Saguaro cacti are the icons of the Sonoran Desert (see post here). The large, multi-armed saguaros are the senior citizens of the cactus world. Saguaros are killed by being struck by lightning, blown over in a wind storm, and by people. But the largest cause of death is freezing. Last winter we had an unusual hard freeze, but its effects may take several years to become evident.

Freezing weakens the cactus, making it more susceptible to infection. A healthy middle-aged saguaro can stand a few hours of temperatures as low as 10 degrees F in mid-winter. On the other hand, 12 hours of 20 degrees F in late fall causes damage and death. The first sign of trouble is dark scaly skin which cuts down on the cactus’ ability to carry on photosynthesis.

The chief agent of infection, a bacterium, Erwinia cacticida, is carried by the caterpillar of a moth. The bacterium causes rot upon which the caterpillar feeds. The rot turns the flesh of a weak saguaro into a smelly, black liquid.

According to the Arizona-Sonora Desert Museum, a dying saguaro:

becomes an oasis to numerous insects and other arthropods, providing food, moisture, shaded habitat or an enticement for predators seeking live food…. Carve off a piece of tough outer skin of a decaying saguaro, and you’ll find the innards teeming with life. Move the whole saguaro, and spiders race from beneath it, escaping exposure to sunlight.

The sheer size of insect populations is always amazing. A small chunk of rotting saguaro (about 1 cubic foot) was examined at the University of Arizona; it yielded 413 individual arthropods, including adult and larval beetles, larval flies, pseudoscorpions, and mites. Compare that small portion to the size of a whole saguaro and you instantly understand why insects are this planet’s dominant life form.

A closer look at the fauna in a rotting saguaro will expose an ecosystem with grazers on fungi, such as the feather-winged beetles Acrotrichis and Nephanes, and such recyclers of plant matter as the flattened, leathery syrphid fly larvae Volucella, the neriid cactus fly maggots Odontoloxozus longicornis, and numerous phytophagous mites. This habitat is no longer solid plant material, but is now quite aquatic in nature. Several hydrophilid beetles, Agna capillata and Dactylosternum cacti, may be seen swimming awkward strokes through the muck. They feed on a wide assortment of organic material, from fungus and dead plant matter to castoff exoskeletons and dead insects. What better pond is there in the desert?

And what would an ecosystem be without predators? They swarm to this bountiful table in hordes—all sizes of rove and hister beetles, each staking claim to a prey size suitable to its mandibles. Most are colored red, so an observer may readily spot these hungry terrors of the bug world as they stalk their prey through the saguaro rot. One must be careful, however, when searching through this habitat, as another great predator of the desert, the scorpion, may be lurking in a hidden recess with its pinchers and sting at the ready. (By the way, Hister beetles are used in forensic entomology to determine time of death.)

And, of course, there are spiders, snakes, lizards, and rodents waiting to prey upon the inhabitants of the rotting saguaro. High school biology teachers, why be satisfied with dissecting frogs when you could lead your class in dissecting a very aromatic and messy chunk of decaying saguaro that is teeming with life?

Besides bugs, there are also minerals. A mature saguaro can contain as much as 200 pounds of the mineral weddellite (CaC₂O₄·2H₂O). According to Laurence Garvie (American Mineralogist, 2003):

After the death of the saguaro, a series of minerals crystallize in the rotting flesh. These minerals form from elements released from the decay of the cactus by microorganisms and thus is a type of biologically induced mineralization. During the initial stages of decay, authigenic Mg- and Ca-bearing minerals crystallize from elements released by the putrefying flesh and include lansfordite (MgCO₃·5H₂O), nesquehonite (MgCO₃·3H₂O), several polymorphs of MgC₂O₄·2H₂O including glushinskite, monohydrocalcite (CaCO₃·H₂O), calcite, vaterite, and several unidentified Mg-bearing phases.

Even in death, the saguaro provides habitat for life and a learning experience for us.　

See also:

A Desert Christmas cactus

Agave, a plant of many uses

Brittlebush and chewing gum

Can You Get Potable Water From a Cactus?

Chiltepin peppers, spice and medicine

Creosote Bush, a Desert Survivor

Desert Tobacco, a pretty but poisonous desert plant

Edible Desert Plants – Barrel Cactus Fruit

Jojoba oil, good on the outside, bad on the inside

Mesquite Trees Provide Food and a Pharmacy

More on Mesquite

Ocotillos and the Boojum

Palo Verde Trees Will Turn the Desert Golden

Saguaro Cactus Icon of the Sonoran Desert

The Old Man and the Totem Pole

This is the time of year when saguaros bloom and produce fruit. For many generations the Tohono O’odham people have harvested the fruit using long poles made of saguaro ribs. The fruit may be eaten raw or it may be cooked down to a sweet syrup. In the O’odham tradition, some of the syrup was fermented into wine used in a ceremony to herald in the summer monsoon. Dried seeds from the fruit, up to 5,000 per fruit, are rich in protein and fat and can be ground into meal.

