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

Hypotheses on Saguaro Population Change

Some saguaro populations fluctuate markedly in density and age structure (Figs. 3-6). This is especially true and even characteristically so in the cold-limiting parts of the range of this subtropical species (see Harper 1967; Steenbergh and Lowe 1976). Regarding such fluctuations, as seen for example at Saguaro National Monument, we comment briefly here on: (1) overgrazing; (2) climatic change; (3) "disease"; and (4) environmental limiting factors.

The first three hypotheses, more or less distinct and with relatively long histories, have originated in the Tucson area. In various combinations these basically differing hypotheses attempt to explain these fluctuations of saguaro populations in various localities. We briefly discuss here some of their more important limitations as they relate to the interpretation of the experimental results and observations of others, and to our own investigations since 1951 on the role of freezing weather and other limiting factors in the regulation of saguaro populations.

The grazing-plus-rodents hypothesis

Wilder and Wilder (1939:160) were the first to suggest the possibility that the sparsity of young saguaros in some nonrocky habitats might be related to ". . . the effects of erosion and cattle grazing . . ." Mielke (1944:4), reporting on investigations at Saguaro National Monument (east), noted the sparsity of young saguaros and, concerning conditions necessary for establishment, states "This cannot take place in the presence of over-grazing or of the large rodent population, particularly of ground squirrels, now on the Monument."

In regard to saguaro populations including those at Saguaro National Monument, the question was further examined and discussed by Niering et al. (1963) and Niering and Whittaker (1965). They concluded that in some habitats grazing-caused alteration of the plant community results in increased density of rodent populations that destroy young saguaros, that this leads to slow decline and disappearance of saguaro populations in these habitats resulting from failure of the saguaro to reproduce, and that rodent populations may tend to keep saguaro populations at lower levels on the bajada than on rocky slopes. Further, they concluded that when, in some habitats, the effects of grazing are far advanced and rodent populations are high, these effects are largely irreversible.

A report on the influence of rodents and lagomorphs on the survival of transplanted saguaro seedlings at Saguaro National Monument (east) by Turner et al. (1969) concluded that ". . . there is no conclusive evidence that lagging reproduction of the saguaro forest at the monument is a result of increased feeding by small mammals on cacti."

Burgess (1964) examined saguaro populations at Tonto National Monument, Arizona, and discussed grazing in relation to observed bimodal age-class structure of the saguaro population at that location. He hypothesized that historic grazing may be responsible for the relative sparsity (10%) of saguaros 30-75 years old, and that the much larger proportion of younger plants (33%) may be correlated with protection from grazing during the previous 24 years.

Obviously, overgrazing produces changes in the natural environment that are seriously detrimental to the establishment and survival of young saguaros. However, the hypothesis may overestimate the ultimate impact of rodents, and fails to accord sufficient importance to the controlling effect of recurring catastrophic freezes in these marginal habitats.

The climate change hypothesis

Climatic change (together with grazing) has been proposed as a possible cause of the lack of young saguaros in the "valley portions" of Saguaro National Monument. Turner et al. (1966:102) suggested that failure of saguaro "population recovery" in these habitats may be dependently related to a decline in shrubs and trees; that "General forces, such as grazing or climatic change, which cause the loss of perennials first affect the nurse plants, and then the influence of these forces is relayed to the saguaro populations."

The question of historic changes in saguaro populations and associated vegetation in Saguaro National Monument has been explored by Hastings and Turner (1965). Photographs they provide support earlier observations that the saguaro population in the flat habitat of the Cactus Forest area and the adjacent rolling hill habitat of the east monument was not maintaining itself. Further, they observed that ". . . in the rocky foothills and along the lower slopes of the Tangue Verde Mountains . . . the saguaro seems to be re-populating and the plant communities appear to be stable." The report also provides documentation of fluctuations in the status of associated perennial plant species populations and offers the observation that ". . . the giant cactus is dying out, but so, in many parts of Saguaro National Monument, is much of the rest of the community associated with it" (Hastings and Turner 1965:197).

Important as these observations are, they are not explained in relation to those authors' interpretations of climatic change. The observed die off of large saguaros is attributed to "bacterial necrosis," and they offer that ". . . the disease has probably always been a major cause of death" (Hastings and Turner 1965:195).

The possibility that the sparseness of young saguaros in these populations is related to the reported climatic trends could be assumed. However, Hastings and Turner (1965) drew no specific conclusions concerning the relationship of the reported climatic changes to the observed sparsity of young saguaros in these unstable populations, or to observed changes in the associated plant community.

