From the start, the odds against survival are great. Out of all the seeds that a saguaro produces in its life, few will survive to adulthood.
Seeds and young saguaros have the best chance for survival if they are cared for by nurse trees such as palo verde and mesquite. Saguaro seedlings that grow under these sheltering plants are shaded from the desert's intense sunlight, blanketed from winter cold, and hidden from rodents, birds, and other animals that eat them. Rocks provide similar protection for young saguaros. Saguaros do best on bajadas -- gently sloping outwash plains at the foot of desert mountains.
Growth of a Green Giant ... a saguaro's growth is extremely slow. Growth occurs in spurts, with most of it taking place in the summer rainy season each year. By the end of a year the saguaro seedling may measure only 1/4 inch. After 15 years, the saguaro may be barely a foot tall. At about 30 years saguaros begin to flower and produce fruit. By 50 years the saguaro can be as tall as 7 feet. After about 75 years it may sprout its first branches, or "arms." The branches begin as prickly balls, then extend out and upward.
By 100 years the saguaro may have reached 25 feet. Saguaros that live 150 years or more attain the grandest sizes, towering as high as 50 feet and weighing 8 tons, sometimes more, dwarfing every other living thing in the desert. These are the largest cacti in the United States. A strong but flexible cylinder-shaped framework of long woody ribs supports their huge bulk.
Death and Rebirth ... Saguaros may die of old age, but they also die of other causes. Animals eat the seeds and seedlings, lightning and winds kill large saguaros, and severe droughts weaken and kill all ages. The saguaro is vulnerable during every stage of its life.
Where there is a balance of life and death, saguaro forests thrive. Until recent years deaths have greatly outnumbered the growth of new young saguaros in some forests within the park. What has caused the decline in these areas?
Biologists believe that severe freezes are the park's major cause of saguaro deaths. The saguaros here are at their extreme northern and eastern range, where the coldest winter temperatures most often occur.
Humans, too, have played a part in the decline. Livestock grazing, which continued from the 1880s until 1979, devastated some cactus forests. Seedlings were killed outright by trampling or were unable to find suitable places to grow because the ground had been compacted and nurse plants killed.
Today, with grazing eliminated, recovery of the saguaro is underway in several areas. Thousands of young saguaros have taken hold, and they are thriving. Still, natural forces, vandalism, and cactus rustling -- the theft of saguaros for use in landscaping -- continue to take a toll on the park's saguaro forests.