Saguaros commonly reach 40 feet tall, a few reach 60 feet to 80 feet. Growth rate depends on rainfall and soil conditions. In Tucson, which averages 12 inches of rain per year, saguaros take about 10 years to get 2 inches high and 30 years to get 2 feet high. Arms may form when the saguaro is 50 to 100 years old and 7 to 12 feet high. Saguaros growing on sandy alluvial fans tend to be bigger and have more arms than plants growing on steep, rocky slopes. The saguaro’s range is restricted almost entirely to southern Arizona and western Sonora. This range is defined by rainfall (too little in the Mojave desert) and by freezing temperatures.

Anatomy

(paraphrased from A Natural History of the Sonoran Desert by Arizona-Sonora Desert Museum press.)

The saguaro (Carnegiea gigantea) is covered by thick, waxy skin that waterproofs the surface and restricts water loss through evapotranspiration almost exclusively to the stomates (pores for gas exchange). The outer surface is folded into pleats (commonly called “ribs,” but not to be confused with the internal, woody ribs. The pleats allow the stem to expand in girth during water uptake without stretching and bursting. Areoles, the roundish pads from which the spines and usually the flowers are produced, are distributed at 1-inch intervals along the ridges of the ribs. Each areole bears a cluster of about 30 spines up to 2 inches long. The spines help protect the plant from herbivorous animals and provide shade to reduce water loss. The lower trunks of old saguaros lose their spines and develop dark, corky bark.

Immediately beneath the skin is a thin layer of chlorophyll-containing tissue where most photosynthesis takes place. The deeper interior, most of the bulk of the plant, consists of the water storage tissue. A fully hydrated saguaro contains 90% water and a large plant weighs about 80 pounds per foot. This water-bearing tissue helps protect the cactus from temperature extremes.

Heat is absorbed through the surface during the day and is stored, resulting in a small temperature rise that does not reach a lethal level. At night, the heat is slowly radiated back into the air. This same thermal inertia usually keeps the tissues above freezing on cold winter nights.

(You cannot tap into a saguaro to get a drink of water. See my post “Can You Get Potable Water From a Cactus?” to find out why.)

A cylinder of 13 to 20 woody ribs occur near the center of the cactus stem, running the length of the main stem and branching into the arms. In the upper part of the stem, the ribs are separate; as the stem ages the ribs continue to grow and fuse into a latticed cylinder. The outside of these woody ribs contain the vascular, or water transporting, part of the plant.

A tap root extends down about 2 feet. The rest of the extensive root system is shallow, rarely more than 4 inches, and radiate outward about as far as the cactus is tall.

Flowers and pollination

Flowers occur near the top of the main stem and on the tips of arms. The white flowers are about 3 inches in diameter and smell like ripe melon. Each flower opens at night and remains open until mid-afternoon the next day. Nectar produced at night attracts bats. A second batch of nectar is produced in the morning to attract birds and insects. Bats and white-winged doves are the main pollinators. The doves, Gila Woodpeckers, and House finches disperse the seeds.

After the flowers are pollinated and fruits mature, the fruit opens to expose its red interior leading some people to think there are red flowers atop the saguaro.

  The Boots

Many large saguaros contain holes excavated by Gila Woodpeckers and Gilded Flickers. The birds go in and down, removing the fleshy part of the cactus. The cactus produces scar tissue, calus, which quickly becomes very hard and impervious to bacterial infection. This material, often in the rough shape of a boot, survives after the cactus dies and rots away. Woodpeckers generally excavate a new hole each year, leaving the hole for other cavity-nesting birds.

Mortality

Most saguaro seedling die from drought, frost, and predation. Seedlings up to a foot tall are eaten by rodents and rabbits.

Some mature saguaros are killed by lightening strikes or blown over by high winds. However, the chief agent of mortality in the Arizona Upland is freezing. Mortality by freezing depends of the seasonal timing and duration. A healthy middle-aged saguaro can stand a few hours of temperatures as low as 10 degrees F in mid-winter. On the other hand, 12 hours of 20 degrees F in late fall causes damage and death.

According to Natural History cited above, a frost-damaged saguaro may survive for another decade or longer, but eventually weakens until it can no longer resist infection. “Bacterial rot caused by Erwinia cacticida turns the flesh of weakened plants into an odoriferous black liquid.” Healthy saguaros can ward off small infections by walling off infected parts. The bacterium is carried by a moth whose maggot-like caterpillar burrows into the cactus and feeds on the rot it introduces. Many living saguaros have round, half-inch scabs on their surface. These are the caps of contorted tunnels left behind by the caterpillar.

Back in the 1990′s some biologists published a paper about the “brown decline” and impeding doom of saguaros. These scientists were unfamiliar with saguaro biology. There was a great freeze in December, 1978, which affected some of the old saguaros and this is what the visiting biologists focused on. They ignored the large population of smaller, younger saguaros.

