Heat Beating Desert Adaptations of the Cactus

My first exposure to the desert appeared on the Loony Tunes cartoon, Road Runner. I obsessed over the “thlup, thlup” of the Road Runner’s tongue as he zipped away, including it into my grade school gym activities. In the background of every show stood the iconic two-armed Saguaro Cactus looking like a waving spectator Road Runner vs. Wile E. Coyote banter. Even Chrysler Corporation created a powerful yet cost-effective Plymouth 1970s muscle car after the speedy bird. To survive in the desert, organisms must follow a similar formula of optimizing performance while regulating costly temperature extremes and water scarcity.

Three potential desert adaptations ripe with biomimicry inspiration include: dodge, manage, and store. While a mobile organism, can dodge the heat, the cactus is left with managing heat and storing water. When thinking of cactus, the most common image is of spines. The spine distribution on the cactus top is denser to shade to the direct sun exposure. At night these spines insulate the cactus from radiated heat loss managing the extreme temperature swings between summer days and winter nights. These spines also act as fog catchers directing water droplets into their cells.

The iconic corrugated side of the cactus creates a myriad of advantages to tolerate the heat and store water. The peaks of each corrugation shade adjacent fibers more than could a standard cylindrical shape. The temperature differential between the cooler valleys and warmer peaks creates upward, convective cooling currents in the valley. This upward airflow is separated from external wind flow due to a boundary layer formed at the ridges. The peaks are exposed to turbulent airflow for cooling.

Inside the valley, the density and size of the stomata are larger than at the peaks. Stomata are pores that open and close based on light, pressure, temperature, and stress factors to intake CO2 necessary for photosynthesis. This stomata arrangement, when open, takes advantage of the cooler and moister air. This daytime photosynthesis process is augmented by an optional nighttime process inherent in desert plants called CAM (Crassulacean Acid Metabolism). The CAM process is a two-stage process in which the cactus opens its stomata at night to store the CO2 as an acid. The plant then processes the CO2 into sugar during the sunlight without opening its stomata and losing water.

The cactus’ corrugated shape creates a mechanism for expansion, a key to storing water during infrequent desert precipitation. Complementing this expandability, the internal structure of the cactus evolved. Radially-arranged cylindrical fibers run bottom-to-top, creating a hollow center. Resembling a birdcage, the structure provides bending strength and torsional flexibility and allows fluid to pass into the expandable, corrugated perimeter. Given its water content, cacti weighing up to 4800 lb and grow to 60 feet tall, this cage-like structure is critical to its thermal and moisture adaptations.

Cage structure inside a dead Saguaro cactus

How can humans create technology for restorative climate change solutions with so many unique attributes of the cactus? Architects can immediately see the advantage of buildings that adapt to the hot and arid climates. The Qatar Minister of Municipal Affairs & Agriculture building is one example. Passive cooling and thermal regulation designs emulating the cactus could reduce air conditioning energy consumptions. Engineers looking to design reusable liquid storage with structure can look to the Saguaro and barrel cactus for material-efficient, multi-functional designs. How would you use cacti inspiration?

References:

Kim, Kiwoong, Kim, Hyejeong, Park, Sung Ho; and Lee, Sang Joon (2017) “Hydraulic Strategy of Cactus Trichome for Absorption and Storage of Water under Arid Environment” Front Plant Science 8:1777

Lewis, Donald A. and Nobel, Park S. (1977) “Thermal Energy Exchange Model and Water Loss of a Barrel Cactus, Ferocactus Acanthodes” Plant Physiology 60: 609-616.

https://www.desertmuseum.org/kids/oz/long-fact-sheets/Saguaro%20Cactus.php

https://inhabitat.com/qatar-cactus-office-building/

Photos by Nature Comes Standard, LLC