THE FATE OF THE SEED: DISPERSAL, ATTRITION, AND GERMINATION (continued)
The saguaro is a subtropical species and its seeds germinate only in the summertime. Germination occurs principally in July and August during the southwestern summer monsoon. Thus, successful germination and early establishment depend on a short period of coincident high levels of moisture and warmth during but a few weeks of the desert year. We are not surprised, therefore, to find that this highly successful desert species has acquired adaptive strategies that fit a set of germination requirements to an environment characterized by a persistently high evaporation which is only partially offset by scanty and often uncertain rainfall.
Seedfall is completed just prior to the normal arrival of summer rains. In exceptional years, the two eventsseedfall and rainfallactually overlap. The first summer rains contribute importantly to seed survival by dispersing seeds and by washing them into locations that offer concealment from predators. Little or no germination, however, results from these late June and early July rains. Rigid light and moisture requirements interact to inhibit germination and prevent losses that would result from germination at that time, for sporadic early monsoon rains seldom provide a sufficiently reliable supply of moisture to promote germination and support the initial establishment of seedlings. The increasing frequency of rains during the second half of July offers progressively more favorable conditions for seed germination.
The highly variable spatial and temporal distribution of summer rainfall importantly affects the number of saguaro seed germinations occurring at a given location (see Shreve 1914; Humphrey 1933; Mallery 1936). Further, the number of germinations varies in accord with the characteristically large year-to-year variability of monsoon precipitation patterns. Our experimental and field observations, however, indicate that the most favorable conditions for saguaro seed germination do not necessarily occur during years with the highest total summer precipitation. Rather, natural germination is associated with temporal clustering of summer rainstorms that provide continuously high moisture levels at and near the soil surface during the 2- to 3-day germination period.
Most germination takes place during the period from mid-July through the first week of August. Then, with continuing availability of high surface moisture resulting from short-spaced rainstorms and supplemented by dewfall, germination proceeds to completion within a period of 2-3 daysinitial establishment of the seedlings takes place at a time when temperatures and moisture are optimum for their growth.
The high temperature limitations on saguaro seed germination operate to restrict natural germination to shaded sites where periods of high moisture are sufficiently prolonged to permit germination (Table 10; Fig. 21). During wet periods such sites seldom reach temperatures high enough to inhibit germination (Alcorn and Kurtz 1959; McDonough 1964). Inhibition of germination by low temperatures effectively restricts germination to the warm-wet periods of June, July, August, and September in the overall distribution of the saguaro.
Genetically controlled variability in the time required for completion of the germination process further insures that the entire annual seed crop will not respond to conditions that are marginal for seedling establishment. Approximately 10% of seeds tested under ambient summer light and temperatures required 4 days or more of continuous exposure to free water for completion of germination. Thus, even under conditions that permit extensive germination, a substantial portion of the germinable seeds remain viable and available for subsequent germination.
The light requirement may seem to present an apparent survival disadvantage because it seemingly dictates that germination must occur at or near the soil surface where the predation pressures are greatest and requisite moisture conditions are least apt to prevail. However, in view of the characteristic development pattern of the monsoon rains, an important adaptive strategy can be recognized. In the presence of insufficient moisture to complete germination and initial establishment of the seedling, the light requirement acts to stop the germination process without loss of seed viability. Buried seeds retain their viability, and will germinate quickly when exposed by subsequent rains, animal digging, other natural forces, or experimental exposure.
The light requirement insures germination sufficiently near the soil surface for the globular seedling to emerge and immediately receive adequate light for subsequent growth (Fig. 24A). Sensitivity to far-red light may also permit better utilization of energy from nocturnal re-radiation thus hastening completion of the germination process.
Field observations indicate that in nature germination of shallowly buried seeds accounts for a substantial portion of the annual saguaro seedling crop. This observation appears to conflict with knowledge that germinating seeds must receive exposure to light after they have imbibed water, that little or no germination (<1%) will occur in total darkness. Extensive laboratory tests using a variety of environmental, mechanical, and chemical treatments have revealed no mechanism by which germination might occur naturally in the absence of light (Alcorn and Kurtz 1959; McDonough 1964).
Based on successful use of acid treatments in promoting dark germination, McDonough (1964) suggested that passage through the digestive tracts of animals might promote germination in the absence of light. However, we have attained no success in the dark-germination of saguaro seeds recovered from the feces of native rodents or birds.
In fact, the natural germination of buried saguaro seeds requires no complex mechanism for explanation. In view of the brief exposure and low intensities of light required for germination as found by all investigators, the natural translucence and gravelly surface of the soils of the saguaro habitat will admit sufficient light in most instances to permit germination of seeds buried to a depth of 5-10 mm (0.2-0.4 inches), the approximate maximum depth from which the seedling can emerge. A further mechanism for satisfying the light requirement exists in the soil churning and washing action occurring during typical summer rain storms. Such action permits burial of seeds after wetting and exposure to light.
Germination of saguaro seeds requires contact with free water and exposure to light (Alcorn and Kurtz 1959; Alcorn 1961a). Under ambient summer light and temperatures in a laboratory environment, initial germination of saguaro seeds maintained in continuous contact with free water occurs approximately 48 hr after initial wetting and 50% germination is reached in approximately 72 hr (Table 12; Fig. 23; also see Keswani and Upadhya 1969). However, within the natural environment of the saguaro such prolonged periods (3 days) of continuously available water are a rare, almost nonexistent occurrence during July and August when most natural germination takes place. Rather, natural germination commonly follows two or more shorter periods of water availability with interspersed drying conditions at the soil surface resulting from the occurrence of two or more distinct rainstorms within a 2- to 5-day period (Steenbergh and Lowe 1969).
In a humid atmosphere, saguaro seeds hygroscopically imbibe and retain moisture required for germination. Such pre-exposure to high relative humidity effectively speeds the germination process and reduces the required period of seed contact with free water. First germination of seeds so pre-conditioned can take place after 24 hr of wet contact and 50% germination can occur within 48 hr. In a near-saturated atmosphere, maximum hygroscopic imbibition (approximately 20% of the air-dry seed weight) is reached in approximately 20 hr (Table 11; Fig. 22).
In an experiment to determine the effect of such treatment on the required period of contact with free water, treated seeds reached 50% germination in 46.5 hr, 21.5 hr sooner than untreated (air-dry) seeds (68.0 hr) (Table 12; Fig. 23). Hygroscopic imbibition can reduce by approximately one day the required pre-germination period of seed contact with free water.
Germination under natural conditions, therefore, need not depend upon the rare occurrence of a continuously saturated soil surface over a prolonged period of time; but rather it can be sustained by a period of high relative humidity followed by a relatively brief period of saturation. Thus, hygroscopic imbibition allows rapid germination with intermittent high moisture levels commonly associated with the characteristic summer storm patterns of the Sonoran Desert.
Requisite environmental conditions for saguaro seed germination occur far beyond the bounds of the plant's natural distribution. Only at the western boundaries of its occurrencein the lower Colorado River Valleywhere summer rainfall is seldom adequate to promote germination, does the range of the species appear to be limited primarily by germination requirements. With that exception, therefore, the factors that limit its distribution and control populations along the margins of its range must act during the post-germination stage of the plant's development.
Last Updated: 21-Oct-2005