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

The Outlook

The outlook for the future of saguaros at Saguaro National Monument is not an optimistic one. Neither, however, does it conform to the grim predictions of early extinction that some have offered. Barring unlikely sudden environmental changes in the habitats of its major occurrence within the monument, the saguaro will continue to survive long after our concern with the question is terminated.

There are today thousands of young saguaros at Saguaro National Monument, many of which will survive beyond a human lifetime. Many more of these, however, will perish during that time. Certainly, there will not be in our time—and perhaps never again—a return to the condition that inspired Homer Shantz (1932) to describe it as the finest stand of the giant saguaro in the world.

Generally, even in the absence of further adverse influences of man, we can expect that the density of large saguaros in most, if not all, habitats within the monument will be lower than in the recent past. In some habitats, however, we can expect that with the complete elimination of livestock grazing an increase in the density of young saguaros will accompany a natural regeneration of the damaged plant communities.

Within the monument, continuing short-term climatic variations can be expected to produce corresponding fluctuations of saguaro populations but not to cause their early extirpation. Generally, this will effect changes in density, with unfavorable winter temperatures eliminating young plants from the least favorable microsites.

Here at the cold-limited margin of its range, saguaro density is limited ultimately by the number of available sites suitable for the survival of young plants during critical freezing periods. During a series of favorable winters when there are no prolonged periods of subfreezing temperatures, young plants will become established in relatively large numbers, occupying microhabitats offering varying degrees of protection from freezing. The subsequent occurrence of a catastrophic freeze will then eliminate those individuals that become established on relatively unprotected sites. The survivors will be those occupying sites offering the greatest protection from freezing.

The number of sites suitable for germination and initial growth of the seedling far exceed those suitable for its survival through the first winter of life. During its first years of life, the probability that a young plant will survive is dependent mainly upon the winter-cold moderating effectiveness of its physical environment. Further, as the plant increases in size, the size of the effective microenvironment needed by the plant also increases, i.e., the plant grows out of its original microenvironment—pebble, dead branch, or small shrub. Its survival then depends upon the presence of a larger protective canopy—a rock, larger shrub, or tree.

As the cold-limited boundaries of its range are approached, survival of the young plant to the age of first flowering is increasingly dependent upon the presence of multistoried vegetation or massive outcroppings of rock. Conversely, it may be observed that in the warmer portions of its range, in southwestern Arizona and in Sonora, the saguaro increasingly occurs in association with low shrubs, without benefit from or dependence upon tree canopy.

Climatic trends

A small but significant climatic change toward somewhat warmer and drier conditions is "written" into diverse records (see Kincer 1946; McDonald 1956; Schulman 1956). A long record of catastrophic freezes accompanying this change during the last century is "written" into the existing age-class structure of the saguaro populations in the northern portions of the Sonoran Desert, and indeed, in the form of freeze-caused scars, in the plants themselves.

The continued occurrence of catastrophic freezes does not, as it might appear, conflict with the observed trend toward "warmer and drier conditions" previously noted. Rather, there may be an overall trend, at least in this region, toward an increasingly continental climate, and conversely, a decrease in the maritime influence upon the climate of the region.

While it is not evident in this stage of our on-going analysis of the freeze-scar record in saguaro populations that a climatic trend is recorded in those scars, it is clear that recurring catastrophic freezes are a long-standing characteristic of the climatic environment of this northern portion of the Sonoran Desert. Observed changes in the age-class structure of these populations may indicate a climatic trend. A continued decline of these saguaro populations would suggest a shift toward increasingly frequent and/or severe extremes of winter cold. In view of the relatively brief length of the climatic record, and long life span of the saguaro—approximately twice the length of the climatic record—such a conclusion is highly tenuous.

Population trends

. . . on passing from lower to higher latitudes or altitudes the number of consecutive hours of freezing becomes gradually greater until the point is reached at which days without a mid-day thaw are first encountered; there is then a sudden rise from about 22 hours of frost to from 36 to 42 hours, according as the fall and subsequent rise of temperature are abrupt or gradual. In other words this factor is unique in that the curve expressing its changes of intensity possesses a sudden vertical rise, or indeed a number of such rises. The line along which this takes place in the severest of winters is bound to be an important limit of plant distribution, at least it is so along the line of sudden rise which lies nearest the absolute frost line.—Forrest Shreve, "The influence of low temperatures on the distribution of the giant cactus." 1911:139.

Regardless of any other changes that may occur, this statement by Forrest Shreve describes the single most important natural environmental factor—past, present, and future—affecting saguaro populations at Saguaro National Monument and elsewhere in the colder portions of its natural distribution in Sonora and Arizona.

Saguaro populations and other cold-limited Sonoran Desert plant species will continue to be controlled by recurring catastrophic freezes (see Steenbergh and Lowe 1976). Any changes in the frequency, intensity, or duration of these recurring catastrophic freezes will result in a response by the affected populations. Any trend in these populations will follow the trend of those particular parameters of the winter climate that exert the greatest effect on the youngest members of the plant populations.

A continued trend toward an increasingly continental climate would be accompanied by a continued response in saguaro and other warm-desert plant populations in the colder portions of their geographic distribution: (1) shrinking the absolute limits of their occurrence along the cold-limited boundaries of their distribution; (2) eliminating populations now occupying habitats with marginally cold winter environments; and (3) reducing the density of populations occupying habitats with warmer, more favorable winter environments. In each instance the response would decrease along an environmental gradient from the coldest to the warmest winter environments of the species' distribution and as the distributional limits imposed by other factors are approached.

Our grossly inadequate knowledge either of climatic trends or the long-term dynamics of saguaro populations, however, justifies no such long-term projections concerning future saguaro populations.

Long-term climatic trends, however, are not necessarily the only or most plausible explanation for observed changes in saguaro populations. It is clear that the dramatic fluctuations of saguaro populations in the colder portions of their range that have been observed during this century are unquestionably attributable to catastrophic freezes. However, we have no information indicating that such climatic events are not a long-standing, "normal" characteristic of the regional climate that has similarly affected many past generations of saguaros in the same habitats where they presently occur.

With its high reproductive potential and long life span, the saguaro is well adapted to maintain itself in an environment in which occasional freezes that result in catastrophic die-off are followed by regeneration of the population during intervening periods of climatic remission. It is entirely possible that we have observed in these saguaro populations but one phase of the normal fluctuation of populations whose stability must be measured not in years, but in generations.

It may well be that the "problem" that we have observed—the "decline" of specific saguaro populations—is but a limitation imposed by our life span, too brief to permit our recognition of an adaptive strategy of a longer-lived species that by natural selection has evolved a near-perfect system for survival in an environment of recurring catastrophic climatic events. If that likely explanation is correct, then the real "problem" is neither in biology nor management, but in a limited perspective, and the only "solution" lies in the perspective offered by the time scale of evolution.