The giant saguaro begins its life as a shiny black seed no bigger than a period on this page. But what it lacks in size it more than makes up for in numbers. One saguaro produces tens of thousands of seeds in one year, and as many as 40 million in a lifetime of 175 to 200 years. But from the start the odds against survival are great. Out of all the seeds that one saguaro produces in its life, probably only one will live to adulthood.

Seeds and young saguaros have the best chance for survival if they are "cared for" by nurse trees such as paloverde 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 one foot tall. By 50 years the saguaro can be as tall as seven feet. After about 75 years it may sprout its first branches, or "arms. The branches begin as prickly balls, then extend out and upward. This is also the stage of life when the saguaro begins to flower and produce fruit and seed.

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  and sometimes more  dwarfing every other living thing in the desert. These are the largest cacti in the United States. Their huge bulk is supported by a strong but flexible cylinder-shaped framework of long woody ribs.

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. But in some forests in Saguaro National Park deaths have greatly outnumbered the growth of new young saguaros.

What has caused the decline in these areas?

• Biologists believe killing freezes are the number one cause of saguaro deaths in the park. The saguaros here are at the extreme northern and eastern edge of their range, where the coldest winter temperatures most often occur.
• Man, too, has played a part in the decline. Livestock grazing, which continued from the 1880s until 1958, devastated some forests. Many seedlings were killed outright by trampling or were unable to find suitable places to grow because the ground had been compacted and nurse trees killed.

Today, with grazing eliminated, recovery appears to be underway in several areas, where thousands of young saguaros have taken hold and are thriving. Still, natural forces, vandalism, and cactus rustling  the theft of saguaros for use in landscaping  continue to take a toll on the parks saguaro forests.