The Mighty Saguaro Cactus

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Where does one begin with a plant like the saguaro cactus (Carnegiea gigantea)? It is recognized the world over for its iconic appearance yet its native range is disproportionately small compared to its popularity. It is easily one of the most spectacular plants I have ever encountered and I will never forget the sound the wind makes as it blows over its spiny pleated trunk. It would be impossible to sum up our collective knowledge of this species in one article, however, I feel that some form of an introduction is necessary. Today I want to honor this icon of the Sonoran Desert.

The saguaro is the only member of the genus Carnegiea, which is part of a subtribe of cacti characterized by their columnar appearance. Despite its unique taxonomic affinity, the evolutionary origins of this cactus remains a bit of a mystery. Though it is undoubtedly related to other columnar cacti of the Americas, a proper family tree seems to be just out of our reach. Due to lots of convergent and parallel evolution as well as conflicts between genealogies and species histories, we still aren't sure of its evolutionary origins. What we do know about this species on a genetic level is nonetheless quite interesting. For instance the saguaro has one of the smallest chloroplast genomes of any non-parasitic plant and we aren’t exactly sure why this is the case.

Saguaro are long lived cacti. Estimating age of a cactus can be rather tricky considering that they don’t produce annual growth rings. This is where long term monitoring projects have come in handy. By observing hundreds of saguaro throughout the Sonoran Desert, experts believe that saguaro can regularly reach ages of 150 to 170 years and some individuals may be able to live for more than 200 years. Amazingly, it is thought that saguaro will not begin to grow their characteristic arms until they reach somewhere around 50 to 100 years of age. That being said, some saguaro never bother growing arms. It all depends on where the conditions they experience throughout their lifetime.

Growth for a saguaro depends on where they are rooted. Under favorable conditions, a saguaro can grow to heights of 50 feet or more, with the world record holder clocking in at a whopping 78 feet in height. Such growth becomes all the more impressive when you realize just how agonizingly slow the process can be. Studies have shown that juvenile saguaro only put on about 1.5 inches of growth in their first eight years of life.

Despite preconceived notions about the hardy nature of most cacti, saguaro have proven to be rather specific in their needs. They are limited in their growth and distribution by the availability of water and warm temperatures. Saguaro, especially young individuals, cannot tolerate periods of prolonged frost. Additionally, germination and seedling survival occur most frequently only during the wettest years. In fact, one study showed that successful years for reproduction in these beloved cacti were tied to volcanic eruptions that cooled the climate just enough to allow the young saguaro to become established.

Outside of volcanic eruptions, saguaro appear to have friends in the surrounding vegetation. Studies have shown that saguaro seedlings seem to do best when growing under the shade of trees like the palo verde (Parkinsonia florida), ironwood (Olneya tesota), and mesquite (Prosopis velutina). The microclimates produced by these trees are much more favorable for saguaro growth than are open desert conditions. In essence, these trees serve as nurseries for young saguaro until they are large enough to handle more exposed conditions. Their nursery habits are not mutually beneficial however as research suggests that saguaro eventually compete with the trees that once protected them for precious resources like nutrients and water.

Saguaros outgrowing their palo verde nurse tree. 

Saguaros outgrowing their palo verde nurse tree. 

At roughly 35 years of age, a saguaro will begin to flower. Flowers are small compared to the size of the cactus but they are abundant. Most flowers are produced at the apex of the cactus and it is thought that the growth of saguaro arms is largely a way of increasing the reproductive potential of large individuals. The flowers are cream colored and night scented. They open in the evening but will stay open and continue to produce nectar well into the morning hours.

Though a wide variety of animals will visit these flowers, the main pollinators are bees during the day and lesser long-nosed bats at night. Interestingly, it has been found that certain amino acids within the nectar of the saguaro can actually help female bats sustain lactation while raising their young, making them a valuable food source for these flying mammals. Catering to such a broad spectrum of potential pollinators is thought to have evolved as a means of increasing seed set. Each saguaro ovary contains many ovules and the more pollen that makes it onto the stigma, the more seeds will be produced.

