ESTABLISHMENT AND SURVIVAL (continued)
Drought-caused mortality of saguaro seedlings is related importantly to the date of germination and subsequent conditions for growth. Seedlings which sprout during July and early August usually attain sufficient size and volume during the summer growth period to survive through the autumn and fore-summer drought periods. During occasional years with heavy September and/or October rains, survival is further aided by additional growth.
Few seedlings that result from late summer germination (late August and September), however, survive to the start of the second year of life. Such seedlings do not develop sufficient water-storage capacity in the brief remaining growth period to survive prolonged periods of drought. Poorly developed seedlings that survive the arid after-summer are usually eliminated by the more severe drought conditions of the following arid fore-summer (Steenbergh and Lowe 1969). Our observations on winter mortality of seedling and young juvenile saguaros, however, support the conclusion that low levels of hydration increase resistance to freezing (Soule and Lowe 1970).
Desiccation and moisture stress resulting from freeze-caused injury to seedlings and young juvenile saguaros are important causes of apparent drought-kill during the arid fore-summer (Steenbergh and Lowe 1976). Although the death of such plants finally results from critical moisture stress, their death must be attributed to freezing rather than to drought.
The results of the 1967 and 1968 seed-broadcasting experiments offer a reasonable measure of seedling establishment in the principal desert habitats at Saguaro National Monument during years with relatively mild winters (Tables 16-22; Figs. 28-30). During the two winters of those experiments, the lowest recorded minimum temperatures at the University of Arizona weather station were 22°F (-5.6°C) and 30deg;F (-11°C) 1deg;F (0.6°C) and 9°F (5.0°C) respectively, above the 77-year mean (21.1°F; -6.1°C) for that station.
The ultimate suitability of these habitats for the establishment and survival of young saguaros is determined, however, not by any climatic average but, rather, by the extreme minimum temperatures of the most severe winter that they must endure. Catastrophic freezes, a common occurrence in this region, critically limit survival not only of seedlings but of survivors from previous years as well (Steenbergh and Lowe 1976). At those times, the highest probability for continued survival of the young plant becomes abruptly and decisively associated with specific topographic situations (sensu microhabitats) that effectively moderate the intensity and duration of critical winter minimum temperatures, i.e., south-facing slopes, rocky footslopes, and the upper portions of adjoining valleys (Fig. 31). These are winter-warm microenvironments. Moreover, saguaros at the highest elevation (approximately 1585 m; 5200 ft) are growing only against south-facing (nocturnal re-radiating) rock outcrops (Fig. 32A). Conversely, in the colder microenvironments of north-facing slopes and areas of cold-air drainage and accumulation, there is a high rate of freeze-caused mortality; there is little or no survival of young saguaros in such habitats (Fig. 32B).
On the upper bajadas (nonrocky, "flat" habitats), winter thermal microenvironments and freeze-caused mortality are intermediate between those of north-facing and south-facing slopes, and with increased distance from adjoining footslopes these habitats become progressively less favorable for the survival of young saguaros. Furthermore, as distance from the rocky footslopes increases, winter survival becomes increasingly dependent upon the presence of a multistoried (cold-moderating) canopy of associated shrubs and trees (Fig. 33).
Topography and slope exposure
Distinct differences in first-year saguaro survival occur in topographically different habitats of the two sections (east and west) of Saguaro National Monument. Field experiments comparing saguaro seedling survivorship in rocky habitats at the east monument clearly demonstrate that pre-winter survival on south-facing slopes is consistently higher than on adjacent north-facing slopes, and that the relationship continues throughout the first year of life (Tables 16-22; Figs. 28-30). Further, first-year survival in flat (nonrocky) habitats is lower than on south-facing slopes, but higher than on the north-facing slopes. Survival in the rolling-hills habitats is intermediate between that in flat and rocky habitats, and there, as on the steeper rocky slopes, seedling survival on the south-facing slopes is higher than on the north-facing slopes.
Due to lack of germination in some experimental plots, survivorship data from two west monument habitats are incomplete; in 1967, there was no germination at the "flats" site, and, in 1968, there was none in the open plot at the "rocks, south" site. However, synthesis of the available data for these years suggests that conditions for pre-winter seedling survival in the west monument habitats differ significantly from those of analogous habitats in the east monument. Data from the west monument (1967 experiments, and from 1968 exclosures) indicate that north-facing slopes offer a slightly more favorable environment for pre-winter survival of seedlings than do south-facing slopes. Compared to the east monument, the west monument offers a hotter and more arid environment. In that environment, relatively higher availability of moisture on north-facing slopes may well explain the higher rate of pre-winter survival in those habitats.
