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.
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.