Tree of the Month: Desert “Willow”

After a short drive on a rough 2-track through creosote scrub, I parked at a dry sandy wash below the object of my desire—a young lava flow, maybe just 10,000 years old. But as soon as I got out of the car, all thoughts of geology fled. They were driven off by a huge tree, flamboyant with flowers.

Huge and flamboyant by desert standards for sure!

Based on the impressive green (not gray-green) canopy and large trumpet-shaped flowers, I concluded the tree was an escapee, an ornamental that somehow managed to get established in the wash. Such verdure and floral showiness are out of place in the desert. Later I stopped at Kelso, the old railroad depot in the middle of the Mojave, where helper engines were stationed to help trains up the steep Cima grade (more here). I immediately noticed the same trees lining the parking lot. Yes! definitely an escaped ornamental. But then a vague memory began to take shape.

The depot now houses the Visitor Center for Mojave National Preserve. In the bookstore, I read about trees in desert washes. Then it all came came back: Chilopsis linearis, the desert willow, was one of the key species of desert washes that we committed to memory forty years ago. If we saw it on our field trip to the Mojave, that memory is long gone. But I’m sure we did not see it in flower … at least not flowering in such profusion. That I would have remembered.

The desert willow is unusual in other ways. Not really "unusual" of course, but rather surprising … as I knew so little about its life. Since that day we met beside the lava flow, I’ve learned a lot.

First, desert willow is not a willow, not even close. If it were, it would be in the genus Salix in the Salicaceae (willow family). Instead, the genus is Chilopsis in the Bignoniaceae (catalpa family, mainly tropical). The common name is understandable. Desert willow is associated with stream channels (though usually dry), and the leaves look superficially like those of several familiar willows (e.g., Salix exigua, narrowleaf willow).

Chilopsis linearis is the only species in the genus, and is the only native member of the catalpa family in California (source). It’s native to south-central Texas, south to Nuevo Leon and Zacatecas, and west to southern California and Baja California.

At first glance, desert washes look just as dry as the surrounding uplands. But they’re not. After the occasional heavy rain, torrents of water rush down the wash, and quite a bit gets stored underground. This explains the heavier growth of wash-specific shrubs and trees on the banks. But desert willow often grows in the middle of washes, where it must bear the full brunt of flash floods. It can do this because it’s so well-anchored with its strong deep root system. And even in the searing heat of a Mojave summer, its roots provide enough groundwater to support a full compliment of leaves until the first hard frost.

Desert willow in mid-wash, with young lava flow and somewhat older cinder cone behind.

Desert willow is tough in other ways. Atchley and colleagues (1999) noted that water transport in trees and gas exchange in leaves (e.g., uptake of carbon dioxide for photosynthesis) change very little during times of low water availability:

“Desert willow can maintain near normal water potentials and gas-exchange rates during periods of low water availability (Odening et al., 1974; de Sozya et al., in review). This obligate riparian species appears to be highly adapted to minimizing its physiological responses to the often extreme environmental changes common to arroyo habitats.” (italics added)

Flowers typically are most abundant in May and June, with stragglers hanging on through the summer. If there’s enough late summer rain, another burst of flowers may follow. The seed pods are conspicuous, being up to ten inches long (35 cm). They stay on the tree, and split to produce abundant tiny winged seeds.

Seed pod from last year, still on tree.

Desert willow seeds (source); photo by Russ Kleinman. Scale is metric.

Turns out my first impression wasn’t totally wrong. Desert willow is used as an ornamental. In fact, many cultivars have been developed. It’s also popular for soil stabilization projects in the America Southwest.

Cultivar 'Rio Salado' at the Springs Preserve garden in Las Vegas (courtesy Stan Shebs).

This is my contribution to June’s tree-following gathering, kindly hosted by The Squirrelbasket. Obviously I’m not following a given tree. I have just about given up on the one I chose for 2017: Sabalites powellii, an extinct palm. If things had gone as planned, I would be visiting its ancient habitat today!! And you would read about it next month. But the Fickle Finger of Fate intervened, specifically the Wyoming weather. After finishing field work, I drove to the nearest town and checked the 3-day forecast for Fossil Butte National Monument: heavy rain, rain and snow with a low of 34º F (1º C), then more heavy rain. So I drove home. Maybe in September …

Sources (in addition to links in post)

Atchley, MC, et al. 1999. Arroyo water storage and soil nutrients and their effects on gas-exchange of shrub species in the northern Chihuahuan Desert. J. Arid Environments 43: 21–33. PDF

USDA Forest Service. Fire Effects Information System (FEIS). Chilopsis linearis. https://www.fs.fed.us/database/feis/plants/shrub/chilin/all.html (accd 12 June 2017).

