Saguaro Cactus Facts
by Alethea Steingisser
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Saguaro Cactus Facts by Alethea Steingisser (This page is still being edited
for ease of use-all the info is correct, but not |
INTRODUCTION
Saguaro cacti are by far the most notable and revered vegetative feature of
the American Southwest. Their apparent dominance over the landscape and resemblance
to the human form has been a source of inspiration for artists, writers and
ecologists as well as a source of fascination and beauty to visitors. The saguaro’s
popularity is no doubt a measure of its ability to stand tallest above the desert
landscape with its outstretched arms reaching toward the heavens, creating an
aesthetically pleasing scene as if there had not ever been one there before
its appearance.
I, too, have fallen prey to the lure of the saguaro. For it is
the saguaro’s beauty that I write this essay; to become more familiar with
nature’s artwork and to understand how such a fascinating specimen has
come to inhabit the arid lands of the American Southwest. The following paper
is an attempt to describe and to better understand the saguaro cactus. A variety
of factors regarding the saguaro cactus will be discussed including its physical
description, location and distribution and method of reproduction
is most likely due to the immense stature of the saguaro when compared with
other, smaller vegetation of the desert floor. The saguaro’s apparent dominance
over the relatively smaller species of cactus and other types of vegetation
upon the desert floor. The abundance of the saguaro is in reality much less
than that of other cactus species such as the jumping cholla and the barrel
cactus.
PHYSICAL DESCRIPTION
The main portion of the saguaro cactus is its main trunk. This single stem is
shaped like cylindrical column and is generally in the range of 1 to 3 feet
in diameter. The stem is green in color and is grooved from top to bottom by
protruding ribs with intervening depressions between each rib. The number of
ribs ranges from 12 to 30 and each is approximately 1 to 1 and 1/2 inches high.
Upon each rib in a vertical line extending from tip to base are areoles and
protruding spines. Areoles are the succulent equivalent of leaf buds. These
are circular areas from which the thorny spines protrude.
Each areole houses approximately 15-30 spines that are generally about an inch
long (Benson: 1982).
The structure of a saguaro is enforced by a strong but somewhat flexible woody
skeleton. The skeleton extends from base to tip and is composed of cylindrical
rods that converge toward the center height of the saguaro and become less condensed
above (McGinnies, 1981).
In saguaros of sufficient height and age, branches grow far above the base of
the stem. The number of branches is variable and can range anywhere from 1 to
20 on a single saguaro. The branches appear identical to the main trunk of the
saguaro; the only difference being that they bend and curve in a variety of
directions, most often upward toward the sky. Secondary branching upon the initial
branches is uncommon, but has been documented in very old saguaros c.
The saguaro is the tallest species of the succulent family in the United States.
Exceptionally tall saguaros have been measured at heights exceeding 50 feet,
but the average height is between 10 to 30 feet. Each cactus can weigh several
tons due to the extremely high percentage of water weight. 75-95% of a saguaro’s
weight is attributed to stored water (McGinnies, 1981). Like most cacti, the
great percentage of stored water makes the saguaro extremely sensitive to frost
damage and greatly influences its locational extent.
WATER LOSS
Saguaro cacti are ideally suited to survive in arid climates and have adapted
certain physical characteristics that help them take full advantage of the paucity
of available moisture. First, the outer portion of the saguaro is composed of
a relatively thick and impermeable layer. This helps the cactus to reduce the
quantity of internal moisture lost by transpiration and also allows it to live
where the supply of liquid water is minimal. Every drop of water that the saguaro
absorbs is utilized and not allowed to be wasted. Saguaros can retain enormous
quantities of water and do not need to constantly replenish the supply. In addition,
they are designed to fluctuate with regard to water content and can lose up
to 60% of their water content without dying (Benson, 1982).
The thick and compact nature of the main stem and secondary arms or branches
of the saguaro serve to keep the surface area of the cactus at a minimum. Less
actual surface area equals less water lost through transpiration.
The saguaro also protects itself from water loss due to injury. Injuries can
occur from a variety of reasons including holes pecked by woodpeckers, vandalism
and bacterial invasion to name a few. In most plants, if an injury occurs, the
wounded area is left alone. Moisture evaporates from the wound causing the wound
and the immediate area to become dried out and shriveled. For a saguaro living
in very high temperatures and low humidity, moisture loss through an open wound
could be fatal. To protect itself, the saguaro produces a layer of cork to seal
off the wounded area from the environment. Another tactic used to stop evaporation
from a wounded area is to cover the area with slime from its interior that then
dries out rapidly and provides a sealant, protecting the cactus from further
moisture loss (Benson: 1982).
