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.
TABLE 29. Effect of flower bud removal on apical growth (cm) of
unbranched saguaros in nonrocky (flat) habitat at Saguaro National
Monument (east). New flower buds appearing on experimental plants (69A
and 61C) were removed each week beginning 20 April 1970. Plants 69B and
69D were allowed to complete normal development of reproductive growth.
Precipitation for the two summer growth seasons in Table 32.
|Apical growth and fruits
||Growth change (cm)|
aIncomplete counts taken on 8 July 1969 after start of fruit drop.
bBetween 20 April and 25 June 1970, when the last buds developed, a total
of 196 buds were removed from plant 69A, and 242 buds were removed from plant 69C.
Freezing Effects on
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.
TABLE 30. Effect of freezing on growth. Total stem height (cm)
and one-year height growth increment (cm) for young saguaros (N =
17) in flat (non-rocky) habitats at Saguaro National Monument (east),
1969-73. Data graphed in Fig. 42.
|Stem height and
annual increase (cm)|
TABLE 31. Regression equations for 1-year apical growth (cm) on
stem height (cm) for young saguaros (N = 17) at Saguaro National
Monument (east). Data in Table 30, graphed in Fig. 42.
|1||17||1969-70||log Y = -0.2894 + 0.6340 log X||+0.8676|
|2||17||1970-71||log Y = -0.6453 + 0.9762 log X||+0.9488|
|3||17||1971-72||log Y = 0.0559 + 0.4219 log X||+0.9110|
|4||17||1972-73||log Y = 0.1753 + 0.3964 log X||+0.9080|
|2-YEAR GEOMETRIC MEANS|
|5||17||1969-70, 1972-73||log Y = -0.988 + 0.5358 log X||+0.9321|
|6||17||1970-71, 1971-72||log Y = -0.3079 + 0.6533 log X||+0.9536|
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 yearthe 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.
Last Updated: 21-Oct-2005