See also:

Edible Desert Plants – Barrel Cactus Fruit

The Creosote Bush, a Desert Survivor

Mesquite Trees Provide Food and a Pharmacy

The Old Man and the Totem Pole

Palo Verde Trees Will Turn the Desert Golden

On Saturday, Oct. 2, 2010, the National Park Service hosted a symposium at the Arizona-Sonora Desert Museum which considered “How might climate change affect Saguaro National Park.”

The keynote speaker for the morning session was Dr. Jonathan Overpeck from the University of Arizona, a lead author for the IPCC, and one of the scientists mentioned peripherally in the “climategate” emails. He spent a few minutes addressing that.

Overpeck said that the southwest was “ground zero” for climate change. He showed a graph of global temperatures from 1880 to present. I’ve seen him use that graph before. This is a case of cherry-picking to enhance a point. He starts the graph just as the planet began to warm from the “little ice age” so yes, the apparent temperature rise can be depicted as dramatic, especially if one expands the vertical scale of the graph. If, however, he had begun the graph 1,000 years ago during the Medieval Warm Period, the recent rise would have been seen for what it is, just part of the natural cycles.

Overpeck predicted that as temperatures rise, the southwest would become drier because the jet-stream which brings us winter rains will move north and its storms will move north with it. (The summer rains will have little change because they are drawn from the south.) Perhaps that will happen, and the year 2010 is an anomaly with an unusually high winter rainfall. My rain gauge recorded twice as much rain this past winter as I got during the summer monsoon.

Overpeck repeated several times that he has high confidence that human carbon dioxide emissions are responsible for the global warming we are experiencing. In the Q&A after the talk, I asked him to cite some specific physical evidence that human carbon dioxide emissions have produced significant warming. During his answer he said that climate models work best when carbon dioxide is added in, but that’s not evidence. He also made what I thought was an extraordinary statement. He said that most climate scientists (of his group) believe that carbon dioxide is responsible for global warming “because they can’t think of anything else” that would cause such warming. That too, is not evidence. In other words, a lead author of the IPCC climate studies cannot cite any physical evidence that human carbon dioxide emissions have a significant effect on global temperature. Rather, it must be so because they think it so.

Three talks dealt with saguaro populations. These studies are conducted on specific, small plots within the National Park; some were established as long ago as 1935. Each plot is periodically visited and saguaros are counted and measured. The assumption is that these plots are representative of the population as a whole, however, each plot may represent only a micro-climate, not the whole, and may be subject to special local conditions. For instance, in one plot, pack rats were eating the saguaros. The saguaro population is cyclic and depends on, among other things, the amount of precipitation, the number and severity of days of frost, and the health of palo verde trees which act as nurse plants to young saguaros. Some plots showed increases in the number of saguaros while others showed decreases. The National Park Service performs a saguaro census every ten years. Among the plots they studied there was a general increase in saguaro population over the last ten years. For more information on the NPS monitoring program, see http://science.nature.nps.gov/im/. See more details from Tony Davis’ article in the Arizona Daily Star.

Dr. Donald Miles of Ohio State University reported on lizard populations. His study involved study plots similar to those in the saguaro studies. Since lizards are ectothermic, rising temperatures may limit their hunting time since the lizards cannot be outside if it is too hot. Miles reported that certain lizards have become “extinct” from some study plots and predicted that 66% of species will become extinct in 40 years. He did mention that his extinction models have not been calibrated with actual extinctions. When pressed during Q&A, Miles admitted that the “extinction” really meant that the lizards were not observed in the test plots and that they simply could have moved to better climes.

Dr. Phil Rosen studied reptiles along transects in Organ Pipe National Monument. He found no significant lizard decline, but did find decreased populations of snakes (except for sidewinders).

Kris Ratzlaff, a University of Arizona graduate student, studied lowland leopard frogs and canyon tree frogs in the Rincon Mountains. Her study provides baseline data for future investigations. She found that almost all leopard frogs in the three drainages studied were infected with the chytrid fungus, a problem for frogs world-wide. There was much less infection among the tree frogs.

Dr. Travis Huxman gave the keynote talk of the afternoon session. Huxman is a University of Arizona biology professor and director of Biosphere 2. He noted that rising atmospheric carbon dioxide makes plants more productive and more drought tolerant. The magnitude of this aerial fertilization effect is tempered by the type of plant (fast growing vs. slow growing) and by the amount of water available.

Much of the symposium was about speculative problems. There is, however, one real and current problem that has little to do with climate change: the invasion of exotic grasses such as buffelgrass.

Native grasses are generally confined to higher elevations and cannot survive on the hot desert floor. However, non-native species imported for cattle feed, highway beautification, and landscaping, can survive on the desert floor, and that is the problem. “Buffelgrass grows densely and crowds out native plants of similar size. Competition for water can weaken and kill larger desert plants. Dense roots and ground shading prevent germination of seeds. It appears that buffelgrass can kill most native plants by these means alone.” The other problem is that these exotic grasses fill in the space between native plants and thereby can transmit wild fires.

The take-away from this symposium is that the issues are complex. While we would all like some definitive answers, real science is messy.

For more information on droughts, see my article “Drought in the West.”