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

Effects of Flower Bud Removal on Stem Growth

When a healthy saguaro initiates its first reproductive year of life, abruptly moving into adulthood after some 30 or more years of life in which it attains a growth up to or over approximately 2 m (6.6 ft.) in height, the energy shunted into the production of reproductive structures themselves (buds, flowers, fruits, and seeds) is energy directed away from apical stem growth. To test both the prediction and the cost of reproductive effort in terms of stem-growth increment reduction, an experimental design was structured to compare quantitatively apical growth differences between samples of treated and untreated stems of vigorous healthy saguaros.

The results for carefully paired experimental and control plants in the same stand are given in Table 29. Untreated (control) plants were allowed to complete normal development of reproductive growth. All flower buds started by experimental plants were removed each week beginning 20 April 1970. A small energy increment was required for the plant to initiate the buds that were removed, thus providing a slight underestimate in the "percent growth change" column (Table 29). The data reveal not only that the prediction is true but also the rather dramatic disclosure that approximately 50% of the 4-5 m (13-16 ft) saguaro's potential stem growth for a given year is diverted into its yearly reproductive effort.

Freezing Effects on Growth

Freeze-caused injury depresses the subsequent rate of saguaro growth. Desiccation and volume shrinkage resulting from freeze-caused injury are expressed by decreases in stem heights and diameters (see Steenbergh and Lowe 1976). The effects of the January 1971 freeze on the height and apical stem growth of young saguaros at Saguaro National Monument (east) are shown in Tables 30, 31, and Fig. 42.

Fig. 42. Effect of freezing on apical growth of 17 young saguaros at Saguaro National Monument (east). Regression on logarithmic coordinates of apical growth on stem height. (A) Growth during 4 consecutive years, January 1969 to March 1973. (B) Geometric mean of 2 years apical growth on stem height showing normal growth (1969-70, 1972-73) and combined freeze-effect (1970-71, 1971-72) of January 1971 freeze (see text; Steenbergh and Lowe 1976). Data in Table 30, regression equations in Table 31. (click on image for an enlargement in a new window)

The desiccating effect of freezing is greatest on smaller juvenile plants and is evident in the stem measurements recorded in early April 1971, 3 months after the freeze. At that time, the maximum diameter of all of 14 plants over 9 cm (3.5 inches) in height showed a diameter decrease (rather than increase) from the size recorded one year earlier. Only 9 of the 17 plants, all more than 11.2 cm (4.4 inches) in height, showed a height increase greater than that for the previous year—the actual height growth during 1971 was partially offset and completely obscured in these measurements by shrinkage resulting from freeze-caused desiccation.

Subnormal growth following the 1971 freeze-caused injury is shown in part by the 1972 data. Even these low growth rates, however, are deceptively high, for a large portion of the indicated "apical growth" from April 1971 to April 1972 is, in fact, not growth at all but represents the recovery of freeze-caused volume loss. Thus, the effect of freezing on apical stem growth is expressed in the sum and the mean of the increments for the 2 years from 1970 to 1972 as shown in Fig. 42 and Tables 29, 30.