Britton & Rose
(Engelm.) Britton & Rose
|Natural range of Carnegiea gigantea|
The saguaro (//, Spanish pronunciation: [saˈɣwaɾo]) (Carnegiea gigantea) is an arborescent (tree-like) cactus species in the monotypic genus Carnegiea, which can grow to be over 40 feet (12 m) tall. It is native to the Sonoran Desert in Arizona, the Mexican State of Sonora, and the Whipple Mountains and Imperial County areas of California. The saguaro blossom is the state wildflower of Arizona. Its scientific name is given in honor of Andrew Carnegie. In 1994, Saguaro National Park, near Tucson, Arizona, was designated to help protect this species and its habitat.
Saguaros have a relatively long lifespan, often exceeding 150 years. They may grow their first side arm any time from 75–100 years of age, but some never grow any arms. A saguaro without arms is called a spear. Arms are developed to increase the plant's reproductive capacity, as more apices lead to more flowers and fruit.
A saguaro is able to absorb and store considerable amounts of rainwater, visibly expanding in the process, while slowly using the stored water as needed. This characteristic enables the saguaro to survive during periods of drought. The saguaro cactus is a common image in Mexican culture and American Southwest films.
The saguaro is a columnar cactus that grows notable branches, usually referred to as arms. As many as 49 arms may grow on one plant. They grow from 3–16 m (9.8–52.5 ft) tall, and up to 75 cm (30 in) in diameter. They are slow growing but routinely live to 150 or 200 years old. They are the largest cactus in the United States.
The growth rate of saguaros is strongly dependent on precipitation; saguaros in drier western Arizona grow only half as fast as those in and around Tucson. Saguaros grow slowly from seed, never from cuttings, and grow to be over 40 feet (12.2 metres) in height.[verification needed] The largest known living saguaro is the Champion Saguaro growing in Maricopa County, Arizona, measuring 45.3 feet (13.8 metres) high with a girth of 10 feet (3.1 metres). The tallest saguaro ever measured was an armless specimen found near Cave Creek, Arizona. It was 78 feet (23.8 metres) in height before it was toppled in 1986 by a windstorm. They are stem succulents and can hold large amounts of water; when rain is plentiful and the saguaro is fully hydrated it can weigh between 3,200–4,800 pounds (1,500–2,200 kg).
Saguaros have a very large root network that can extend up to 30 m (98 ft), and long taproots of up to 1 m (3.3 ft) deep.
Saguaros may take between 20 and 50 years to reach a height of 1 m (3.3 ft). Seedlings may only be .25 in (0.64 cm) tall after 2 years.
Inside the saguaro, there are many "ribs" of wood which form something like a skeleton, with the individual ribs being as long as the cactus itself and up to a few inches in diameter. The rib wood itself is also relatively dense, with dry ribs having a solid density of approximately 430 kg/m3, which made the ribs useful to indigenous peoples as a building material. While the ribs of dead plants are not protected by the Arizona native plant law, the Arizona Department of Agriculture has released a memo discussing when it's necessary to obtain written permission before harvesting them because of the importance the decomposition of cactus remains in maintaining desert soil fertility.
The spines on a saguaro are extremely sharp and can grow up to 1 millimetre (0.039 in) per day. When held up to the light or bisected, alternating light and dark bands transverse to the long axis of spines can be seen. These transverse bands have been correlated to daily growth. In columnar cacti, spines almost always grow in areoles which originate at the apex of the plant. A spine stops growing in its first season. Areoles are moved to the side and the apex continues to grow upwards. Thus, older spines are towards the base of a columnar cactus and newer spines are near the apex. Studies are underway to examine the relationship of carbon and oxygen isotope ratios in the tissues of spines of an individual to its climate and photosynthetic history (acanthochronology).
Spines grow to 7 cm (2.8 in) long.
