's dogwoods "bearing exceptionally heavy crops of fruit."

The first storm. The Merced at Pohono Bridge read 13.0 cubic feet per second at 8 am Saturday, provisional, against a median of 28 for the date over 110 years of record; the lowest daily mean on record for October 10 is 5.8, in 1978. Yosemite Falls is dry, the Park Service says. The falls run on the snowpack above 6,000 feet, which releases its meltwater through spring and early summer, and why the falls follow the snow is the whole of their calendar. Storms in late fall rejuvenate some of them, the park says. Monday's forecast is a tenth to a quarter of an inch, so whether this one is enough is the storm's to say.

Before you go: Glacier Point Road reopened at 10 am Saturday, October 10, after its Dome Fire closure, and from October 15 there is no overnight parking on it or on Tioga Road. The rest is on the Park Bulletin.

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The red is made new each fall. Back next Sunday, when there is something to say.