BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Carnegiea gigantea | Saguaro 



GENERAL BOTANICAL CHARACTERISTICS : 
Saguaro is a native, arborescent, stem succulent that grows from 9.8 to
52.5 feet (3-16 m) tall and 5.9 to 29.5 inches (5-75 cm) in diameter
[6,10,64,65].  It is the largest of the columnar cacti growing in the
United States [89].  Saguaro has a deep anchoring taproot (up to 3.2
feet [1 m] deep) and extensive lateral roots (13 to 98 feet [4-30 m]
long) [14,65].  The stems are simple with one to five (rarely up to 49)
lateral, erect branches [6,10].  Saguaro trunks have many prominent
ribs, armed with dense, stout spines that are up to 2.8 inches (7 cm)
long [6,10,32].  Flowers are 3.4 to 4.9 inches (8.7-12.5 cm) long and
occur at the ends of branches [6,32,45].  Fruits are oblong and contain
up to 2,500 seeds [1,10].


RAUNKIAER LIFE FORM : 
   Stem succulent


REGENERATION PROCESSES : 
Saguaro reproduce sexually and are self-incompatible [2].  

Reproduction begins when a plant is approximately 7.2 feet (2.2 m) tall
[29,68].  Growth is extremely slow [47,68].  At the end of 2 years,
seedlings will be about 0.25 inch (0.6 cm) tall [10].  At 3.2 feet (1
m), plants may be 20 to 50 years old [47].

Flowers only develop at branch and main stem tips.  An increase in
branch number increases the reproductive potential of an individual
[37].

Many different animals pollinate the nocturnally blooming saguaro
flowers and are detailed elsewhere [2,39,40].  Flowers are open for less
than 24 hours [2].  A plant averages four open flowers per day for about
30 days [39].  Plants commonly have more than 100 fruits [47,68].

Seeds germinate readily [10].  Conditions for germination are detailed
elsewhere [1,89,91].  Saguaro seeds are short-lived; seed reserves are
not maintained in the soil [58,76].  In random soil samples, relative
densities of saguaro seeds were between 0.3 and 2.0 percent [56].  Less
than 1 percent of the annual seed production germinates due to predation
or moisture stress [15,48].  Heavy seedfall can occur during the summer
rains [68].  Drought during the 12 to 14 months following germination is
the most critical factor threatening seedling survival [48,67].

Perennial shrubs, such as foothills paloverde (Cercidium microphyllum),
are important as nurse plants for facilitating saguaro establishment
[22,29,36,62].  Seed dispersal by frugivorous birds is a primary factor
in saguaro establishment beneath shrub canopies [36].  Saguaro seedling
density significantly (p<0.05) decreased from the center to the outer
edges of nurse plant canopies [29].

On the margins of its range, recruitment has not kept pace with
mortality caused by freezing temperatures and drought [52,76].  Saguaro
has lower fruit production at the drier western edge of its range [52].
In some areas where cattle grazing has not occurred, recruitment
appeared to occur in pulses correlated with above-normal precipitation
[76].


SITE CHARACTERISTICS : 
Compared to other columnar cacti, saguaro occurs over the broadest range
of habitats from gently sloping alluvial flats of the lower bajadas to
steep, rocky, high-elevation volcanic slopes [51,53,74].  The greatest
densities of saguaro are found in the more mesic eastern parts of its
range [37].

Saguaro occurs on shallow soils that are light, coarse-textured, and
rocky.  The soils are underlain at 3.2 to 9.6 feet (1-3 m) depths by an
impervious caliche layer [8,88].  There is usually no differentiation of
the soil into horizons [87,88].  Saguaro is found at elevations from
1,640 to 5,000 feet (500-1,525 m) [35,50,56,86].  Saguaro has been
reported on all aspects [77,88].  However, most individuals occur on
open east- and west-facing slopes, and the fewest occur on north-facing
slopes [6,77,86].

The Sonoran Desert is a warm desert with a semiarid to arid continental
or an arid subtropical climate [27,67,82].  Precipitation is bimodal,
falling December to February and July to August, and amounts vary from
year to year [19,42,51].  Average amounts of rain per year are 6 to 15
inches (152-400 mm) [3,14,42,84].

Common arborescent associated species are Organ Pipe Cactus
(Lemierocereus thurberi), barrelcactus (Ferocactus wislizeni), cholla
(Opuntia spp.), ocotillo (Fouqueria splendens), catclaw acacia (Acacia
greggii), ironwood (Olneya tesota), and jojoba (Simmondsia chinensis)
[8,13,26,50].  Other associated species are littleleaf krameria
(Krameria parvifolia), fluffgrass (Erioneuron pulchellum), tobosa
(Hilaria mutica), Indian-wheat (Plantago insularis), and thelypody
(Thelypodium lasiophyllum) [13,23,61].


SUCCESSIONAL STATUS : 
Obligate Climax Series

Succession in desert systems is difficult to define [36].  Saguaro may
be considered a climax species.  Establishing in the shade of perennial
shrubs, saguaro usually outlives and surpasses the nurse plants.
Regeneration of saguaro continues in suitable areas of its range.


SEASONAL DEVELOPMENT : 
Saguaro flowers from late April to June [1,32,39,45].  Fruits mature
during June and July before the summer rains [6,65].  Most growth of
saguaro occurs during the summer rainy season [6].

Related categories for Species: Carnegiea gigantea | Saguaro