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

Summary and Conclusions

Dispersal. Early summer rains in late June and early July effect a major contribution to the dispersal of saguaro seeds by dislodging seed masses still remaining on the plant, and by washing individual seeds into protected locations that provide concealment from predators and are favorable sites for subsequent germination and establishment. The early rains reduce the attrition of seeds and ordinarily set the stage for germination during subsequent summer rains.

Such seed dispersal to more favorable sites accomplished by gravity and rainfall runoff is mainly effective for downslope dispersal in sloping habitats. Animal consumers, principally birds, which pass undigested viable seeds through their digestive tracts, are the primary agents for seed dispersal upslope and in flat habitats. Obligatory seed-eaters that efficiently digest consumed seeds, however, make little or no contribution to seed dispersal.

Attrition. In the complete life cycle of the saguaro, the greatest mortality occurs during the pre-germination (seed) stage, the 1- to 5-week period between seedfall and the occurrence of germinating summer rains. During that period the major portion of the annual seed crop is consumed by animals—birds, mammals, and insects.

To some degree, the fruits of the saguaro are utilized either as food or a source of moisture by nearly every warm-blooded animal member of the community and by several species of insects. Initially heavy losses to feeding birds, especially doves, occur while fruits are still attached to the plant. The greatest losses, however, accrue after fruits drop to the ground where the diversity of consumer species ranges from ants to coyotes. The relative importance of consumer species varies according to their size and foraging efficiency, diversity of feeding habits, and relative abundance within the particular community.

Generally, obligatory seed-eaters—principally harvester ants, doves, and heteromyid rodents—exert the greatest impact as they are abundant and efficient consumers. However, in habitats where they are abundant (such as the Cactus Forest of the east monument), round-tailed ground squirrels consume the major portion of dropped fruits. The impact of specific saguaro fruit consumers varies greatly from one desert community to another. In flat habitats of the east monument, relatively dense populations of harvester ants and round-tailed ground squirrels quickly remove most of the seeds which reach the ground. However, in flat habitats of the west monument where neither species is abundant, a large proportion of the seed crop remains on the ground, undisturbed throughout the pre-germination period.

In some locations, seeds remain over winter on the ground. However, as a result of destruction by insects, microorganisms, or climatic action, few if any survive or contribute significantly to the following year's germinable seed supply.

Most of the fruits drop to the ground immediately beneath the parent plant. Intensive animal activity sponsored by this abundance of food not only results in heavy attrition of seeds from these sites but also severely limits the suitability of such sites for germination and seedling survival, especially in flat habitats.

In view of abundant germination observed during favorable years, it is unlikely that the lack of young saguaros evident in some habitats is attributable to attrition of seeds.

Germination. The principal germination of saguaro seeds takes place in July and August from seeds of the current year's crop. Optimum conditions for natural germination coincide with the first full development of monsoon storms. Then, during the period from mid-July to mid-August, germination is normally associated with the occurrence of two or more rainstorms within a 2- to 5-day period.

The availability of moisture is the critical determinant in the germination process; temperature and light requirements are readily satisfied within all natural habitats of the saguaro. In continuous contact with free water, germination takes place in 48-72 hr.

The germination process is facilitated by the capability of seeds to absorb moisture hygroscopically. We conclude that this is an important adaptive strategy. Pre-conditioning by hygroscopic imbibition can reduce the critical period of required contact with free water by as much as one day, importantly mitigating the need for a prolonged period of saturation at the soil surface.

The saguaro is dependent in its germination requirements on a physically modified microenvironment produced by trees, shrubs, rocks, or other shade-producing objects. These shaded microhabitats provide moderated daytime temperatures that are within the upper range required for germination and prolong periods of high moisture availability at the soil surface. Ultimately, the number of adequately shaded sites suitable for saguaro seed germination is limited by the physical structure of the community.

Experimental evidence indicates that fewer than 1 in 200 seeds that reach a site where germination can occur survive to the seedling stage. We estimate that the net natural survival to the initial stage of seedling establishment is less than 1 per 1000 seeds produced.

The summer climatic environment of the Tucson area is well within the range necessary for saguaro seed germination; this exceeds, in germination suitability, the drier, more westerly portions of the species' range. At Tucson, depending upon the overall intensity of monsoon development, the year-to-year suitability of conditions for natural germination ranges from poor to near optimum, but some natural germination occurs in all years.

It seems likely that the western limits of saguaro distribution in California, Arizona, and northern Sonora are controlled by insufficient moisture for germination. Elsewhere to the northeast and south, it is clear that summer climatic environments would permit natural germination far beyond the present limits of saguaro distribution. Those limits, therefore, must be determined by factors that operate during the post germination stages of the plant's growth.