The ribs of the saguaro are designed to deal with changes of internal water
content. The shape of the ribs are accordian-like and are designed to expand
and contract with increasing or decreasing water content. This avoids putting
unnecessary strain on the structural components of the cactus (Keasey, 1981).
METABOLISM
Photosynthesis is the process by which most plants make their own food. Simply
stated, photosynthesis involves the plant absorbing carbon dioxide (CO2) from
the atmosphere where it reacts with sunlight to produces carbohydrates. Saguaros
and all succulents have adapted a special method of completing the process of
photosynthesis while also protecting themselves from the harsh environment in
which they live.
The thick, outer coat of the saguaro has numerous pores, called stomata, that
open and close in order to regulate incoming and outgoing moisture and carbon
dioxide. During the heat of the day, the stomata remain closed and do not allow
any moisture to evaporate from the saguaro’s interior. As night falls and
the temperature cools down, the stomata open allowing carbon dioxide to be absorbed.
Since sunlight is necessary for photosynthesis, the saguaro has developed a
method to store the carbon dioxide until the following day by tying it up chemically.
When the sun reappears the following day, the stomata close back up and the
stored carbon dioxide is released to react with the sunlight and produce carbohydrates
as food for the saguaro. This type of metabolism is termed succulent metabolism
and is characteristic of all succulents (Benson, 1982).
The amount of carbon dioxide absorbed at night is directly related to the amount
that will be available the following day for photosynthesis. More carbon dioxide
is absorbed on cooler nights rather than warm and conditions are ideal if the
following day is relatively warm. If the nights are warm, little carbon dioxide
is absorbed and therefore unavailable for photosynthesis the following day.
It becomes clear that saguaros need a marked diurnal fluctuation in climate,
which is typical of the desert climate in which they are live. The rate of photosynthesis
is very slow and the growth rate of the saguaro reflects this (Benson, 1982).
ROOT SYSTEM
The root system of the saguaro is relatively shallow for its massive size. Roots
generally penetrate only 3 feet down into the soil, but spread laterally up
to 100 feet in mature cacti. The roots serve two purposes: they absorb water
and provide stability and support for the saguaro’s massive structure and
further protect it from toppling due to strong winds.
Rains in the desert are typically of short duration and high intensity. The
surface layers of the soil are often the only layers to absorb rainfall and
tend to dry out rather quickly once the sunshine returns. The root system of
the saguaro is ideally adapted to take advantage of these harsh conditions and
serves as the solitary means for the saguaro to absorb moisture. With the onset
of moisture in the soil, the lateral roots produce several rootlets and root
hairs that absorb large amounts of water relatively quickly. When the soil dries
out, the rootlets and root hairs generally die off, but remain within the system
as additional support for the main structure.
The shallow root system is also beneficial when it comes to competition for
water with other nearby plant species. Plants with deeper roots pose not threat
to the saguaro’s survival (Benson).
The characteristics and functions of the roots undergo gradual changes with
increasing age. A young saguaro depends on its taproot for both water absorption
and stability. The taproot is the main portion of the root from where new roots
grow and is located just below the main portion of the plant. As the cactus
matures, additional roots grow from the taproot and spread out vertically. These
new lateral roots take over the function of absorbing moisture which leaves
the taproot responsible for supporting the main trunk of the cactus. Older and
therefore larger saguaros cannot be supported by the single taproot, so portions
of the lateral roots near the taproot thicken up to provide additional structural
support (McGinnies, 1981).
LOCATION AND DISTRIBUTION
Saguaro cacti are extremely succeptible to frost damage due to their high water
content and cannot survive in locations where frost is a natural occurrence.
The saguaro’s state-wide distribution and specific location within the
Arizona landscape are wholly concentrated in frost-free areas where other conditions
such as soil type and moisture conditions are favorable.
The major concentrations of saguaro cacti are located in the southern portion
of the state, beginning near Phoenix and extending southward in concentrated
pockets toward the Mexican border. Additional, less extensive stands of saguaros
are present toward the western border of the state in Yuma County and northward
in Kingman County (Benson: 1969).
The saguaros occupy a specific section of the Sonoran Desert termed the Arizona
Upland region that is known for its great diversity and abundance of plant species.
Saguaro cacti flourish in response to the rugged topography and the increased
amounts of precipitation in this section of the desert. Compared with the surrounding
and more low-lying regions, the Arizona Upland region has a tremendous amount
of topography ranging in elevation from 500 to over 3000 feet. Along with the
increase in topography comes an increase in detrital material eroded from the
mountains and steep to gently-sloping bajadas are common throughout (Benson).
