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Saguaro

Carnegiea (Cereus) gigantea

 
Photo © by Michael Plagens

The vast desert metropolis of Phoenix, Arizona, appears below this saguaro perched on the slopes of Shaw Butte, part of the Phoenix Mountain Preserve. This June 2002 photo shows clusters of green fruit near the arm tips.

Photo © by Michael Plagens

Blooming from May to June and occasionally into mid to late summer.


On your hikes through the Sonoran Desert there is a chance that you will find the calling cards of Cactobrosis fernaldialis! In the photo below are two examples of these curious objects that are clearly of botanical origin but seem to have a most curious shape without obvious purpose. These heavily callused structures were left behind after a saguaro had long died and rotted away. They were originally formed by the cactus in defense against the tunneling caterpillar of a Cactobrosis moth. Just before a caterpillar reaches maturity it chews an escape hatch through the outer cortex of the saguaro - this is the origin of the flattened disc structure (right) seen at one end of the callus. If you examine living saguaros these scars are commonly found on the surface.

Photo © by Michael Plagens

Callused galleries created by Cactoborsis larvae:

The callus on the left offers a curious twist in that the "escape hatch" is missing. Something killed the larva before it completed its development and before it could create the hatch. Most likely this was a parasitic wasp. The wasp matured within the Cactobrosis but used an alternate method of escape. Other callused structures are also left behind after a saguaro has disintegrated. The most familiar is called a "saguaro boot". Once a gila woodpecker has hollowed out a nest, the saguaro heals over the injury creating a tough boot-shaped structure. Heavily impregnated with lignin it resists decay and can offer nest holes to hundreds of birds through the life of the cactus.

UNMISTAKABLE: The Saguaro Cactus is unmistakable, at least within Arizona. One of the Barrel Cactus species might look like a young, unbranched saguaro cactus, except for the barrel's larger circumfrence than a saguaro of the same height. But, also, the ...

... SPINE CLUSTERS are different: the saguaro has about 20 straight and similarly textured spines whereas the barrel cactus species have one stiff curved spine, 3-5 straight stiff spines and a dozen or more soft flexible spines.

RANGE: Southwestern third of Arizona, western Sonora, and a few small areas in southeast California near the Colorado River. On south facing slopes the saguaro cactus occurs up to 1500 m elevation which is considerably higher than the 960 m upper elevational limit of the Sonoran Desert as defined by Shreve (1951). I believe that cold-hardiness has been increasing within the saguaro gene pool thus permitting its migration north and upslope.

FLOWERS: 12 cm or more in diameter with many white waxy petals. Numerous yellow stamens form flower center.

FRUIT: The bright red flesh of the fruits is exposed as the fruits begin splitting open after the third week of June. They peel open along the carple seams giving the look of a new set of red flowers.

Cactaceae -- Cactus Family

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A Saguaro Boot, a former Gila Woodpecker nest hole. Photo © by Michael Plagens Where a saguaro cactus has fallen and much of it recycled into the environment the old woodpecker nest holes remain because of the heavily lignized calous that was produced by the plant in response to the tissue damage.

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Growing tip of a saguaro cactus branch, photo © by Michael Plagens

The apical tips of the branches are the growing points of the plant and are protected against the intense sunlight, cold and herbivorous insects. The new Forelius Ant areoles are covered with dense, soft hairs that are reflective of heat and insolation. They might also serve as a blanket on intensely cold nights. The plant also produces substances (sugars?) attractive to ants, particularily Forelius Ants -- (Forelius pruinosus). The ants protect the growing cactus by repelling herbivores.

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Parts of a living saguaro cactus are indeed not alive; this includes the spines; the woody ribs, and some of the exterior cortex. Encrusting Termites (Gnathamitermes perplexus) specialize in covering dead cellulose on the ground surface and relocating it into their below ground galleries. Thus, their encrusting mud-like material is sometimes found coating the surfaces of saguaros as well as ocotillos, usually a half meter or less from the ground. Although this is dead tissue, it might harm the cactus in the long run because the missing spines leave the plant vulnerable to herbivores like Neotoma rodents. In the photo I've knocked away some of the encrusting carton. If conditions are not too hot, dry or cold the small termites can be found working to remove dead plant material.


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Copyright Michael J. Plagens, page created 9 June 2002,
updated 13 April 2013

 

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