All is not lost

The large, multi-armed saguaros are the senior citizens of the cactus world. Saguaros are killed by being struck by lightning, blown over in a wind storm, and by people. But the largest cause of death is freezing and the effects of freezing may take several years to become evident.
Freezing weakens the cactus, making it more susceptible to infection. A healthy middle-aged saguaro can stand a few hours of temperatures as low as 10 degrees F in mid-winter. On the other hand, 12 hours of 20 degrees F in late fall causes damage and death. 
The chief agent of infection, a bacterium, is carried by the caterpillar of a moth. The bacterium causes rot upon which the caterpillar feeds.
 A dying saguaro becomes an oasis to numerous insects and other arthropods, providing food, moisture, shaded habitat.