The Hard Life of a Desert Giant
                            by Patsy Miller, Ph.D 
 

Saguaros also wear a "hat" to protect them from temperature extremes. The whitish fuzzy cap that crowns a saguaro stem is made up of trichomes or plant hairs that add a layer of thermal protection to the growing point of the cacti. Computer simulations of removal of this pubescence increased the daytime maximum temperature of the top of the saguaro by 10º F and decreased the surface temperature at night by 3.6º F. When the spines were removed the maximum and minimum temperatures were changed by only 1.8º F. So for saguaros, pubescence is more important than spines in providing thermal protection.

When it does rain in the upper Sonoran desert, saguaros are very efficient at capturing and storing available water. Depending on the depth of the soil where the individual is growing, the taproot of a saguaro can extend downward about two feet, but most of its roots are located in the upper six inches of the soil. Shallow roots radiate out from the plant in all directions, can be as long as the plant is tall and spread over a 50-foot radius. These roots allow the saguaro to take advantage of rains that only moisten the top few inches of the soil.

Saguaro roots can start to grow within hours after a rain. About 1/3 inch of rain is enough to stimulate cacti root growth. This rapid growth is because specialized fine roots called rain roots can elongate with little cell division. When the soil dries, rain roots shrivel and are shed; larger, older roots shrink and pull away from the surrounding soil matrix inhibiting water loss to the dry soil during periods of drought.

Saguaros are 98 to 99% water which is stored interior to the ribs in specialized cells called "ground tissue." These cells have very large, water-filled vacuoles and thin cell walls. The water pressing on the sides of the cell walls is what actually holds the saguaro stem erect. After a rainstorm a mature saguaro can soak up 200 gallons of water.

Another way that saguaros cope with desert conditions is that instead of drying out from the outside in, saguaros dry from the inside out. In most plants suffering from lack of water the outer part of a leaf dries out first and photosynthesis stops while the interior remains moist. Saguaros do just the opposite. The water storage tissues in the center of the plant somehow are able to reduce their concentration of solutes (salts) and water defuses along a concentration gradient from this water storage "ground tissue" into surface tissues where photosynthesis is maintained as the plant "dries out" and its diameter slowly shrinks.

We think of the Upper Sonoran Desert as being a very sunny place, so it might be surprising to learn that saguaros actually suffer from light limitation. Measurements indicate that vertical cacti that bear a striking resemblance to telephone poles intercept only about 35% as much light as do horizontal leaves at the summer solstice. Over a whole year, a saguaro only receives about half as much solar irradiance as horizontal leaves. This is much less than the energy they need during daylight hours to process the carbon dioxide that they take up at night when their stomates are open. Since a saguaro growing outside in full sunlight is light limited, it should be obvious that cacti confined to even a brightly sunlit room are slowly degenerating and will eventually die from lack of solar irradiance.

Saguaros can and do cope with the natural challenges to their continued existence, but they have no defenses against the challenges imposed by humans. Habitat destruction is the number one problem facing the natural vegetation of the upper Sonoran desert. For the saguaro, that means removal of nurse plants like the lowly and all too often unloved bursage. Pavement encroachment, cutting lateral roots by digging too close to their base, and worst of all transplantation of large individuals take a toll on saguaros. The five-year survival rate of transplanted saguaros over 15 feet tall is less than 10%. Smaller saguaros can be successfully transplanted if at least one foot of all the major lateral roots is retained, they are planted in well drained soil with their original orientation and are watered for at least the first year.

Just one additional tidbit. Saguaros go by two names in the cacti literature. The species was originally named Cereus giganteus in 1884, but was renamed Carnegiea gigantea in 1908 in honor of Andrew Carnegie.

References

• Dimmitt, M. A. 2000. Cactaceae (cactus family). Pp 183-218. In: Phillips, S. J. and P. W. Comus. A natural history of the Sonoran desert. Univ. of Calif. Press. Pp 628.

• Gibson, A. C. and P.S. Nobel 1986. The cactus primer. Harvard Univ. Press, Cambridge, MA. Pp 286.

• McAuliffe, J. R. 1993. New look at research challenges doom-seers. The Sonoran Quarterly 47:1-11.

• Steenbergh, W. F. and C. H. Lowe. 1986. Critical factors during the first years of life of the saguaro. Ecology 50:825-834.