Arizona-Sonora Desert Museum Digital Library

Description

Ocotillo is a woody shrub 10 to 20 feet (3-6 m) tall. Long, thin, and nearly unbranched, spiny stems arise from a very short trunk or caudex. The stems range from nearly vertical to widely-spreading in different individuals. Dense spikes of tubular, red to red-orange flowers sprout from the stem tips in spring. The flowering season begins as early as February at the lowest elevations in the lowest-elevation and lowest-latitude desert habitats, and as late as May in the grassland and woodland habitats. At the higher elevations some plants bloom again in late summer or fall. Each population blooms for a month.

Range

Ocotillo is the most widespread species in the family. It is common in most of the Sonoran and Chihuahuan deserts and extends north to the southern Mohave Desert, south to central Mexico, and east to central Texas.

Notes

All species in this genus have primary and secondary leaves and stems. The primary stems elongate and produce leaves with stout petioles (leaf stalks). When the leaf blade falls off most of the petiole and the base of the leaf midrib become woody and form a spine. At the base of each spine is a growing point that produces only secondary leaves which lack petioles. These axillary growing points are the secondary “branches,” properly termed short shoots; they grow only a couple of millimeters long during many decades of producing leaves.

Though ocotillos look like (albeit weird) woody shrubs, they behave as if they were cam succulents. They have very shallow roots, as do succulents. Full-grown leaves are produced as quickly as within three days of a summer rain, and fall off after a couple of weeks of dry weather. This rapid leaf production suggests that the thin layer of moist, green tissue beneath the brown cuticle would have CAM-idling metabolism. But it doesn't; it's C₃, as are the leaves. It photosynthesizes during the day in the dry season when the stems are leafless. The idling metabolism of ocotillo remains a scientific mystery.

Ocotillos exhibit an interesting correlation with elevation and geology — specifically, soil types. They occur at their highest elevations (6000 feet, 1800 m) on limestone and at their lowest (sea level) on granite. Desert vegetation in general extends about 1000 feet (300 m) higher in elevation on limestone; the reason is twofold. First, limestone weathers slowly without cracking and makes a very thin soil that favors drought- and sun-tolerant plants. Second, limestone has a high specific heat; it stores more heat than most other rocks and thus offers more frost protection on winter nights. At the lowest elevation extreme, ocotillos are limited by water rather than frost. Granite rock disintegrates to form a gravelly soil that creates a moisture-retaining mulch. At the limit of their ecological tolerance for aridity, ocotillos and some other plants can grow on granite, but not on other, drier soils.

The floral characteristics seem to point clearly to hummingbirds as the primary pollinator, but, as with the saguaro, the story is more complicated. Ocotillos are important to hummingbirds, but the reverse is less true. Several species of hummingbirds migrate northward through the desert in large numbers in spring on their way to breeding grounds as far north as Alaska. Ocotillo flowers are a crucial energy source for this migration, since it is the only desert hummingbird flower that blooms abundantly even in dry years. (Like the nectar-feeding bats, hummingbirds return south along the mountains in late summer, feeding on higher-elevation flowers.)

Many other animals will consume nectar if they can get to it, but their behavior and body forms usually make them ineffective pollinators compared to each plant's primary flower visitors that have coevolved a symbiosis (mutually beneficial relationship) . The long tubes of ocotillo flowers tend to prevent animals with mouthparts shorter than hummingbirds from reaching the nectar at the bottom of the tubes. But some animals have learned to cheat — they steal the nectar (that is, they take the reward without performing the pollination service). Verdins and carpenter bees can often be seen visiting blooming ocotillos, slitting the flower tubes at their bases and sucking out the nectar. It was assumed that this behavior failed to transfer pollen among flowers, a failure demonstrated for other nectar thieves.

But there's a twist in this story. Peter Scott and colleagues from Southern Indiana University discovered that hummingbirds rarely visit ocotillos in the Big Bend area of Texas. But carpenter bees do, and they transfer pollen effectively while crawling around on the inflorescences biting through the flower tubes. So at least in this population, what appears to be a nectar thief is in fact the major pollinator.

Antelope ground squirrels climb into the branches and feed on the flowers and seeds.

Ocotillo is used for fencing, house walls, and ramada roofs by Indians and ranchers. The cut, buried stems often root, creating a living fence. The flowers are soaked in cold water to make a refreshing beverage.

The ocotillo family is a small one of only 13 species restricted to the warm-arid section of North America. Members of this family are odd-looking plants, some even bizarre. They are characterized by spiny stems with bundles of seasonal leaves at each spine. A few species are stem succulents, the rest barely semisucculent. The fouquierias have a curious parallel with the Didiereaceae. The few species of this exclusively Madagascan family closely resemble some of the ocotillos in growth habit, differing from them in growing much larger and having succulent leaves. The didiereas are distantly related to the cacti and not at all to the ocotillos, so this is an example of convergent evolution.