Drought-caused mortality of saguaro seedlings, discussed previously in this chapter, refers to death from desiccation resulting from drying of the soil to beyond the wilting point. As also discussed, such mortality is related to details of topography, slope exposure, and other soil-related features of saguaro habitats.
The plant-available soil moisture is affected importantly by the physical texture of the soil (Table 23). Nowhere is the result of this fact more striking than in the Sonoran Desert home of the saguaro, for saguaros are not found in poorly aerated soils (Shreve 1920; Kramer 1961, 1962). Coarser-textured soils primarily of gravel and rock on bajadas, fans, and rock slopes support more highly diverse desert biotic communities in which the saguaro is a major dominant and where it often forms "saguaro forests" (Fig. 34A); such coarse soils are also relatively well drained. Finer-textured soils that are relatively high in clay, silt, and sand content support fewer species, and so result in desert communities of simpler structure (Fig. 34B); such relatively fine soils are also less well drained (Yang and Lowe 1956; Kramer 1962; Whittaker and Niering 1965).
In experiments with young saguaros we have found that seedling growth stops and death eventually results from prolonged exposure to near-saturation soil moisture values in soils of all textures. In the greenhouse, rotting of young saguaros often results from daily watering during warm weather. In cultured young saguaros, Booth and Alcorn (1959) reported destruction of crowded plants by soft-rot fungi (Fusarium spp.).
The natural association of the saguaro with well-aerated soils and, conversely, its lack of tolerance of saturated soils under warm conditions strongly suggests that the southern limits of its distribution may be determined by interactions of soil characteristics and summer climate. The poorly aerated, predominately clayey soils that characterize the edge of the desert and coastal thornscrub southward from the vicinity north of Obregon to Navojoa may prevent the growth, establishment, and survival of young saguaros there and further southward in Mexico on the west Gulf Coast plain.
It is not surprising that the last saguaro populations southward are located on natural rock piles (Figs. 8, 12B, 35A). One is on Cerro Prieto east of Navojoa along the Navojoa-Alamos road and, further southward, the other is on Cerro Masiaca located northwest of Masiaca. Both populations are in extreme southern Sonora.
As demanded by the hypothesis given, the habitats of both of these southernmost saguaro populations are on disjunctively isolated rocky buttes. These isolated populations are located well above the poorly aerated, fine-textured soils of silts and clays that characterize the intervening stretches of the flood-prone coastal plain. The intervening plain is densely clothed with subtropical thornscrub ("thorn forest") maintained by a relatively high summer-warm rainfall regime (Fig. 35B).
In any year, the major portion of the current seedling crop is destroyed by a sequence of climatic and biotic factors. The operation of individual factors is associated specifically with one or more vegetative seasons as described by MacDougal (1908c, d) (Table 24): humid mid-summer (July to September), arid after-summer (October and November), winter months (December to March), and arid fore-summer (April to June). Most seedling deaths occur during the summer growth period. The rate of seedling mortality drops abruptly with the start of the arid after-summer in September (Tables 16-22; Figs. 28-30). Round-tailed ground squirrels estivate at that time, and insect numbers and activity decrease sharply.
There is large year-to-year variability in the rate of seedling establishment (Tables 25-27; Fig. 36). Such variability results primarily from differences in (1) summer rainfall and (2) winter minimum temperatures. During some years, exceptionally favorable conditions for germination produce an unusually large crop of seedlings; such conditions occurred at Saguaro National Monument (east) in 1964 (Steenbergh and Lowe 1969). Under those conditions many seeds germinate in exposed situations where there is little or no chance for seedling establishment. Under those circumstances a high rate of germination will be followed by high seedling mortality. Thus, a high rate of seed germination does not necessarily result in a correspondingly high rate of seedling establishment.
The relative importance of each of the several factors responsible for the destruction of young saguaros cannot be measured simply in terms of the relative mortality attributable to each cause. Rather, the relative importance of each factor is time-related. The importance of a particular factor lies in both its temporal and spatial impact upon the existing population and the life expectancy of the survivors. Furthermore, evaluation of factor importance must be concerned with the question of whether or not the particular factor operating is density-dependent and age-related.
The life expectancy of a young saguaro increases rapidly with ageit outgrows its predators and becomes increasingly able to survive the periodic hazards of the abiotic environment. However, in the case of recurring catastrophic freezes which operate without regard to population density (density-independent), survival is primarily dependent upon (1) the number of winter-favorable sites available within the habitats, and (2) the number of such sites actually occupied by young plants. Regardless of their numbers, other exposed cohorts in the population will die. Thus, near the cold-limited boundaries of its range, the number and character of physical microhabitats, as determined by the plant community together with the topographic characteristics of the habitats, are the factors that ultimately limit the survival and density of young saguaros.
Last Updated: 21-Oct-2005