CHAPTER 5:GROWTH (continued)
The value for first-year stem height obtained by equation (1) (0.6486 cm; 0.26 inch) provides a basis for conversion of height to age for the population studied at Saguaro National Monument (east) in the saguaro habitats on flat and moderately rolling, nonrocky surface terrain. As noted above and described below, in this population the age of a very young saguaro of a given height can be determined (i.e., estimated) using the log of 0.6486 for log X in equation (2). The saguaro growth curve for young saguaros—saguaros <2.2 m (7.2 ft) height—illustrated in Fig. 40 is described by log where Table 28 provides age data for young saguaros (<2.2 m; 7.2 ft) at Saguaro National Monument (east) as graphed in Fig. 41. The semi-logarithmic plot of height and age (Fig. 41) is described by log where Saguaros at approximately 2 m (6.6 ft) stem height undergo a marked change in growth form. The change in form is associated with a decline in growth rate and with changes in functions associated with the onset of reproduction (see Table 29; Figs. 15, 16, 38, 40, 43, 44). Two events occur at that stage of saguaro development: (1) the plant produces its first blooms and fruits; and (2) a transition from the "club" form of the large juvenile plant to the "wine-bottle" form of the unbranched young adult as the maximum diameter ceases to follow the upward growth of the tip (Fig. 38A).
As discussed earlier and illustrated in Chapter 2, some saguaros in southern Arizona and northern Sonora may start their reproductive life when only approximately 1.5 m (4.9 ft) in stem height. Ordinarily, 50% or more of the population in height-class 2.00-2.49 m (6.6-8.2 ft) flowers and produces seed during most years (Fig. 16). The tabular age of a young 1.5-m (4.9 ft) saguaro is 27 years, and the age range is approximately 31-36 years for height-class 2.00-2.49 m (6.6-8.2 ft). Our data thus estimate age of the 200 cm (6.6 ft) young saguaro at 31 years, and the 2.2 m (7.2 ft) mean stem height at 33 years age at Saguaro National Monument (east; flats and rolling terrain); see Table 28. The data provided here together with our other field observations suggest that earlier changes in the apical growth rate of the young saguaro also are coincident with major changes in form. The data on the apical growth of young saguaros as graphed in Fig. 40 suggest that a reduction in apical growth rate may accompany the transition from the globose juvenile form to the "club" form of the large juvenile that occurs in the vicinity of 5-10 cm (2-4 inch) above-ground stem height. We anticipate answering this question in further investigations on growth rates and form changes in relation to saguaro survival and climatic adaptation.
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