Saguaro National Park

Ecology

The saguaro begins its life as a shiny black seed no bigger than a period. But what it lacks in size it more than makes up for in numbers. One saguaro produces tens of thousands of seeds in a year, and as many as 40 million in a lifetime of 176 to 200 years. From the start, the odds against survival are great. Out of all the seeds that a saguaro produces in its life, probably only one will survive 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.

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—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.

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 major 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. Humans, too, have played a part in the decline. Livestock grazing, which continued from the 1880's until 1979, devastated some cactus 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 plants 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 park's saguaro forests.


Published: 29 Apr 2002 | Last Updated: 7 Jan 2011
Details mentioned in this article were accurate at the time of publication

advertisement

park finder
step one
Where are you going?


step one
What do you want to do?

+ More Activities


GEARZILLA: The Gorp Gear Blog

Receive Gear Reviews, Articles & Advice

Email:
Preview this newsletter »

Ask Questions