From their analysis of weather records back to 1898, Hastings and Turner (1965:287) concluded: "With fluctuations, winter rainfall has dropped markedly and winter temperatures have risen; summer rainfall has remained about the same, or has decreased slightly; . . ." Such trends would doubtless affect saguaro populations in California, western Arizona, and northwestern Sonora where lack of moisture controls the western limits of the species distribution. However, as we will further show in this report, the observed changes in the saguaro populations along the eastern and northern boundaries of its distribution—where the species is limited neither by lack of moisture nor high temperature—cannot be explained either by a decrease in mean winter or summer precipitation or by a rise in mean winter temperatures.

The "bacterial necrosis disease" hypothesis

Catastrophic freeze-caused lethal injury to saguaros is followed by the dramatic breakdown of damaged tissues—often not evident till long after the critical freeze. The bacterial necrosis of such damaged tissues—the so-called "bacterial necrosis disease"—is the long-recognized, natural, and ecologically important process of natural decomposition. The once presumed fatal disease is a result, not a cause, of saguaro death.

"Bacterial necrosis disease" has been called a "major cause of saguaro death" (Hastings and Turner 1965:195). The attrition of saguaros in the Cactus Forest area of Saguaro National Monument (east) attributed primarily to "bacterial necrosis disease" together with failure to "maintain an adequate rate of natural repopulation" has led to the prediction that this plant population will cease to exist by the end of this century (Alcorn and May 1962).

In his original description of the species, Engelmann (1852) remarked on the ". . . decomposition of the fleshy parts" of the saguaro. Numerous others during the following 120 years observed and reported on various causes of saguaro death and described the subsequent process of decomposition and the many diverse organisms associated with that ecologically important process.⁵

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⁵The list of references includes Hooker 1892; Hubbard 1899; MacDougal 1908b; Shreve 1910, 1911, 1920, 1929, 1931a, 1945, 1951; Brown et al. 1942; Lightle et al. 1942; Gill and Lightle 1942, 1946; Brown and Boyle 1944; Mielke 1944; Boyle 1949; Gill 1951; Howes 1954; May and Palmer 1959; Alcorn 1961a, 1961b, 1966; Santana 1961; Alcorn and May 1962; Graf 1965; Takacs 1967; Schuyler 1968.

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Shreve (1910:238) noted that saguaro deaths ". . . are due in at least 90% of the cases to mechanical factors . . ." and are ". . . followed by the work of bacteria and fungi." Shreve (1911) subsequently established that freezing is an important cause of saguaro deaths (see also Thornber 1916). Apparently unaware of this important early work, Lightle et al. (1942) identified a rot-associated bacterium (Erwinia carnegieana) and unfortunately termed the long-known necrosis of the saguaro a "disease."

A comprehensive survey on the condition of saguaro populations at Saguaro National Monument (east), in the Tucson Mountains, and at Organ Pipe Cactus National Monument was begun in 1941 by the USDA Bureau of Plant Industry. At the same time the bureau initiated a massive experimental "sanitation program" involving "surgery" or the removal of all rotting saguaros from a 0.5 mile² area (1.29 km²) of the Cactus Forest portion of the Saguaro National Monument (east).

The unpublished reports of those investigations (Gill and Lightle 1942, 1946; Mielke 1944; Gill 1951) contain a large amount of valuable information relevant to the understanding of the population ecology of the saguaro in Arizona.

Particularly relevant to the question on the factors limiting the establishment and survival of young saguaros is the following important observation by Mielke (1944:3) that "Many seedling saguaros, grown in the greenhouse, have been inoculated with the bacterium, but none of them have ever developed any symptoms of the disease. Rot pockets developed in some older plants inoculated by members of the Plant Pathology Department of the University of Arizona, but the pockets calloused out and the plants did not die."

Conclusions relating bacterial necrosis to vigor and age were offered by Gill and Lightle (1946:4): "Mortality is directly correlated with size, the larger plants being subject to higher death rates . . . the mortality rate rises sharply in plants over 18 feet high.", and by Mielke (1944:1): ". . . only the older saguaros and those apparently lacking in vigor are killed as a result of attack by the rot." Gill (1951:4) concluded that the rot was "linked with overmaturity."

Massive freeze-caused death of saguaros in 1962 followed by bacterial necrosis was reported by Niering et al. (1963) and Lowe (1964, 1966). Steenbergh (1970) reported on rejection of bacterial rot by saguaros and on bacteria] necrosis following lightning kill (1972). Bacterial necrosis of saguaros following catastrophic freeze-kill of saguaros in January 1971 is described by Steenbergh and Lowe (1976).

The relationship between freezing and the collapse and decomposition of vegetable tissue has long been recognized. M. A. de Candolle accurately described the relationship in 1838 (1852; see Chap. VI).

More than a century—125 years—was to pass, however, before de Candolle's important discovery was recognized as the obvious explanation for the reported "epidemics of bacterial necrosis disease of the saguaro."