If you aren’t familiar with the FEIS database, you should be! It contains thorough compilations of plant information (though not for gardening). You’re a lucky person if your species of interest is included.

Plants with no names, and other mysteries

It was like being young again. On the road, driving fast, light fading, finally a sign—the wrong sign. Missed the turnoff 40 miles back. Turn north anyway, drive fast, light fading, hit the brakes, back up, take a narrow deep-rutted two-track winding through creosote scrub with no place to turn around. Keep driving, wondering, light fading, rocky hill visible ahead, suddenly arrive and stop. Flat open area, fire ring, fragrance of creosote bush, tiny car lights snaking through the valley below. Far to the east a golden glow edges the jagged mountain crest and the full moon rises—unexpected, unplanned, perfect.

And look what the morning brought:

Morning at the base of Van Winkle Peak.

The desert was in bloom—not the highly-touted “super bloom” of early spring annuals, but rather shrubs covered in flowers. All the moisture had benefitted them too. Most looked familiar, probably because 40 years ago we novice botanists stood before them chanting in Latin. For a moment I hesitated, caught by the reflex. But how absurd! There’s no requirement to name plants to enjoy them.

Of course I recognized creosote bush—I would have recognized it just by its wonderful fragrance (early travelers struggling to cross the Mojave before dying of thirst probably would have disagreed with “wonderful”).

Vigorous creosote bush almost ten feet tall.

Note resinous aromatic leaves.

A yellow-flowered green-stemmed shrub was the main source of color—desert senna, Senna armata, as I later learned from a plant display at the park Visitor Center. It’s a member of the pea family.

Why doesn’t it have pea-like flowers? Because it’s in the subgroup Caesalpinioideae, with flowers only slightly irregular. But it does have compound leaves like many peas, though it took me a moment to see this, with the widely-spaced tiny leaflets on a twisting axis.

Look close – compound leaves!

Next I found a wild buckwheat (Eriogonum sp.) thick with flowers. This genus is easy to recognize, with clusters of small six-parted flowers, but species are often tough. Wild buckwheats are common and diverse in California deserts. During C. Hart Merriam’s Death Valley Expedition of 1891, botanists Frederick V. Coville and Frederick Funston collected 25 species!

Lucky shot—a preoccupied pollinator stumbled into my field of view.

Next to the wild buckwheat was a dead shrub … or so I thought until violet spots caught my eye. This plant’s a total mystery. I don’t know the genus nor the family. It has sharp-tipped twigs, and the stems, leaves and buds are covered in fine gray hairs. Any ideas?

The low shrub below was another a puzzle. It was covered in yellowish fruit, each with two plump locules (chambers) and a persistent style rising between. It had green stems and tiny leaves. Green stems are not uncommon in the desert; they allow photosynthesis to continue after leaves are dropped in response to drought.

Oh boy, chollas ahead! (Opuntia spp.; mid photo below). I spent a fair amount of time among them—I love photographing cactus spines.

Inside the flowers, the stamens were moving, their yellow anthers swaying erratically. Something was rummaging around down at the base of the red filaments. Is there nectar down there, full of drunken pollinators? Finally I caught a shot of one of the culprits.

What is it?

A few annuals were hanging on, the last of the super bloomers. This little beauty was the most common. It’s a member of the aster (sunflower) family.

I spent two nights and two days in the Mojave National Preserve—not nearly enough, not even close. But then I hadn’t planned to go there at all. Seems I always drive across the Mojave in May, when it’s much too hot to stop, but this year things were different, “unusual.” Thunderstorms, heavy rain, flood warnings and finally snow drove me out of New Mexico. I sped across Arizona hoping for dry tolerably-warm Mojave days. Indeed they were.

Continuing on to the coast, I had lots of time to ponder this change. Why are wonderful surprises rare now? Has my venturing into the unknown declined due to age or due to the hyper-availability of information? I thought about all those hours spent on the web the week before I left. Maybe every long trip should include at least one area with little information and no plan. We’ll see if this plan can be implemented ;-)