The white, waxy flowers appear in April through June, opening well after sunset and closing in mid-afternoon. They continue to produce nectar after sunrise. Flowers are self-incompatible, thus requiring cross-pollination. Large quantities of pollen are required for complete pollination because many ovules are present. This pollen is produced by the extremely numerous stamens, which in one notable case totaled 3,482 in a single flower. A well-pollinated fruit contains several thousand tiny seeds. Saguaros have a redundant pollination system, i.e. full fruit set is possible even if only a fraction of the pollinating species are present.[contradictory]
Main pollinators are honey bees, bats, and white-winged doves. In most years, diurnal visitors, mostly honey bees, are the main contributors for fruit. Other diurnal pollinators are birds such as Costa's hummingbird, the black-chinned hummingbird, the broad-billed hummingbird, the hooded oriole, Scott's oriole, the Gila woodpecker, the gilded flicker, the verdin, and the house finch.
The primary nocturnal pollinator is the lesser long-nosed bat, feeding on the nectar. A number of floral characteristics are geared toward bat pollination: nocturnal opening of the flowers, nocturnal maturation of pollen, very rich nectar, position high above ground, durable blooms that can withstand a bat's weight, and fragrance emitted at night. Further, the amino acids in the pollen appear to help sustain lactation in bats.
Flowers grow 3.4–4.9 in (8.6–12.4 cm) long, and are open for less than 24 hours. Since they form only at the top of the plant and the tips of branches, it is reproductively advantageous for saguaros to grow numerous branches. Flowers open sequentially, with plants averaging four open flowers a day over a bloom period lasting a month.
The ruby red fruits are 2.4 to 3.5 inches (6 to 9 cm) long and ripen in June, each containing around 2,000 seeds, plus sweet, fleshy connective tissue. The fruits are edible and prized by local people.
The fruits are often out of reach and are harvested using a pole (often a saguaro rib) 7 to 16 feet (2 to 5 m) long, to the end of which is attached a smaller pole, crosswise. This pole is used to knock the fruits free.
Saguaro seeds are small and short-lived. Although they germinate easily, predation and lack of moisture prevent all but about 1% of seeds from successful germination. Seeds must wait 12–14 months before germination; lack of water during this period drastically reduces seedling survival. The existence of nurse plants is critical to seedling establishment. Palo verde trees and triangle bursage represent important nurse species. They act by regulating temperature extremes, increasing soil nutrients, and reducing evapotranspiration, among others. While nurse plants reduce summer temperature maximums by as much as 18 °C (64 °F), they are more important in raising winter minimum temperatures – as extended frosts limit the range of Saguaros.
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The saguaro is the only species in the monotypic genus Carnegiea. It was described by cactus expert George Engelmann during his work on the United States and Mexican Boundary Survey, published in 1859. What tribe the saguaro belongs to is a matter of taxonomic dispute. A molecular analysis of the cactus family in 2010 placed the saguaro in the Echinocereinae. The ARS GRIN places it in the Echinocereeae.
Distribution and habitat
Saguaros are endemic to the Sonoran Desert and are found only in western Sonora in Mexico and in southern Arizona in the US – although plants are occasionally found in southeastern California. Elevation is a limiting factor to its environment, as the saguaro is sensitive to extended frost or cold temperatures. No wild saguaros are found anywhere in New Mexico, Texas, Colorado, Utah, or Nevada, nor in the high deserts of northern Arizona. The northern limits of their range are the Hualapai Mountains in Arizona.
Native birds such as Gila woodpeckers, purple martins, house finches, and gilded flickers live inside holes in saguaros. Flickers excavate larger holes higher on the stem. The nest cavity is deep, and the parents and young are entirely hidden from view. The saguaro creates callus tissue on the wound. When the saguaro dies and its soft flesh rots, the callus remains as a so-called saguaro boot, which was used by natives for storage.
Gila woodpeckers (Melanerpes uropygialis) create new nest holes each season rather than reuse the old ones, leaving convenient nest holes for other birds, such as elf owls, tyrant flycatchers, and wrens. In recent years, early-breeding, aggressive, non-native birds have taken over the nests to the detriment of elf owls that breed and nest later.
Harming or vandalizing a saguaro in any manner, such as shooting them (sometimes known as 'cactus plugging') is illegal by state law in Arizona. When houses or highways are built, special permits must be obtained to move or destroy any saguaro affected. Exceptions to this general understanding exist; for example, a private landowner whose property is 10 acres (4.0 ha) or less, where the initial construction has already occurred, may remove a saguaro from the property. This is common when the cactus falls over in a storm, its location interferes with a house addition, or it becomes a potential hazard to humans.
