Leafy Cacti?

Pereskia aculeata

Pereskia aculeata

At first glance, there is little about a Pereskia that would suggest a relation to what we know as cacti. Even a second, third, and forth glance probably wouldn't do much to persuade the casual observer that these plants have a place on cacti family tree. All preconceptions aside, Pereskia are in fact members of the family Cactaceae and quite interesting ones at that.

Most people readily recognize the leafless, spiny green stems of a cactus. Indeed, this would appear to be a unifying character of the family. Pereskia is proof that this is not the case. Though other cacti occasionally produce either tiny, vestigial leaves or stubby succulent leaves, Pereskia really break the mold by producing broad, flattened leaves with only a hint of succulence.

Pereskia spines are produced from areoles in typical cactus fashion.

Pereskia spines are produced from areoles in typical cactus fashion.

What's more, instead of clusters of Opuntia-like pads or large, columnar trunks, Pereskia are mainly shrubby plants with a handful of scrambling climbers mixed in. Similar to their more succulent cousins, the trunks of Pereskia are usually adorned with clusters of long spines for protection. Additionally, each species produces the large, showy, cup-like blooms we have come to expect from cacti.

They are certainly as odd as they are beautiful. As it stands right now, taxonomists recognize two clades of Pereskia - Clade A, which are native to a region comprising the Gulf of Mexico and Caribbean Sea (this group is currently listed under the name Leuenbergeria) and Clade B, which are native to regions just south of the Amazon Basin. This may seem superficial to most of us but the distinction between these groups has a lot to teach us about the evolution of what we know of as cacti. 

Pereskia grandifolia

Pereskia grandifolia

Genetically speaking, the genus Pereskia sorts out at the base of the cactus family tree. Pereskia are in fact sister to all other cacti. This is where the distinction between the two Pereskia clades gets interesting. Clade A appears to be the older of the two and all members of this group form bark early on in their development and their stems lack a feature present in all other cacti - stomata. Stomata are microscopic pours that allow the exchange of gases like CO2 and oxygen. Clabe B, on the other hand, delay bark formation until later in life and all of them produce stomata on their stems.

The reason this distinction is important is because all other cacti produce stomata on their stems as well. As such, their base at the bottom of the cactus tree not only shows us what the ancestral from of cactus must have looked like, it also paints a relatively detailed picture of the evolutionary trajectory of subsequent cacti lineages. It would appear that the ancestor of all cacti started out as leafy shrubs that lacked the ability to perform stem photosynthesis. Subsequent evolution saw a delay in bark formation, the presence of stomata on the stem, and the start of stem photosynthesis, which is a defining feature of all other cacti.

Pereskia aculeata

Pereskia aculeata

If you are as excited about Pereskia as I am, then you , my friend, are in luck. A handful of Pereskia species have found their way into the horticulture trade. With a little luck attention to detail, you too can share you home with one of these wonderful plants. Just be warned, they get tall and their spines, which are often hidden by the leaves, are a force to be reckoned with. Tread lightly with these wonderfully odd cacti. Celebrate their as the evolutionary wonders that they are!

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

Further Reading: [1] [2]

 

 

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] 

The Pima Pineapple Cactus

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The Pima pineapple cactus (Coryphantha robustispina) is a federally endangered cactus native to the Sonoran Desert. It is a relatively small cactus by most standards, a fact that can make it hard to find even with a trained eye. Sadly, the plight of this cactus is shared by myriad other plant species of this arid region. Urbanization, fire, grazing, and illegal collection are an ever present threat thanks to our insatiable need to gobble up habitat we should never have occupied in the first place. 

Deserts are lands of extremes and the Pima pineapple cactus seems ready for whatever its habitat can throw its way (naturally). Plants are usually found growing individually but older specimens can take on a clustered clonal habit. During the winter months, the Pima pineapple cactus shrivels up and waits until warmth returns. Come spring, the Pima pineapple cactus begins anew. On mature specimens, flower buds begin to develop once the plant senses an increase in daylight. 

The flower buds continue to develop well into summer but seem to stop after a certain point. Then, with the onset of the summer monsoons, flower buds quickly mature and open all at once. It is thought that this evolved as a means of synchronizing reproductive events among widely spaced populations. You see, seed set in this species is best achieved via cross pollination. With such low numbers and a lot of empty space in between, these cacti must maximize the chances of cross pollination.

If they were to flower asynchronously, the chances of an insect finding its way to two different individuals is low. By flowering together in unison, the chances of cross pollination are greatly increased. No one is quite sure exactly how these cacti manage to coordinate these mass flowering events but one line of reasoning suggests that the onset of the monsoon has something to do with it. It is possible that as plants start to take up much needed water, this triggers the dormant flower buds to kick into high gear and finish their development. More work is needed to say for sure.

