Ring Of Fire: This was my most-appreciated Facebook post of 2018, liked and shared hundreds of times.

Sometimes ocotillo plants (Fouquieria splendens) are spectacular in autumn, like the one above glowing in the final minutes before sundown on Friday, Nov 23, 2018. But unlike much more famously colorful plants in fall such as quaking aspens or sugar maples, they do not always color up well. Why not?

A particularly large and extravagantly well-branched individual stands backlit by the early winter sun on Thursday, December 13, 2018.

Here’s a probable explanation based upon my 20 years of observation in foliar years both colorful and dull, and occasionally even nonexistent: It depends strongly upon rainfall and the timing thereof. If fall is dry, or the summer monsoon season that runs from late June to mid-September was poor and had below-average rainfall, then there are few or no leaves on the ocotillos to even turn colors at all because the plants are strongly drought-dormant and will lack foliage without sufficient soil moisture to support it. However if there was enough late-season rain to prolong leaves into the traditionally autumnal months, then fall colors will be pretty good and in some cases splendid, just like the species’ Latin name of Fouquieria splendens indicates.

The same ocotillo from a different angle, growing on Alamo Road not far from the town of Yucca, Arizona, where I live.

Let’s look at the example of fall 2018 in my region of western Arizona. Most of the fall of 2017 and the winter and spring of 2018 were very dry, and the summer monsoon was rather poor, creating a Category 3 Extreme Drought according to the US Drought Monitor. However there was some relief as the desert’s autumn season began, when it rained decently well in early and mid-October 2018, about 6 to 8 weeks ago. (Shout out to Hurricanes Rosa and Sergio, 2018!) This was beneficial, because it was still warm enough last month to bring the long-dormant ocotillo plants into leaf again.

By contrast, true winter rains in the cold season after about December 1 and on into around early March will not coax them into activity, so had rains arrived later (such as in late December when I write this post) the ocotillos would not have leafed out. They would definitely benefit from the moisture, but not by presenting foliage to the chilly nights and short days. It is one thing for them to retain leaves they put forth weeks ago when it was warmer, and it is another to expect them to generate brand new foliage when many nights drop to near and occasionally below freezing.

Ocotillos growing on Archway Ridge at D:F Ranch, which is where most of these photos were taken on November 23, 2018.

Ocotillos are also active in summertime with July-September monsoonal rains, should they arrive. If summer weather becomes too hot and dry again for them to remain green after a midsummer leafing out, they will enter drought-induced dormancy and lose their leaves. This summer transition to leaflessness is usually pretty fast and the leaves dry out and drop quickly, without much fanfare.

I have not really observed a significantly pretty color change in ocotillos being forced into dormancy during summer heat – they are usually a dull yellow or tan and drop off hastily without lingering. This is because of the imperative to save water – why drag the leaf drop process out when preserving scarce water is important to survival? Ocotillos entering summer drought dormancy just dehydrate and drop the leaves without delay.

Ocotillo plants on Archway Ridge exhibit a range of colors from still-green to yellow, orange, and red. Individuals differ genetically, just like people and any other organisms do, and how quickly they turn colors and what shades they take on vary, as well as how fast they turn. Local microclimatic variations in soil, moisture retention, and exposure must also play a role.

However autumnal rains follow a significantly different pattern. When ocotillos leaf out in late September or early October, they undergo a very gradual dormancy process in November. Rather than a quick turn-and-drop process as happens in hot weather, the plants transition slowly into their winter barren state, motivated by short days and cooling nights, but not necessarily as much by drought.

A gold-leaved ocotillo in bright fall plumage frames an adolescent saguaro at D:F Ranch.

Why might drought not play much of a role in autumn? I believe that this is because late-season rains last much longer, since the shortening days and cooling nights don’t evaporate it from the desert soils nearly as quickly as in long, hot summer days. Therefore ocotillos are quite capable of utilizing the soil moisture to keep photosynthesizing during the warm, but no longer hot, days. Drought stress is not what is driving the dormancy under this set of conditions – cooling nights and shortening days are. So the leaves persist for weeks longer than they would in hot summer dry spells, gradually changing colors.

This individual’s leaves have more of an orange tint, probably caused by a higher proportion of red carotenoid pigments to the yellow xanthophyll pigments.

The slowness of this procedure enables the plants to show their hidden leaf colors, which are inspired by pigments leading to shades of yellow, orange, russet, and red. And this is what makes ocotillos the fall foliage stars of the Sonoran Desert. It may not quite be New England, but it’s special when it happens, because we don’t get to see this every year.

Very red ocotillos have carotenoids as their dominant alternate pigments.

As with other plants, all of these secondary red and yellow leaf pigments are hidden by chlorophyll, which obviously is green and the primary photosynthetic molecule. Plants have other chemicals that assist in photosynthesis (like the carotenoids and xanthophylls) or serve other biotic functions such as absorbing UV light or preventing sun damage.

I’ve been waiting for a good, clear late afternoon to go see the fall foliage on the ocotillo plants. All of these photos were taken up on Archway Ridge on my property, about a quarter mile from my house. Taken less than an hour before sunset, it was a rewarding photo session amongst the plants I adore and am fortunate to live with.

Let’s get as many cool desert plants into one image as possible: Nolina bigelovii, Fouquieria splendens, Carnegiea gigantea, Yucca brevifolia, Ferocactus acanthodes….

Many visitors comment upon how little this actually looks like a stereotypical desert. I agree with them.

There aren’t too many places where the natural ranges of Joshua trees overlap with that of ocotillos. I am pleased to legally inhabit one such spot.

Looking up Archway Ridge to the south as the setting sun illuminates the scene from the west.

Above are just a few photos of the rocky feature for which I named Archway Ridge. Cleverly, I named it D:FR Arch. Look, it’s on my property, it didn’t have another name, and I will egotistically name it for my operation. Besides, I could have called it Emming Arch, which seems a bit much. Unless of course future historians decide to give it that name after I die….

The sun goes down on another desert day.

A vehicle raises a dust cloud on a local ranch road below my vantage point up on Archway Ridge.

A study in orange and black.

Subtle twilight colors.

And finally, the sun went down, ending the day of November 23, 2018. I am so fortunate to live here.