In 1982, a man was killed after damaging a saguaro. David Grundman was shooting and poking at a saguaro cactus in an effort to make it fall. An arm of the cactus, weighing 500 pounds (230 kg) fell onto him, crushing him and his car. The trunk of the cactus then also fell on him. The Austin Lounge Lizards wrote the song "Saguaro" about this death.
Contrary to published statements, there is no law mandating prison sentences of 25 years for cutting a cactus down; however it is considered a class 4 felony with a possible 3-year, 9-month maximum sentence.
Invasive species, such as buffelgrass and Sahara mustard, pose significant threats to the Sonoran Desert ecosystem by increasing the rate of fires. Buffelgrass outcompetes saguaros for water, and grows densely. It is also extremely flammable, but survives fire easily thanks to deep root systems. Saguaros did not evolve in an environment with frequent fires, and are thus not adapted to fire survival. Most Sonoran desert ecosystems have a fire return interval of greater than 250 years; buffelgrass thrives at fire return intervals of two to three years. This has lead to the reshaping of the Sonoran Desert ecosystem and threatens the survival of the saguaro.
- Native Americans ate the fruit fresh and dried, making it into preserves and drinks.
- The saguaro is an important source of food and shelter for the Tohono O’odham. Saguaro spines are sometimes used as sewing needles and the ribs are used to make harvesting tools.
- The ribs of the saguaro were used for construction and other purposes by Native Americans. An example can be seen in the roofing of the cloisters of the Mission San Xavier del Bac on the Tohono O'odham lands near Tucson.
- The Seri people of northwestern Mexico used the plant, which they call mojépe, for a number of purposes.
- The ribs of dead saguaros are used as building material.
- Cactus boots, excavated by birds[which?] and taken from dead saguaros, have been used by native peoples as water containers.
- The O'odham tribes have a long history of saguaro fruit use. The Tohono O’odham tribes celebrate the beginning of their summer growing season with a ceremony using a fermented drink made from the bright red fruit, to summon rains vital for their crops.
The saguaro is often used as an emblem in commercials and logos that attempt to convey a sense of the Southwest, even if the product has no connection to Arizona or the Sonoran Desert. For instance, no naturally occurring saguaros are found within 250 miles (400 km) of El Paso, Texas, but the silhouette is found on the label of Old El Paso brand products. Though the geographic anomaly has lessened in recent years, Western films once enthusiastically placed saguaros in the Monument Valley of Arizona, as well as New Mexico, Utah, and Texas. The Dallas, Texas-based band Reverend Horton Heat pokes fun at this phenomenon in their song "Ain't no Saguaro in Texas".
Needles, Paradise Valley, Arizona
Flowers, Scottsdale, Arizona
Unusually-formed crested or cristate saguaro near Kino Bay, Sonora
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Cavities in saguaro cactuses in the Southwest are common. Both gilded flickers and Gila woodpeckers make these cavities for nesting, but they often choose different locations on the cactus. The stouter bills of the gilded flickers allow them to cut cavities through the wooden ribs near the top of the cactus where the ribs converge. Gila woodpeckers stay at midlevel on the cactus where the ribs are separated enough to cut a cavity between them. Cavities in saguaros are cut out by these birds the year before they are inhabited. The excavated cactus secretes a fluid that hardens into a scab, thus preventing water loss, which could kill the cactus, as well as waterproofing the inside of the next cavity.
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Although they do not use them immediately, waiting first for the sap to harden, Gila woodpeckers excavate cavities in cacti and trees as nesting sites. Females typically lay two broods a year of three to five eggs, which incubate for 14 days. Once abandoned, the cavities are occupied by reptiles, rodents, and small birds like kestrels, elf owls, flycatchers, and wrens. In the desert, the woodpeckers perform the important ecological function of removing unhealthy flesh from the saguaro cactus. Some insects on which it feeds carry diseases, harmless to the bird, which damage the cactus and leave discolorations. The marks signal larvae to the bird, and as it excavates the insects, it also cuts away the diseased tissue. As the sap hardens, the cactus is healed, and the excavation becomes a convenient nesting site.
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