Their huge bulk is supported by a strong but flexible cylinder-shaped framework of long woody ribs. 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 Monument deaths have greatly outnumbered the growth of new young saguaros.
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.