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

Summary and Conclusions

For the saguaro, the greatest hazards to survival are associated with the pre-juvenile, seedling stage of life. Establishment is ordinarily reached at an age of 12-14 months.

The mean life expectancy of the newly emerged seedling is on the order of 2-6 weeks. Less than 1% of seedlings survive the first year of life to become established juvenile plants.

The end of the establishment period and the beginning of the juvenile stage is marked by the advent of monsoon rains of the summer following germination. Rapidly increasing in size and root development during that second summer of growth, the young plant attains a substantially increased drought tolerance and an increased ability to survive insect attack.

The tiny, succulent, weakly rooted saguaro seedling is highly vulnerable to destruction by a broad variety of abiotic and biotic agents. These include freezing, drought, rodents, and insects. The relative importance of each factor varies with the habitat, the season, and from year to year.

Our data indicate that biotic factors account for the greatest number of deaths of saguaro seedlings. The observations indicate that this is true in southern parts of the range (Sonora) and in the northernmost parts (Arizona). In the Tucson area and elsewhere along the northern and eastern limits of the plant's range in Arizona and Sonora, it is the extremes of the climatic environment that ultimately control the establishment and survival of young saguaros.

Within the absolute distributional limits imposed by the macroclimate, establishment and survival of young saguaros are dependent upon the physical protection provided by close association with other vegetation, detritus, and rocks. Acting singly or in combination, these moderate the limiting extremes of drought and winter cold and, in addition, reduce the probability of discovery and destruction by animals.

The density of young saguaros is limited by the availability within the community of suitable microhabitats as determined by the physical characteristics of the vegetation, soil type, and topography.

The probability of seedling survival increases with distance from reproductive adult saguaros and the associated activity of foraging animals. In nonrocky habitats, young saguaros rarely occur in close association with mature plants; rocks offer physical protection that permits some establishment in close proximity to mature saguaros.

Differential freeze-caused seedling mortality associated with differences in the intensity and duration of subfreezing temperatures occurs in topographically different habitats. In Saguaro National Monument and elsewhere in southeastern (and north-central) Arizona, sub-freezing winter temperatures are the primary control on seedling survival.

Differential survival of seedlings occurs in rocky and nonrocky habitats, and on north- and south-facing slopes. Seedling establishment is highest on south-facing slopes, lowest on north-facing slopes, and intermediate in flat habitats.

Little or no establishment of saguaro seedlings occurs on north-facing slopes. In these habitats, higher soil moisture levels that favor pre-winter seedling survival in relatively arid environments are offset by subsequent freeze-caused winter mortality.

The southern limits of saguaro distribution may be controlled by summer climate in relation to summer soil moisture potentials. Seedlings cannot become established in poorly aerated or saturated soils.