A Festive Anniversary Week in the Sonoran Desert. Part 5: Saguaro National Park – East, Rincon Mountains Unit; All saguaro, all the time!

(December 11-17, 2022)

December 14, 2022

We certainly enjoyed visiting Saguaro NP – East. Most of the Rincon Mountains Unit is wilderness and only accessible on foot; no dogs allowed.  But the cactus Forest Loop Drive was scenic, with views of the Rincon Mountains to the East, and saguaro everywhere!

Really couldn’t seem to get enough saguaro! The highlights of this day were finding another crested saguaro while hiking an area Luna could enjoy, standing next to some shoulder high fishhook barrel cactus, and enjoying the Phainopeplas with their shimmery black feathers.

If you’re up for more saguaro botany, read on.  This post will be dedicated to this largest of all North American cactus, Carnegiea gigantea. (Beware. This post is long, so settle in!)

Mini-Cactus Intro

Did you know that all cacti (cactuses) are succulents, but not all succulents are cacti? It’s true! Take the case of the aloe …. classified as a succulent, but definitely not a cactus. It’s all about the areoles.  Only cacti have areoles.

And to clear up some terminology …… cacti is the proper plural for cactus – in Latin.  But you’re equally correct by using the English plural for cactus ….. cactuses.

There are around 1800 species of cactus, nearly all endemic (native) to the New World, where they range from southern Canada to southern South America. Although prickly pear cacti are found around the world, they became established and naturalized following the introduction of the species, primarily to cultivate and harvest the cochineal bug for its brilliant red dye (See my post from last summer about cochineal https://wordpress.com/post/a-curious-nature.com/1341).  Only one cactus species is native to East Africa, Sri Lanka and Madagascar…. the mistletoe cactus, Rhipsalis baccifera. Why this species is native to these areas of the Old World is a mystery, because it is a widespread native species from southern Florida, and throughout Central and South America. But that’s a story for another day! 

Saguaro ….. What Makes them Tick (aka Saguaro Botany and Ecology 101)

A Keystone Species

Like trees in a forest, the saguaro is a keystone species. This means that nearly every other organism in its range is ecologically intertwined with this cactus. 

Saguaro provide homes for many desert creatures. Gila woodpeckers and gilded flickers excavate nest holes in the cactus’s fleshy stems, and after their young fledge, other birds move in.  Elf owls, house finches, ash-throated flycatchers, and purple martins all occupy these abandoned holes, as do invertebrates. Birds like red-tailed hawks and the cactus wren build nests in the angles between the mainstem and arms, and use the cactus as hunting and resting perches.  

In drier areas of the Sonoran Desert, pack rats, jackrabbits, mule deer and bighorn sheep eat the saguaro’s flesh when other water sources are not available.

Saguaro that have died, whether on the ground or still standing, become homes for snakes, rodents, lizards and other invertebrates.

In late April through early June, the top or crown of the saguaro’s mainstem and arms sprout a profusion of creamy white flowers that open only at night, and last less than 24 hours. These showy flowers, which smell like overripe melons, are self-fertile, but must be cross pollinated to develop their fruit. So with such a short window of availability, it’s like a feeding frenzy.  Nectar feeding bats (the lesser long-nosed and the Mexican long-tongued) feed and pollinate at night, while during the day the flowers are pollinated by birds such as the white-winged dove, and bees. 

In late summer, ripening fruit provides moisture and energy-rich food. Bats readily eat the bright red fleshy fruits, consuming some of the 2,000 black seeds with the meal. The seeds eventually pass through their digestive system, but are usually deposited in their roost caves where germination is unsuccessful. When the fruit and seeds are eaten by a coyote or cactus wren, the seeds also pass through their digestive system but become distributed throughout the desert. However, if doves or quails eat the seeds, they are are lost, becoming completely consumed in their digestive systems.

Saguaro Features

Exterior

Skin: The epidermis (skin) is covered with a thick, waxy cuticle that waterproofs the surface and restricts loss of water vapor (transpiration)

Areoles are modified branches. They are light colored (in the saguaro), cushion-like roundish pads where spines, branches and flowers are produced. The areoles are distributed at 1-inch intervals along the ridges of the pleats.

Spines are modified leaves. They serve as the primary and secondary defense against herbivory, radiate heat from the stem during the day, and collect and drip condensed water vapor to the ground during cooler nights.  Each areole bears up to 30 spines, some 2” long. The stout, needle-like to awl shaped, straight or curved spines are found on the lower 8 feet of the mainstem; above this level the areoles produce flexible bristles (glochids) which are fine, sharp and barbed, resembling splinters. The sharp spines are not detachable from the areoles; the glochids readily detach.  The lower trunks of old saguaros lose their spines and develop dark, corky bark.

