Saguaro Cactus Icon of the Sonoran Desertby Jonathan DuHamel on Jun. 23, 2011, under Natural History
This is the time of year when saguaros bloom and produce fruit. For many generations the Tohono O’odham people have harvested the fruit using long poles made of saguaro ribs. The fruit may be eaten raw or it may be cooked down to a sweet syrup. In the O’odham tradition, some of the syrup was fermented into wine used in a ceremony to herald in the summer monsoon. Dried seeds from the fruit, up to 5,000 per fruit, are rich in protein and fat and can be ground into meal.
Saguaros commonly reach 40 feet tall, a few reach 60 feet to 80 feet. Growth rate depends on rainfall and soil conditions. In Tucson, which averages 12 inches of rain per year, saguaros take about 10 years to get 2 inches high and 30 years to get 2 feet high. Arms may form when the saguaro is 50 to 100 years old and 7 to 12 feet high. Saguaros growing on sandy alluvial fans tend to be bigger and have more arms than plants growing on steep, rocky slopes. The saguaro’s range is restricted almost entirely to southern Arizona and western Sonora. This range is defined by rainfall (too little in the Mojave desert) and by freezing temperatures.
(paraphrased from A Natural History of the Sonoran Desert by Arizona-Sonora Desert Museum press.)
The saguaro (Carnegiea gigantea) is covered by thick, waxy skin that waterproofs the surface and restricts water loss through evapotranspiration almost exclusively to the stomates (pores for gas exchange). The outer surface is folded into pleats (commonly called “ribs,” but not to be confused with the internal, woody ribs. The pleats allow the stem to expand in girth during water uptake without stretching and bursting. Areoles, the roundish pads from which the spines and usually the flowers are produced, are distributed at 1-inch intervals along the ridges of the ribs. Each areole bears a cluster of about 30 spines up to 2 inches long. The spines help protect the plant from herbivorous animals and provide shade to reduce water loss. The lower trunks of old saguaros lose their spines and develop dark, corky bark.
Immediately beneath the skin is a thin layer of chlorophyll-containing tissue where most photosynthesis takes place. The deeper interior, most of the bulk of the plant, consists of the water storage tissue. A fully hydrated saguaro contains 90% water and a large plant weighs about 80 pounds per foot. This water-bearing tissue helps protect the cactus from temperature extremes.
Heat is absorbed through the surface during the day and is stored, resulting in a small temperature rise that does not reach a lethal level. At night, the heat is slowly radiated back into the air. This same thermal inertia usually keeps the tissues above freezing on cold winter nights.
(You cannot tap into a saguaro to get a drink of water. See my post “Can You Get Potable Water From a Cactus?” to find out why.)
A cylinder of 13 to 20 woody ribs occur near the center of the cactus stem, running the length of the main stem and branching into the arms. In the upper part of the stem, the ribs are separate; as the stem ages the ribs continue to grow and fuse into a latticed cylinder. The outside of these woody ribs contain the vascular, or water transporting, part of the plant.
A tap root extends down about 2 feet. The rest of the extensive root system is shallow, rarely more than 4 inches, and radiate outward about as far as the cactus is tall.
Flowers and pollination
Flowers occur near the top of the main stem and on the tips of arms. The white flowers are about 3 inches in diameter and smell like ripe melon. Each flower opens at night and remains open until mid-afternoon the next day. Nectar produced at night attracts bats. A second batch of nectar is produced in the morning to attract birds and insects. Bats and white-winged doves are the main pollinators. The doves, Gila Woodpeckers, and House finches disperse the seeds.
After the flowers are pollinated and fruits mature, the fruit opens to expose its red interior leading some people to think there are red flowers atop the saguaro.
Many large saguaros contain holes excavated by Gila Woodpeckers and Gilded Flickers. The birds go in and down, removing the fleshy part of the cactus. The cactus produces scar tissue, calus, which quickly becomes very hard and impervious to bacterial infection. This material, often in the rough shape of a boot, survives after the cactus dies and rots away. Woodpeckers generally excavate a new hole each year, leaving the hole for other cavity-nesting birds.
Most saguaro seedling die from drought, frost, and predation. Seedlings up to a foot tall are eaten by rodents and rabbits.
Some mature saguaros are killed by lightening strikes or blown over by high winds. However, the chief agent of mortality in the Arizona Upland is freezing. Mortality by freezing depends of the seasonal timing and duration. A healthy middle-aged saguaro can stand a few hours of temperatures as low as 10 degrees F in mid-winter. On the other hand, 12 hours of 20 degrees F in late fall causes damage and death.
According to Natural History cited above, a frost-damaged saguaro may survive for another decade or longer, but eventually weakens until it can no longer resist infection. “Bacterial rot caused by Erwinia cacticida turns the flesh of weakened plants into an odoriferous black liquid.” Healthy saguaros can ward off small infections by walling off infected parts. The bacterium is carried by a moth whose maggot-like caterpillar burrows into the cactus and feeds on the rot it introduces. Many living saguaros have round, half-inch scabs on their surface. These are the caps of contorted tunnels left behind by the caterpillar.
Back in the 1990′s some biologists published a paper about the “brown decline” and impeding doom of saguaros. These scientists were unfamiliar with saguaro biology. There was a great freeze in December, 1978, which affected some of the old saguaros and this is what the visiting biologists focused on. They ignored the large population of smaller, younger saguaros.