Yellow and Red (Shallow Thoughts)

Yellow and Red

Sometimes it seems like yellow is the color of fall.

First, in late summer, a wide variety of sunflower appear: at the house we get mostly the ones with the uninspiring name of cowpen daisy (Verbesina encelioides). The flower is much prettier than its name would suggest.

Then the snakeweed (Gutierrezia sarothrae) and chamisa (Ericameria nauseosa) take over, with their carpets of tiny yellow flowers. (More unfortunate names. Chamisa has a mildly unpleasant smell when it's blooming, which presumably explains its unfortunate scientific name; I don't know why snakeweed is called that.)

No one ever seems to bother about the lowly cottonwood (Populus fremontii), or to drive long distances to see them. But they're turning gold in all the river bosques, and personally, I think they're just as beautiful as aspens.

But the aspens (Populus tremuloides) are the popular kids (as opposed to poplar kids, ha ha). Everybody wants to see the aspens. There are several places over in the Sangre de Cristos that are famous for aspens, though I never seem to manage to be there at the right time, and the Sangres often suffer from infestations of tent caterpillars that strip the aspen leaves before they have a chance to turn.

Meanwhile, on the Los Alamos side of the valley, our Jemez mountains are vastly under-appreciated. Pajarito mountain, in particular, has one of the best aspen displays I've ever seen, nearly every year. This year it's especially good.

Pajarito has a lot of aspens, because they're fire succession plants: after the recent big fires (Cerro Grande in 2001 and Las Conchas in 2011) burned down a lot of the ponderosas, aspens were quick to take over many of the burned areas.

You can tell there are a lot of aspens on Pajarito from the name of some of the trails: Aspen for Trouble, Half Aspen, and Aspeñola (a pun on the nearby town of Española).

One of the reasons the Pajarito aspens are so special is that it's one of the few places to reliably see red aspens. East Coast people apparently sneer at Southwestern fall color because it's all monotonous yellow, not enough reds in it. But every now and then you see a stand of aspen with a lot of red in it. On Pajarito there are several such areas that turn the mountainside into a blaze of color. (I'd have said it looks like the mountain is on fire, but when you live in a town where several of your friends lost their houses to forest fires, that's not something you say casually.)

They're at their peak right now. I went for a short walk on the mountain on Wednesday, and decided I needed to go back Thursday to get a closer look at some of those red groves.

I've always wondered what makes only a few aspens turn colors other than yellow. The photography blog In Light of Nature explains in More than yellow!: the red color comes from anthocyanins, which are also responsible for red apple skins and red grapes. When the sugar in a plant's sap is unusually high, it will generate more anthocyanins and, once the chlorophyll is gone, will make the leaf turn red rather than the yellow or brown of carotene. Anthocyanins are sensitive to pH: the more sugar there is in a plant's sap, the more acidic it will be, and the redder the anthocyans will appear.

There's a more technical explanation over at the forest service's Chemistry of Fall Colors, though alas all the images that apparently are meant to appear on that page are missing. And here's a full paper on the subject: Anthocyanins in Autumn Leaves of Quaking Aspen in Colorado.

Update, a day later: today's EPOD (Earth Science Picture of the Day), "Colorful Leaf Layers", includes a link to a fascinating article, Why Is Red Fall Color Nearly Absent in Northern Europe but Prevalent in North America?. I hadn't even known that red fall color was more common in North America than in Europe, but it turns out the explanation (or at least one possible theory) relates to the prevailing direction of mountain ranges.

But if you'd rather just look at pretty pictures, here are more photos of fall color in the Jemez, including larger versions of all the photos featured here. Fall Color in the Jemez Mountains.