Ecology of the Saguaro: II
NPS Scientific Monograph No. 8
REPRODUCTIVE GROWTH (continued)
Reproductive growth of a healthy saguaro begins when
the plant is approximately 2.2 m (7.2 ft) tall (Table 4; Figs. 15A, 16).
The first blooming may result in the production of only one or two
fruits. The number of fruits increases rapidly, however, in succeeding
years, and production commonly exceeds 100 fruits per year by the time
the plant has attained a height of 4.5 m (14.8 ft) (Table 3; Fig. 13).
At that time, the first arm buds develop on vigorous individuals. As
early as the third spring thereafter, when they have attained a length
and diameter of about 15 cm (5.8 inches), the spherical arms begin to
produce fruits (Fig. 15B).
TABLE 4. Reproductive status of young saguaros at Saguaro
National Monument (west), based on presence or absence of reproductive
structures (buds, flowers, or fruits) for all plants 14 m
(3.313.1 ft) ht in transects in flat habitat (20 X 700 m; 66 X
2297 ft) and rocky habitat (20 X 350 m; 66 X 1148 ft) 10 June
The mean density of saguaros 1-4 m ht was approximately four times
greater m the rocky habitat (78.6 stems/ha; 31.4 stems/acre) than in the
flat habitat (18.6 stems/ha; 7.4 stems/acre). Data graphed m Fig.
Fig. 15A. Reproductive growth of a
saguaro begins when the plant is approximately 2.2 m (7.2 ft) tall (see
Fig. 16). The first blooming commonly results in the production of only
one or two fruits. The production of fruits increases rapidly in
succeeding years and commonly exceeds 100 fruits per year by the time
the plant has attained a height of 4-5 m (13.1-16.4 ft). Photographed
17 July 1973.
Fig. 15B. Five small arms produced 113
of the total annual crop of 182 fruits on this 5-m (16.4-ft) saguaro.
During the fourth spring after their first appearance, three visible
30-cm (12-inch) arms bear a total of 103 ripening fruits. Two third-year
arms behind (16 and 8 cm; 0.5 and 0.6 ft) produced an additional four
and six fruits, respectively.|
Arm buds emerge during the summer growth period when the plants have
attained a stem height of 4-5 m (13.1-16.4 ft) or more. Reproductive
growth on arms has been observed the third spring thereafter when the
arms have attained a diameter of about 15 cm (5.8 inches).
The function of arms on the saguaro is to increase the number of fruits,
and hence the number of seeds, produced by the plant. Photographed 25
Fig. 16. Heights and percentage of
saguaros with reproductive structures (buds, flowers, or fruits) at
Saguaro National Monument (west); N = 81. The sample includes all plants
1-4 m (3.3-13.1 ft) tall in transects in flat habitat (20 X 700 m; 66 X
2297 ft, N = 26) and rocky habitat (20 X 350 m; 66 X 1148 ft, N = 55).
The numbers above each bar are the reproductive (upper number) and
nonreproductive plants (lower number) in each height class.|
In this population the approximate mean height at the age of the first
flowering is 2.25 m (7.4 ft). Data in Table 4.
The number of fruits borne on the arms increases
rapidly in succeeding years. The function of arms on the saguaro is to
increase the number of fruits, and hence, the number of seeds produced
by the plant.
Mature fruits weigh about 50 g (1.8 oz) each and
contain approximately 2250 seeds (Table 5; Fig. 17). Under natural field
conditions at Saguaro National Monument, the authors have obtained up to
60% germination success from broadcast seeds. Under controlled
conditions, seed viability closely approaches 100%. Heit (1970) obtained
98% germination of wild Arizona seeds in the laboratory (14 days) with
alternating daily temperatures (20-30°C; 68-86°F) and exposure
to light (8 hr per day) with seeds in closed dishes and a uniform
moisture supply. McDonough (1964) obtained similarly high germination
percentages of 90-100% with fresh seeds and noted the lower values
reported by Alcorn and Kurtz (1959).
TABLE 5. Size, weight, and seed content of ripe saguaro fruits
(N = 18) collected 21 June 1968, Saguaro National Monument (west). The
mean seed weight is 1.3 mg (0.02 grain).
|Measurement||Mean (± SE)
||Range||95% Confidence limits|
|Length (mm)||70.28 ± 1.99||5783||63.9776.59|
|Max. Dia. (mm)||38.39 ± 0.52||3443||37.3039.48|
|Volume (cc)||52.22 ± 1.95||3664||48.0953.31|
|Weight (g)||52.55 ± 1.91||37.0963.99||48.5156.59|
Seeds per fruit (air-dry)
|Weight (g)||3.01 ± 0.15||1.793.89||2.693.33|
|Numbera||2263.00 ± 108||14883124||20352490|
aBased on 100% count of seeds from 2 fruits and 200 seed samples from
the remaining 16 fruits.
Fig. 17A. Ripe saguaro fruit; seed mass
drying in open receptacle with dried flower still attached. The
receptacle splits open upon ripening, exposing the sugary, bright red
matrix containing the small black seeds (see Fig. 17B). Photographed 15
Fig. 17B. Seeds from a single saguaro
fruit. A common dressmaker's pin (lower left) provides scale. Each
seed is about 1.5 mm (0.06 inch) long and weighs approximately 1.3 mg
(0.02 grain). An average size saguaro fruit during an average year
contains 2000-2500 seeds. Photographed 15 July 1974.
According to Shreve (1931b), the saguaro begins
reproductive growth at age 50-75 yearsa conservative estimate, see
Tables 4 and 28and lives 150-175 years. Based on an estimated
average production of 200 fruits per year, a healthy saguaro produces a
total of 40 million viable seeds during its 100-year reproductive life
span. Saguaro populations at Saguaro National Monument and elsewhere in
the Tucson area are characterized by high densities of reproductive
saguaros (Gill and Lightle 1942; Alcorn and May 1962; Niering et al.
1963). In bajada stands at Saguaro National Monument, Niering et al.
(1963) reported densities of reproductive size saguaros (plants greater
than 2 m ht; 6.6 ft) of 101 stems/ha (2.5 acres) in the east monument,
and 154 stems/ha in the west monument.
Assuming an average annual productivity of 200 fruits
per plant (Thackery and Leding 1929), the annual reproductive potential
of the east monument stand is 40.4 million seeds/ha (2.5 acres) and for
the west monument, 61.6 million seeds/ha (2.5 acres). It is evident that
the heretofore reported sparsity of young saguaros in these stands
cannot be attributed to a lack of viable seeds.
Last Updated: 21-Oct-2005