Interior

Immediately beneath the waxy skin is a thin layer of chlorophyll-containing cells. Because the saguaro doesn’t have any actual leaves, photosynthesis is carried out by the vertical stem and arms.  

Going deeper:  Most of the bulk of the plant consists of water storage tissue or cells called parenchyma. But more about that later! 

And deeper still: Near the center of the stem is a cylinder of 12-30 woody ribs running the length of the main stem and branching into the arms. In the upper part of a stem the ribs are separate; as the stem ages the ribs continue to grow and fuse into a latticed cylinder.

Underground

Saguaro Roots

Because the Sonoran desert is typically dry and only receives occasional rainfall, to efficiently and quickly collect rainwater, the saguaro grows a maze of shallow, fibrous roots within 3-5” of the surface. This network of roots radiates out from the mainstem as far as the stem is tall. That means a 40 foot tall saguaro is likely to have roots spreading out in an 80 foot diameter! Another adaptation that helps saguaro take advantage of every drop of rain during a storm, is its ability to grow “rain” roots; an additional network of temporary fibrous roots that can develop in a matter of hours but wither away when the storm is over.      

Water Storage

Saguaro are water hogs. When well hydrated, 75-85% of their weight consists of water. Just one foot of of a large stem can weigh over 80-90 pounds; an entire saguaro can weigh more than two tons! 

Where is all of this water stored, I wondered. Parenchyma! Parenchyma is the water-storage tissue making up the bulk of the plant just beneath the waxy epidermis (skin) and the layer of cells working hard on photosynthesis. And saguaro have a greater number of parenchyma cells than almost all other plants.  All of that water not only hydrates this giant cactus, but protects it through thermal regulation, helping to stabilize the saguaro’s internal temperature, especially hot days and freezing nights.

Ok, that’s fascinating. But for saguaro to store so much water in a water starved desert …. how is that even possible?    It’s all about the roots and the accordion-pleated skin.

Accordion-style Pleats

How can the saguaro absorb hundreds of pounds of water so quickly, and not burst? The plants’ outer skin, which is folded into pleats, readily expands in girth to accommodate the rapid uptake of water.  This expansion means the skin of a well hydrated saguaro has shallow folds, without deep furrows. The same mechanism that allows the skin to expand, also causes the skin to contract as the saguaro loses water, resulting in deeply furrowed pleats. Because the saguaro has several evolutionary characteristics that prevent it from rapid water loss between storms, it is able lose up to two-thirds of its water weight and still survive.

Key Features and Anomalies 

Saguaro Arms

Saguaros may begin to grow arms when the plant is between 50 and 100 years of age, usually just above the stem’s maximum girth at about 7 to 9 feet above-ground. A few saguaros have been observed with as many as 50 arms; many never grow arms. Saguaro arms always grow upwards. The drooping arms seen on many saguaros is a result of wilting after frost damage; their growing tips will turn upwards in time. There is a myth that arms are produced so as to balance the plants, but research shows arm-sprouting to be random. Many saguaros can be found with several arms all on the same side of the main stem.

The number of arms and overall size of a plant seem to be correlated with soil and rainfall. Saguaros on bajadas (dry washes) with finer, more water-retentive soils tend to grow larger and produce more arms than do those on steep, rocky slopes.

Saguaro Boot

Saguaros make excellent nesting places for many birds, especially the Gila woodpecker and gilded flicker, both of which excavate nest holes in the fleshy stems.

The plant seals the wound with a callus-like scar tissue that quickly becomes very hard and impervious to microbial infection. Callus tissue decomposes more slowly than most of the rest of the saguaro and can be found on the ground among the debris of dead plants. Because of their shapes the callus remains of nesting holes are called “saguaro boots.” 

Crested and Wavy-ribbed Saguaro

Occasionally abnormal growths occur, the best-known type called a “crest”.  This fan-shaped crest results when the actively growing and dividing tissue at the tip or crown of the saguaro begins branching and branching until it can branch no more.  The cause is not known. It may be the result of a genetic mutation, or may be triggered following damage to the growing point. These fan-shaped crests aren’t harmful; the saguaro will continue to produce flowers and fruit. Crests are occasionally found in nearly all plant species; the phenomenon is especially noticeable in saguaros because of their size. Other saguaro anomalies occasionally encountered are ribs that undulate or spiral.

Wow! That’s a wealth of information gleaned from many scientific sources. After hours and hours of fun research, I believe the saguaro is the most researched cactus ever! What say you?

If you’ve stayed with me through this entire post, bravo! Like myself, you are now wiser about many things “Saguaro.” Thanks for coming along, again., and again, and again!

Stay tuned for Part 6: Mt. Lemmon – Tucson’s Majestic Backdrop and a Cool Summer Escape