Seed dispersal for this species comes in the form of a species of hare called the antelope jackrabbit. Jackrabbits consume Pima fruits and disperse them across the landscape as they hop around. However, seed dispersal is just one part of the reproductive process. In order to germinate and survive, Pima pineapple cacti seeds need to end up in the right kind of habitat. Research has shown that the highest germination and survival rates occur only when there is enough water around to fuel those early months of growth. As such, years of drought can mean years of no reproduction for the Pima.

Taken together, it is no wonder then why the Pima pineapple cactus is in such bad shape. Populations can take years to recover if they even manage to at all. Sadly, humans have altered their habitat to such a degree that serious action will be needed to bring this species back from the brink of extinction. Aside from the usual suspects like habitat fragmentation and destruction, invasive species are playing a considerable role in the loss of Pima populations. 

Lehmann lovegrass (Eragrostis lehmanniana) was introduced to Arizona in the 1930's and it has since spread to cover huge swaths of land. What is most troubling about this grass is that it has significantly altered the fire regime of these desert ecosystems. Whereas there was once very little fuel for fires to burn through, dense stands of Lehmann lovegrass now offer plenty of stuff to burn. Huge, destructive fires can spread across the landscape and the native desert vegetation simply cannot handle the heat. Countless plants are killed by these burns.

Sometimes, if they are lucky, large cacti can resprout following a severe burn, however, all too often they do not. Entire populations can be killed by a single fire. What few plants remain are frequent targets of poaching. Cacti are quite a hit in the plant trade and sadly people will pay big money for rare specimens. The endangered status of the Pima pineapple cactus makes it a prized target for greedy collectors. 

The future of the Pima pineapple cactus is decidedly uncertain. Thankfully its placement on the endangered species list has afforded it a bit more attention from a conservation standpoint. Still, we know very little about this plant and more data are going to be needed if we are to develop sound conservation measures. This, my friends, is why land conservation is so important. Plants like the Pima pineapple cactus need places to grow. If we do not work harder on setting aside wild spaces, we will lose so much more than this species. 

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

Further Reading: [1] [2] [3]

A Truly Bizarre Cactus From The Amazon

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When we think of cacti, we tend to think of dry deserts and sandy soils. Few of us would ever jump to the trunk of a tree, nestled in a humid rainforest, and experiencing periodic inundation. Yet, such a habitat is the hallmark of one of the world's strangest species of cactus - Selenicereus witii. In more ways than one, this species is truly aberrant.

Whereas epiphytic cacti aren't novel, the habits of S. witii surely push the limits of what we know about the entire cactus family. Despite having been discovered in 1899, little attention has been paid to this epiphytic cactus. What we do know comes from scant herbarium records and careful observation by a small handful of botanists.

S. witii is endemic to a region of central Amazonia and only grows in Igapó, or seasonally flooded, blackwater forests. It makes its living on the trunks of trees and its entire morphology seems particularly adapted to such a harsh lifestyle. Unlike most cacti, S. witii doesn't seem to bother with water storage. Instead, its stems grow completely appressed to the trunks of trees. Roots emerge from near the spine-bearing areoles and these help to anchor it in place. 

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Because they are often exposed to bright sunlight, the stems produce high amounts of chemical pigments called betalains. These act as sun block, protecting the sensitive photosynthetic machinery from too much solar radiation. These pigments also give the plant a deep red or purple color that really stands out against the trunks of trees. 

Like all members of this genus, S. witii produces absolutely stunning flowers. However, to see them, your best bet is to venture out at night. Flowers usually begin to open just after sundown and will be closed by morning. And my, what flowers they are! Individual blooms can be upwards of 27 cm long and 12.5 cm wide (10 in by 5 in)! They are also said to produce an intense fragrance. Much of their incredible length is a nectar tube that seems to be catered to a specific group of sphinx moths, whose proboscis is long enough to reach the nectar at the bottom.

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The seeds of S. witii are just as aberrant as the rest of the cactus. They are rather large and shaped like a kidney. Cross sections reveal that most of their size is devoted to hollow air chambers. Indeed, the seeds float like tiny pieces of cork when placed in water. This is likely an adaptation resulting from their preferred habitat.

As mentioned above, S. witii has only been found growing in seasonally flooded forests. What's more, plants only occur on the trunks of large trees right at the high water line. In fact, the highly appressed nature of its stems seems to suggest that this species can withstand periodic submergence in fast flowing water. The seeds must also cope with flooding and it is likely that their buoyant nature aids in seed dispersal during these periods. 

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All in all, this is one weird cactus. Although it isn't alone in its tropical epiphytic habit, it certainly takes the cake for being one of the most derived. Aside from a few publications, little attention has been given to this oddball. It would appear that the seasonal flooding of its preferred habitat has simply chased this cactus up into the trees, the environmental demands of which coaxed out strange but ingenious adaptations from its genome. The good news is that where it does occur, S. witii seems to grow in high numbers.

Photo Credit: [1] [2] [3] [4] [5]

Further Reading: [1]

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]