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Geo-Joint: The Saguaro Cactus

Posted on February 22 2017

The saguaro cactus, Carnegiea gigantea, can’t be missed. Not only are they everywhere in western design, symbolic of the desert landscape, but in real life, they are huge. The largest cactus in the United States, they can tower above the surrounding landscape, a bold form in a climate where the residents tend to be smaller, struggling in the blazing sun and scarce water. Though saguaros can be a quick shorthand statement for “the desert,” they actually grow only in the Sonoran Desert. That desert covers southwestern Arizona, a portion of California, some of northwestern Mexico, and the majority of Baja California. However, the saguaro grows almost entirely within Arizona and the mainland portion of the Sonoran Desert in Mexico.

The most striking thing about a saguaro is, of course, its size. Up to 60 feet tall, they may grow either as a tall singular trunk, or as they age, they may sprout branches after their first half-century. These usually but do not always curve upward, making them look like people (or Gumbies) with their arms reaching for the sky. Some of them have more than 25 arms stretching out. These giants of the desert are mature saguaros, and they have been where they are for a very long time. It can take a saguaro ten years just to reach a height of one and a half….inches! Their rate of growth increases over time, but the big multi-armed specimens you see may be as much as 200 years old.

Living as they do in areas of extreme temperature, saguaros, like most other cactus, do not have leaves as we commonly know them. Through evolution, their “leaves” have become the sharp spines the plant is covered with. The plant structures that do the work of leaves for them, the stomata, are all over the cactus’ body surface. Leaves on plants in more temperate regions take in carbon dioxide through stomata, to perform photosynthesis and create sugars. But precious moisture may be lost doing this in a desert environment. To avoid this, cactus open their stomata at night, collecting carbon dioxide and storing it to process with the following day’s sunlight. The waxy surface of the plant also protects against water loss. Saguaros have other strategies for high-heat survival. The distinctive shape of a saguaro itself is no accident. By growing vertically and pole-like, their sides are maximally exposed to the sun for photosynthesis in the cooler morning and evening hours, while the blazing sun of the midday principally impacts only the top of the cactus.

The life of a saguaro is crucially dependent upon the sporadic and occasionally torrential rainfall found in the Sonoran Desert. When rain does fall, especially during the monsoon season of late summer, the saguaro will drink as fast as possible before the rain all runs off, storing the water in its large body. It is aided ln this by the accordion-shaped arrangement of ribs on its exterior. They swell with water in the good times, and as the water is used up, the plant contracts along those accordion folds, simultaneously shrinking the sun-exposed area when conditions are hottest. During the season of greatest hydration, a saguaro can weigh 3200 to 4800 pounds. Surprisingly, the root structure it uses to Hoover up the hard rain may only go four to twelve inches deep, though it may also spread out as far from the trunk as the trunk is tall. This ensures a huge collection area, and helps support the tall cactus in high winds, as well. The plant also has a taproot that extends straight down, but not more than two or three feet.

Reproduction for the saguaro is another hurry-up-while-you-can affair. White three-inch flower blossoms appear at the ends of the main trunk and arms, and open at night when bats will come to drink their nectar. The following day, bees and various bird species will take their turn at the flowers. And that’s it. One night and the next day, and the flowers are done. They have to hope their overripe melon fragrance has attracted enough activity for successful polination. If so, oblong red fruit will be produced. Packed with thousands of little black seeds, the birds, tortoises, coyotes, javelinas, and others who eat them will spread the genes of the saguaro far and wide. The Native Americans of the desert, when they came into the picture, also found the fruit to be a valuable treat, and became another useful sower for the saguaro.

So the seeds travel as the animals wander, but saguaros have their limits. While they can stand the desert heat and dryness, they have only so much tolerance for cold. They don’t do well much above 3500 feet in elevation, as frost will cause them too much damage. Saguaros have a big cousin, the cardon cactus, which grows south of its range, in mainland and peninsular Mexico. Similarly shaped, this completely different species (Pachycereus pringlei) can reach 70 feet and live over 300 years, besting the performance of the saguaro. However, the two give each other space, rarely growing close by. The cardon, for all its vigor, cannot stand even as much cold as the saguaro can, so it is restricted to more southerly regions.

Like the Giving Tree, saguaros are long-suffering but always providing. Besides their flowers and fruit, they offer precious shade in the harsh desert heat, and woodpeckers will burrow holes into the plant to create nests. Former woodpecker nests are happily taken over in subsequent seasons by a host of other birds from flycatchers to cactus wrens to owls and others. Once a saguaro dies, dries up, and falls, the lining of the hollows in which these nests are made is left behind, a structure called a saguaro boot. Native Americans found that these gourd-shaped plant remains could function as canteens. They also used the woody ribs of the cactus as building material. So the stately saguaro stands tall not only as a handsome icon of the Sonoran Desert, but thanks to its generous provisions, as a vitally important member of the desert ecology.

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