Saguaro cactus

The saguaro (Carnegiea gigantea) is the best-known cactus species belonging in the Echinocereeae tribe. It is the state symbol of Arizona. It is the tallest cactus in the world; it can reach up to 15 metres in height. The diameter of its trunk is 40–60 cm; after rainfall, its mass can reach up to 5–6 tonnes. The saguaro cactus can live for 150–200 years. It is native to Arizona, the Mexican state of Sonora, and the southeastern semideserts of California. Due to the mountain ranges, in these regions, precipitation is low and irregular (200–300 mm annually), and temperatures can rise to 45 °C. It can store large amounts of water, enabling the plant to endure long periods of drought, lasting for even years.

The saguaro initially grows straight upwards, then its side branches begin to appear when it reaches 50–100 years of age. 12–14 blunt pleats run along its trunk. During heavy rainfall, the saguaro absorbs as much water as its root system can hold. To accommodate this large amount of water, the trunk, which is pleated like an accordion, expands. During drought, the saguaro uses its stored water, causing its pleats to contract.

As an essential function of the trunk is photosynthesis, it contains photosynthetic tissue rich in chloroplasts. Its spines are developed from areoles that are vestigial leaves densely growing along the ribs. There are approximately 15–28 (in some cases, even 50) spines on an areole. Besides protecting the cactus from herbivores, the spines reduce the temperature of the stem by casting shadow over its surface. They also collect condensed water vapour during cool nights and drip it down to the base of the plant.

Its white flowers grow in clusters at the top of the stem. They begin to bloom at night, open the following day completely, and close again later that same day. They are pollinated by nectar-feeding bats, birds and insects.

Its fruits are elongated, reddish-green berries measuring 7–8 cm. When the fruits ripen, they split open, revealing red, juicy flesh containing small black seeds. Ripe fruits are an excellent source of food for animals, which play an important role in seed dispersal.

Saguaro cacti provide habitat to a wide range of animals. The Gila woodpecker builds its nest almost exclusively in cavities it excavates for this purpose in the cactus. Larger birds make twig nests between the branches of the saguaro. The withered trunk and woody ribs of the cactus are used as beams or for firewood. The ribs are also used to collect the fruits of the plant.

Gila woodpeckers (Melanerpes uropygialis) excavate holes in the trunks of saguaro cacti to make nests. As the wound created by the bird heals, a so-called saguaro boot is formed, which is a callous-like hard shell impregnated with lignin. The cactus covers the hole with resinous sap, making it waterproof. The cavity is ready for the bird to build a nest inside after about a year. A single cactus can contain more than one nest cavity, and a saguaro boot can have several openings. Dried saguaro boots survive long after the cactus dies. The Seri people once used them to store water. Today, collecting saguaro boots is prohibited in Arizona.