Botany

Saguaro



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When you think of the desert southwest, you probably picture a tall, stately cactus with branches looking like arms, silhouetted by the sunset. This is the giant saguaro.

Saguaros are native to parts of the Sonoran desert, in an area that covers southwestern Arizona and northwestern Mexico. They do not usually grow above 4,000 feet elevation, as they do not tolerate freezing temperatures. Occasionally they will be found on south-facing slopes at higher elevations.

Saguaros can live for hundreds of years and they grow very slowly, only about one inch per year. Because saguaros grow so slowly, they begin life in the shade of a “nurse” plant, often a mesquite or palo verde tree. This is why you will not usually notice them until they are larger. They can reach heights of up to fifty feet and they grow more arms as they age. Saguaros normally produce their first branches between fifty and seventy years of age. In areas of lower rainfall, they may not grow branches for up to one hundred years.

The structure of a saguaro is a marvel of natural engineering. It is supported by a tap root to hold it in place, assisted by a network of smaller roots that radiate out from the stem to a distance equal to the height of the cactus. The stem has a thick waxy exterior to retard water loss and accordion pleats. Inside are woody ribs which help to support the weight of the plant and spongy tissue for absorbing water. A mature saguaro can weigh as much as six tons. When it rains and a large quantity of water hits the ground all at once, the wide array of shallow roots is able to soak up the water and bring it up into the stem. The accordion pleats allow the stem to expand to hold up to a ton of water after a heavy rain. As the water is used, the pleats contract again. A large water reserve can last the cactus for months.

Saguaros bloom between April and June. The tops of the stem and branches produce a large number of flowers that bloom in the evening and close the next afternoon. Not all the flowers bloom on the same night. The flowers are about three inches across and have a tube at the base that fills with nectar. Like many night-blooming plants, the flowers are white, to attract birds, insects, and bats. Saguaros must be cross-pollinated, so they rely on these animals for pollination.

After the flowers die, Saguaros produce bright red fruits with thousands of tiny black seeds. The fruit was prized by Native American peoples for its sweetness and thirst-quenching ability, since it ripens in the heat of summer. It provides food for many species of birds and mammals.

The official name is carnegiea gigantea: “carnegiea” is an homage to industrialist Andrew Carnegie and “gigantea” is, as you would expect, because it’s huge.

Certain saguaros have a rare fan-shaped mutation known as a cristate or crested form. Scientists are not sure what causes it, but they have speculated that it may be due to frost damage or lightning. This occurs in only about one of 250,000 plants.

Various birds use the saguaro for shelter. The gila woodpecker and the gilded flicker dig out burrows in the stem of the cactus. When they abandon their nests, the space may be adopted by elf owls, purple martins, or other birds. Red-tailed or Harris hawks will sometimes build stick nests between the saguaro arms.

Saguaros are an icon of the desert landscape, but they are much more. This cactus is an excellent example of adaptation to a harsh environment with extreme temperatures. And, just think, some of these long-lived saguaros may have witnessed all the drama of the "Wild West" - gold-rushes, covered wagons, gunslingers, and all.

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