Saguaros prefer the shallow, rocky, well-drained soils of the upper bajadas
and are most commonly found on their south-facing upper slopes up to elevations
of 3,500 feet. The coarse soils of the upper bajada are ideal for root penetration.
The spaces between the angular fragments in the soil provide pathways for the
roots to grow and also allow water to penetrate the surface rather quickly.
The increase in topography in the Arizona Upland is also responsible in part
for the increased precipitation of the region. The higher elevations are extremely
efficient at intercepting moisture-laden storm systems. Annual total precipitation
ranges from 4 to 11 inches and the rainfall is biseasonal, occurring during
the winter and summer seasons.
As mentioned previously, saguaros prefer to inhabit the south-facing slopes
of the upper bajadas. This is due to the saguaro’s inability to cope with
frost. South-facing slopes receive increased solar radiation compared to north-facing
slopes. This location ensures them protection from the winter cold due to the
southerly position of the sun. Where northern slopes are shaded for part of
the day, the southern slopes receive a larger percentage of sunlight hours between
sunrise to sunset, although the angle of the solar rays upon the south slope
will vary with the time of day and with season. Toward their northern distributional
limit, saguaros are almost exclusively located on south to southwest facing
slopes. Toward their southern distributional range, saguaros can also be found
growing upon northern slopes in some locations, although their size and extent
may not be as great as on the southern slopes.
REPRODUCTION
The method of reproduction for saguaro cacti is by seed production.
Generally, only saguaros greater than 8 feet tall or approximately 50 years
of age produce flowers. Flower production is not dependent upon seasonal precipitation
totals; saguaros can produce flowers each year regardless of rainfall due to
their large capacity for water storage and slow growth.
Flower and fruit production occurs in the late spring.
By mid-April, flower buds begin to develop within areoles located around the
tips of the saguaro’s arms or main trunk. By May, the flowers begin to
bloom and there is often a continuous bloom throughout the month. Saguaro flowers
are night-blooming and remain open only until the next day, never to open again.
Generally, the flower begins to open just after sundown and is completely open
by midnight. By about noon on the following day, the flower closes up. Each
flower is approximately 3 inches wide with creamy white colored petals and produces
an extremely sweet nectar within. The nectar, along with the color of the petals,
attracts a wide variety of animals and insects that come to feed upon the nectar
and, in the process, serve to cross-pollinate the flowers. In order for the
saguaro flowers to bear fruit, they must be fertilized by pollen from a different
cactus and the fertilization must occur in the short period when the flowers
are open. Bats, birds, butterflies, moths, wasps and other insects all play
a large role in cross-pollination.
Fruit begins to form immediately if fertilization is accomplished and is generally
ripe within one month. By June, the flower petals dry up and fall off exposing
the growing fruit within. The fruit continues to grow until it splits open revealing
the fleshy, sweet interior and numerous tiny black seeds. The seeds are dispersed
by a variety of animals who come to feast upon the fruit. Several species of
birds, insects, rodents and larger mammals such as coyotes, skunks and badgers
all consume the saguaro’s fruit as a portion of their diet (Keasey). The
seeds are also very nutritious and are often directly eaten with the fruit by
the larger organisms. In this case, seeds go through the digestive tract of
the animal and are deposited at a new location. Seeds are also dispersed by
ants, rodents, birds and other animals who physically take the seeds away from
their original location. Those seeds that survive may or may not have a possibility
of germinating in their new location.
The saguaro is able to maintain its population by producing extremely large
quantities of seeds within the fruits. Each fruit can produce up to 2,000 seeds
and it is not uncommon to have over 30 fruits per stem on an individual saguaro.
Considering that there are often several blooming stems per individual and there
are several individuals within any particular area, the quantity of seeds produced
within any given blooming period is well into the millions. Not many seeds need
to survive and germinate to ensure the saguaro’s survival. In fact, the
variety of specific conditions that must be met for a seed to germinate are
numerous and complex and ensure that only a minute percentage of saguaro seeds
will germinate. Of those that germinate, only a small percentage will survive
and continue growing to maturity.
A saguaro seed may lie dormant on the ground for up to three years if it has
not been damaged by frost or trampling or washed to an unfavorable location.
If favorable conditions occur, growth can begin within 48 hours. Seeds only
sprout when four conditions are met: it is located in proper soil with ad
As described previously, saguaros grow best in coarse, well-drained soils located
on south-facing slopes. This is also true for the saguaro seed. The seed needs
to be in a soil and location that is ideal for its continued growth. Seeds in
unfavorable soils or locations may germinate but will probably not survive the
seedling stage. Seeds that are buried 1/8 of an inch beneath the surface have
the best chance of survival.
