Incredible plants: ocotillo

A flowering ocotillo in the western Arizona desert,
April 2017.

Primary productivity in desert environments is low. Water is the limiting resource for desert vegetation, whereas sunlight is available in overabundance. Desert plants generally only attain relatively short stature, and either flourish in annual bursts of growth (when rains come) or grow slowly over years and decades, responding opportunistically to rainfall availability over the longer term. Plants like cacti or the iconic Joshua Tree (Yucca brevifolia) of the southwest follow the latter strategy. In the life cycle of the ocotillo, one of my favorite desert plants, this species sort of embodies a bit of both strategies.

Ocotillo (Fouquieria splendens) is indicative of the Sonoran Desert, the hot low-elevation deserts of southeast California, eastern Arizona, and northwest Mexico. In terms of growth habit, the plants are one of the larger of the woody desert species in the southwestern US, reaching up to 10 m in height. They may live a century or two.

Germination of a young ocotillo requires a significant summer rainfall event of several centimeters. Most young seedlings will succumb to drought, frost or herbivores. Survivors become larger and woody. The stems of adult plants have a mottled grey and black surface with furrows. Sharp grey foreboding spines are abundant all over the stems.

Ocotillo leaves are short, spoon-shaped, and of two types. The primary leaves emerge from a petiole (which when dried, becomes the numerous spines lining the stems), while secondary leaves emerge in clusters directly from the base of the spines of the primary leaves and don’t have associated spines of their own. Secondary leaves are produced episodically and gorw in profusion very shortly after a good rain soaks the soil. The plant doesn’t invest for the long-term in these secondary leaves, rather they are produced without a cuticle (allowing easy CO₂ uptake but making them susceptible to water loss). After a few weeks mining the soil for water, the plant sheds its temporary leaves and will go physiologically dormant during a subsequent period of drought. Dormant plants retain living cells internally, but shallow roots die off and secondary leaves are lost.

Bare stem (left) on a plant from Joshua Tree National Park, CA, Feb 2012 and secondary leaves (right) on a plant from Arizona, April 2017. 

Inflorescence, western Arizona, April 2017.

Ocotillo are placed in their own plant family, the Fouquieriaceae, a small group of woody species endemic to Mexico and the southwestern US. The family is believed to have evolved in the subtropics during the Miocene (5-24 mya). The species grows below 2500 ft and occurs from California and Baja California eastward to Texas. The species is not tolerant of freezing temperatures which may be one factor limiting its occurrence in the Mojave and Great Basin deserts to the north where winter-time frost is likely.

Flowers are produced on branch tips. From a distance they appear as red flames. Flower petals are up to 2.5 cm long and are fused into a trumpet-shaped tube. A tuft of red stamens emerges beyond the corolla extending the length of the flower. Hummingbirds enjoy the nectar produced by the flowers.

References

Baldwin BG et al. (eds). 2012. The Jepson Manual. Higher Plants of California. 2^nd ed. University of California Press, Berkeley, CA.

Munz PA. 1962. California Desert Wildflowers. University of California Press, Berkeley CA.

Pavlik BM. 2008. The California Deserts. An Ecological Rediscovery. University of California Press, Berkeley, CA.

Leaf-less ocotillo in the Sonoran desert portion of Joshua
Tree National Park, Feb 2012.