SAGUARO Ecology of the Saguaro: II NPS Scientific Monograph No. 8

Summary and Conclusions

We estimate that a healthy saguaro produces on the order of 40 million viable seeds during an average reproductive life span of 100 years. For a population to maintain itself or to grow, one of those seeds must, within that period, germinate and survive approximately 30 years or more, to the age when it becomes a reproductive member of the population.

The peak of saguaro fruit ripening and seedfall occurs in late June or early July prior to the arrival of germinating summer rains. Variations in the date of that peak as well as the size of the current year's seed crop are related to year-to-year variations in climatic factors.

Early or late occurrence of peak seedfall, regulated respectively by early or late arrival of warm spring temperatures, importantly affects probabilities for seed survival. Early seedfall lengthens the period of seed exposure to decimating factors. Late seedfall shortens that exposure and accordingly affects the number of germinable seeds remaining on the ground at the time of germinating summer rains.

Drought does not have a significant diminishing effect upon the size of the annual seed crop, fruit production being dependent upon energy and moisture reserves stored within succulent stem tissues. Moreover, prostrate saguaro stems completely severed from their roots are capable of producing buds and flowers for at least 2 years.

Recurring winter freezes that frequent Arizona and northern Sonora effect a significant reduction in reproductive growth. Damage to incipient buds caused by such freezes directly reduces the number of fruits produced the following spring. Freeze-damage may affect flower production on the same plant for one or more years after the event, although to a lesser degree.

Freezing permanently diminishes the reproductive base of the population by selectively removing the largest and potentially most productive members from the population. The most severely damaged individuals collapse within the year. Others, moribund or dead, appear to senesce, often over a period of several years, each year producing diminishing numbers of fruits by utilizing stored energy reserves from the succulent stem until terminated by the ultimate collapse of the stricken plant.

Reproductive potential in spectacular stands of large saguaros in the Tucson area—including those at Saguaro National Monument—and elsewhere near the northern and eastern limits of the distribution of the plant in both northern Sonora and northern and eastern Arizona is more than adequate to maintain existing population levels. The density of large, reproductive individuals within stands along the edge of the species range far exceeds that in other portions of the plant's range where stands exhibit more normal distribution of age classes. The relative sparsity of young saguaros within the stands at Saguaro National Monument cannot reasonably be attributed to a lack of viable seeds. The causes of the wide departure from stable age distribution must instead be sought in the factors that control the survival of the seed, its germination, and survival of the plant to the age of reproduction.