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A lesser long-nosed bat pollinates a saguaro bloom.

A lesser long-nosed bat pollinates a saguaro bloom.

Due to their size and abundance, it is easy to understand why the saguaro is such an ecologically important species in the Sonoran Desert ecosystem. In essence, they function similar to trees in that they serve as vital sources of shelter and food for myriad desert animals. Woodpeckers, especially the gila and the gilded flicker, regularly hollow out and build nests in saguaro trunks. These hollows are subsequently used by many different bird, mammal, and reptile species. The flowers and fruits are important sources of food for wildlife.

Gila woodpecker with its nesting hole.

Gila woodpecker with its nesting hole.

Gila woodpecker holes become homes for other birds like owls. 

Gila woodpecker holes become homes for other birds like owls. 

On rare occasions, woodpecker holes can even become home to other cacti!

On rare occasions, woodpecker holes can even become home to other cacti!

I sincerely hope that this brief introduction does at least some justice to the wonderful organism that is the saguaro cactus. The Sonoran Desert would be a shell of an ecosystem without its presence. What’s more, it has played a significant role in the culture of this region for millennia. Though it appears quite numerous on the landscape, the long-term status of the saguaro is cause for concern. Numerous declines have been reported throughout its range. With its slow growth rates and infrequent recruitment events, the saguaro can be quite sensitive to rapid changes in its environment. Luckily it has received special protection laws throughout its US range.

Photo Credits: [1] [2] [3] [4]


Further Reading: [1] [2] [3] [4] [5] [6] [7] [8] [9] 

In the Wake of Volcanoes

Recruitment windows are any period of time in which seeds germinate and grow into young plants successfully. Needless to say, they are a crucial component of of any plants' life cycle. For some species, these windows are huge, allowing them ample opportunity for successful reproduction. For others, however, these windows are small and specific. Take for instance the saguaro cactus (Carnegiea gigantea) of the American southwest. These arborescent cacti are famous the world over for their impressive stature. They are true survivors, magnificently adapted to their harsh, dry environment. This does not mean life is a cakewalk though. Survival, especially for seedlings, is measured by the slimmest of margins, with most saguaro dying in their first year. 

Hot, dry days and freezing cold nights are not particularly favorable conditions for young cacti. As such, any favorable change in weather can lead to much higher rates of successful recruitment for a given year. Because of this, saguaro often grow up as cohorts that all took advantage of the same favorable conditions that tipped the odds in their favor. This creates an age pattern that researchers can then use to better understand the population dynamics of these cacti. 

Recently, a researcher from York University noticed a particular pattern in the cacti she was studying. A large amount of the older cacti all dated back to the year 1884. What was so special about 1884, you ask? Certainly the climate must have been favorable. However, the real interesting part of this story is what happen the year before. 1883 saw the eruption of Krakatoa, a volcanic island located between Java and Sumatra. The eruption was massive, spewing tons of volcanic ash into the air. Effectively destroying the island, the eruption was so large that it was heard 1,930 miles away in western Australia. 

The effects of the Krakatoa eruption were felt worldwide. Ash and other gases spewed into the atmosphere caused a chilling of the northern hemisphere. Records of that time show an overall cooling effect of more than 2 degrees Fahrenheit. In the American Southwest, this led to record rainfall from July 1883 to June 1884. The combination of higher than average rainfall and lower than average temperatures made for a banner year for saguaro cacti. Seedlings were able to get past that first year bottleneck. After that first year, saguaro are much more likely to survive the hardships of their habitat. 

The Krakatoa eruption wasn't the only one with its own saguaro cohort. Further investigations have revealed similar patterns following the eruptions of Soufriere, Mt. Pelée, and Santa Maria in 1902, Ksudach in 1907, and Katmai in 1912. What this means is that conservation of species like the saguaro must take into account factors far beyond their immediate environment. Such patterns are likely not unique to saguaro either. The Earth functions as a biosphere and the lines we use to define the world around us can become quite blurry. If anything, this research underlines the importance of a system-based view. Nothing operates in a vacuum. 

Photo Credit: Geir K. Edland

Further Reading: [1] [2]