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

Geographic Variation in Growth

The rate of growth can vary significantly and even widely between saguaro populations separated by relatively short distances, especially where elevation, topography, and/or soil structure also vary greatly over short distances. For example, several different effective environments characterize different desert areas and habitats within Saguaro National Monument, and between the east and west divisions of the monument (Table 32).

Still greater differences in growth rate can be predicted for populations separated by greater distances and growing under more widely distinctive effective environments, for example, as indicated by the climatic data for Tucson, Ajo, and Yuma, Arizona (Table 33). We predict large order growth-rate differences between Tucson populations and populations in the Yuma desert (Gran Desierto) area. There the saguaros on bajadas and valley fill are essentially a desert riparian species population with low marginal density under 3.38 inches (86 mm) annual rainfall and 1.57 inches (40 mm) summer (May-October) rainfall (Green and Sellers 1964:493, Yuma; Fig. 9B).

Organ Pipe Cactus National Monument

Measurement of saguaro growth over a period of 26 years in populations at Organ Pipe Cactus National Monument provides an important sample for the study of the Species that is summarized in Table 34 and graphed in Figs. 43 and 44. The curve in Fig. 44 is fitted by multiple linear regression as in the case for the Saguaro National Monument data (Fig. 40). As at Saguaro National Monument and elsewhere, the asymptote of the growth curve for Organ Pipe Cactus National Monument is at approximately 2 m (6.6 ft) stem height (Fig. 44). As noted above, important changes in form and function—and important consequences—take place in the saguaro in southern Arizona when the plant reaches a stem height on the order of 2 m (6.6 ft).

The data on apical growth of saguaros at Organ Pipe Cactus National Monument,² shown in Table 34 and graphed in Fig. 44, provide, for the first time, a basis for estimating mean growth rates and age-height relationships of saguaro populations whose growth rates are not under the dominant depressing influence of catastrophic freezing. Growth rates for this population, situated close to the ecological and geographic center of the species range, differ markedly from those for saguaro populations at Saguaro National Monument and elsewhere near the cold-limited boundaries of the species range.

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²Lightle (1941) field data and re-measured in 1967 to obtain a 26-year apical growth increment.

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Fig. 43. Apical growth on stem height (1941) of 93 saguaros at Organ Pipe Cactus National Monument, Arizona, on uniform (arithmetic) scale. Note the reversal in the rate of growth that occurs at aproximately 200 cm (6.6 ft) height. Data in Table 34. (click on image for an enlargement in a new window)

Fig. 44. Regression on logarithmic coordinates of 26-year apical growth on 1941 stem height (Lightle 1941) for 93 saguaros at Organ Pipe Cactus National Monument, Arizona. Note asymptote of curve at approximately 2 m (6.6 ft); see text. Data in Table 34. (click on image for an enlargement in a new window)

A comparative view of a significant difference in growth rate between two saguaro populations in southern Arizona—at Saguaro National Monument (east) in southeastern Arizona and Organ Pipe Cactus National Monument in south-central Arizona—is provided in Fig. 45. The differences in growth-rate characteristics for these two populations, as shown in Fig. 45, are attributable to differences in temperature and precipitation. Gross differences in parameters of the climate at Tucson and Ajo (Table 33) relate importantly to observed differences in saguaro growth rates and to height-age relationships for saguaros in these two different climatic environments.

Fig. 45. Regression on logarithmic coordinates of 26-year apical growth on height for young saguaros at Saguaro National Monument (east) and Organ Pipe Cactus National Monument (see Tables 8, 33, and 34 and Figs. 38, 39, 42, and 43). See discussion in text.

As compared to the climatic environment of Saguaro National Monument and the Tucson locality, the climatic environment of Organ Pipe Cactus National Monument and Ajo locality is a much warmer and drier one that results in the slower growth of juvenile saguaros. In its overall effects on the growth of larger plants, however, the warmer environment at Organ Pipe Cactus National Monument offers conditions for saguaro growth more favorable than those at Saguaro National Monument (see Hastings and Alcorn 1961). There, the effects of catastrophic freezing offset the potentially greater benefits of substantially higher plant-available moisture and lower transpiration losses.

Tempe, Arizona

Investigation of saguaro growth in a natural population of saguaros in the Phoenix, Arizona, area was conducted by Marshall and Earle (Marshall 1955) on South Hill in the Desert Botanical Garden, Tempe. Their data for 6 years (1949-54) are given in Table 35. The data appear to be in general accord with our observations and those reported by others on growth rates for young saguaros in this portion of the extreme range of the species.

Tucson, Arizona

In the Tucson area, other investigations of saguaro growth have been reported by MacDougal and Spalding (1910), Shreve (1910), Hastings (1961), and Hastings and Alcorn (1961).

MacDougal and Spalding (1910) measured and reported yearly apical growth increments for individuals in a saguaro population on Tumamoc Hill in the Tucson Mountains. The data are for years 1905-8 and, though long overlooked, they are the first growth data obtained and reported for the species (see Table 36). It is a peculiarity that Shreve (1910), reporting in the same year from the same laboratory, included no reference to these important data.

The growth record provided by MacDougal and Spalding (1910:30, Table 8) contains the highest natural rate of growth ever reported for the saguaro. During the 9-month period from 23 January 1907 to 22 October 1908, one individual (No. 19) in that sample grew from a height of 244.4 cm (96.2 inches) to 301.6 cm (118.7 inches), an average of 28.6 cm (11.25 inches) per year. It is noteworthy in relation to that unusually high rate of growth that the July to September precipitation during the 1907 and 1908 summer growth periods was 8.53 inches (217 mm) (Green and Sellers 1964: Tucson) and 12.05 inches (306 mm) (MacDougal and Spalding 1910, on site), respectively. Both values are substantially above the 5.01-inch (127 mm) Tucson mean for these 3 months (Green and Sellers 1964).

Shreve (1910) provided none of the data used to obtain the age-height relationships he reported for saguaros growing on the northern slope of Tumamoc Hill in Tucson (Table 37). Using his data, a plot of height on age, however, shows an exponentially linear age-height relationship, with a distinct increase in the growth rate occurring at a height of approximately 24 cm (9.4 inches). Our results, however, do not suggest the occurrence of such a growth-rate change at that height (Tables 28, 30; Figs. 40-42).

The growth rates reported by Hastings and Alcorn (1961: Figs. 5 and 6) also showed the reversal in the annual apical growth rate occurring at a height of approximately 2 m (6.6 ft), typical of saguaro growth-form ontogeny thus far known (see Tables 34 and 38; Figs. 43 and 44).