Saguaro seeds require high temperatures to germinate. This requirement is related
to the above description of location, due to the fact that the highest temperatures
are located on the south-facing slopes. The seeds and seedlings are also intolerant
of frost which limits their germination and survival on north-facing slopes.
Adequate moisture and protection from direct sunlight are also necessary for
seed germination. Again, these conditions are highly related to location. North-facing
slopes possess these qualities but are too cold and for the seedling’s
continued survival and the cold would probably damage the seed outright before
germination occurred.
The ideal location for a saguaro seed to germinate is under the protection of
another plant. The plant, referred to as a nurse plant, provides shade, insulation
from cold, protection from winds and enriched soil for the seedling. In addition,
the nurse plant’s roots help bind the soil together which helps to prevent
the seedling from being washed away during rainy periods. The nurse plant also
physically hides the seedling from animals who feed on young seedlings such
as woodrats and cactus mice. In the Sonoran Desert, palo verde, bursage, ironwood
and ocotillo often serve as the nurse trees to young saguaros. In time, the
saguaro grows large enough to survive on its own and may eventually kill its
nurse tree by exhausting all of its water supplies. A seed may also germinate
and grow under the protective cover of a rock. The rock serves as a heat source
and can keep the seedling warm because it is able to radiate the heat it has
absorbed throughout the day.
GROWTH
The growth rate of a saguaro is variable and is dependent upon factors such
as temperature and moisture. Generally, the first portion of a saguaro’s
lifespan is characterized by very slow growth. It can take up to 5 years for
a saguaro to reach 1 inch. As the saguaro ages, its growth rate increases. By
30 years of age, the saguaro grows only to 3 feet tall. By 60 years of age,
it has more than quadrupled its height to 16 feet tall. Almost 100 years later,
the saguaro is between 35-50 feet tall (McGinnies: 1981).
Changes in growth are accompanied by changes in the shape of a growing saguaro.
The saguaro seedling begins its life as a bulbous mass of succulent tissue.
As it grows, it becomes more globular and rounded. By the time it is 1 inch
tall, the seedling is well-rounded and has developed vertical ribs. By 10 inches
tall, the seedling is shaped like a club with the thickest portion at the top
of the cactus. By 8 feet tall, the saguaro is single-stemmed and appears as
an elongated club shape, still retaining the thickest portion at the top of
the cactus. More ribs have grown in to allow an increase in stem diameter. It
is as this height that saguaros have generally reached reproductive age and
can bear fruit at the stem apex. At 16 feet tall, the saguaro takes on a bowling
pin shape, with the portion of the stem about 1/3 from the top becoming slightly
decreased in diameter and the portion just below this being the thickest. It
is at this time that arm buds begin to appear. By 32 feet tall, the saguaro
has large, well-developed arms. The diameter of the upper main stem continues
to decrease as it grows in height. By this time, the saguaro is well over 100
years of age (Steenbergh and Lowe: 1983).
Arms generally do not develop until the saguaro is 75-100 years of age and 12-20
feet tall. They generally grow from the thickest portion of the main trunk located
about 8 feet above the ground. Once they develop, saguaro arms grow at the same
rate as the main trunk. Growing numerous arms is highly beneficial to the survival
of the species of the saguaro as a whole, as each arm increases the quantity
of fruits and therefore seeds that can be produced by an individual saguaro
(Steenbergh and Lowe: 1983).
Death
Adult saguaros die from numerous hazards such as frost, fire, high winds, bacterial
rot, urbanization and vandalism. The primary cause of death is frost and is
the only aspect of saguaro death that will be addressed in the present discussion.
Because the saguaro is composed mainly of water, it is highly succeptible to
damage caused by frost. Frost and freezing temperatures cause irreversible damage
to the soft tissues of the saguaro. The damaged tissue rots and can weaken surrounding
tissues, causing stems to shrivel or become wavy and arms to droop toward the
ground. A prolonged freeze can permanently damage the growth cells at the tip.
If the saguaro is able to recover, new arm buds can grow out of the damaged
tip, but the connection between the two is not continuous and appears like a
sausage link. Irreversible frost damage and death occurs when the saguaro is
no longer able to protect healthy tissue from the bacteria decomposing the rotten,
frost-damaged tissue. The decomposition spreads into the healthy tissue and
destroys it, turning the rotten tissue into a black gelatinous mess that oozes
out and runs down the exterior of the cactus. Saguaros can live from a few months
to 9 years after the fatally damaging frost (Steenbergh and Lowe: 1983).