**Saguaro**

**Adapting to the Desert**

No plant quite like it grows anywhere else in the world. The saguaro
(Carnegiea gigantea), the tree-like cactus towering over the slopes at
Tonto National Monument, makes its home only in the Sonoran Desert.

Saguaros were flourishing when the Salado lived here, from AD 1150 to
1450. Because they are long-lived, saguaros now in the monument may be
great-grandchildren of those the Salado saw.

The Salado used saguaros in various ways. They built roofs with its
wood-like ribs. They ate the fruit or made a number of products, just as
today's desert dwellers do. They also may have used the syrup and cakes
made of dried fruit pulp as trade items.

Scientists say the landscape is much the same today as it was during the
300 years the Salado lived in this region, although there were more
grasses and trees then, and a river ran through the valley now filled in
by Roosevelt Lake.

Sharing a common habitat, saguaros and Salado adapted to this desert
environment in similar ways. They both collected water from great
distances. The Salado brought water from the river and from springs in
the canyons to their cliff dwellings. Saguaros soak up water up through
extended root systems and store it in their cylindrical trunks.

Both saguaros and Salado developed specialized systems for processing
food from desert elements. The saguaros take advantage of the
temperature extremes to help convert sun and moisture into sugar. The
Salado processed nutritious foods from the wild plants they gathered in
the desert.

Overpopulation might have pressured them into stripping the desert. An
exhausted food supply would have left them vulnerable to internal
strife, disease, and famine.

Yet an obvious difference exists between the adaptation methods of
saguaros and the Salado; the plant was successful, the people were not.
Saguaros have existed in the Sonoran Desert for 10,000 years. Despite
the advantages of human intelligence and mobility, the Salado
disappeared. Archeologists speculate that the Salado depleted their
environment.

Saguaros owe their continued existence to fine-tuned adjustments to
desert conditions.

Only one or two of the millions of seeds a saguaro produces during its
lifetime will germinate. The seeds must land on well-drained, rocky
soil, preferably on south-facing slopes under "nurse plants" that
provide crucial shade.

Those seeds not eaten by birds or packrats sprout after the summer
rains, when temperatures range between 60 and 95° F. Saguaros grow very
slowly. They might grow three feet tall in 14 years, and take 20 years
to double that. Between 60 and 75 years, they begin to sprout arms.
Saguaros can reach heights of 50 feet and weigh several tons.

When they are about 50 years old, saguaros start to flower, producing
about 200 blossoms annually. The white blossoms, perched on the tops of
columns and branches, open at night. The flowers, which remain open for
less than 24 hours, are pollinated by moths and bats.

The saguaro's skin is coated with a wax-like substance that prevents
water loss through evaporation. In addition, thousands of needle-like
spines create shade and help break up wind currents. Downward pointing
spines act like rain gutters, guiding moisture to the roots. Because of
their waxy coating, saguaros cannot photosynthesize in the usual way.
They open their pores at night, letting in air. The cold desert nights
facilitate the conversion of carbon dioxide into an acid, which the
cactus stores and turns into food the next day under the hot desert sun.

Saguaros soak up water through shallow root systems, which radiate from
the trunk to a distance roughly equal to the plant's height. Pleated
ridges running the length of the trunk act like accordions, expanding
and contracting in response to the water supply. Pulpy tissues and the
supporting skeleton help with water distribution.

The Sonoran Desert has been experiencing colder temperatures since the
turn of the century. Scientists are studying the saguaro's reaction to
this climate change.

Research has shown that cold temperatures have occurred intermittently
for several thousand years. These temperatures may be cold enough to
stunt or kill many desert plants.

Preliminary studies at Tonto show a high proportion of young saguaros,
indicating a healthy population and creating optimism for their future.