Sonoran Twin B Getaway Part 2: Why So Spiky?

I’d like to ask you to start today’s post with a Thought Experiment. That’s where you think about something, and do sort of a what-if? about the thing you’re thinking about. No, no, not yet- I have to tell you what to think about.

Tangent: I love thought experiments, and here’s why. I love science, and I love the idea of being all kind of science-y and conducting experiments. But I’m also lazy and disorganized, and Real Experiments involve things like materials and effort and time and patience and preparation and what-not. So for me, Real Experiments- with the exception of zero-preparation stream-of-consciousness experiments*- are pretty much off the table.

*i.e. “What will happen if I ride my bicycle down these stairs?”, or “How will my spouse react if I am 2 hours late?”, or ”If I keep nodding, will my coworkers assume I understand what they’re talking about?”

But Thought Experiments are right up my alley. I can conduct them anyplace, anytime, with no preparation, fuss, risk of injury or clean-up.

Nested Tangent: And in fact I do. If you are one of my several lurking coworker readers, you should know that I frequently conduct thought experiments while at work, usually while in meetings with you. You’ll be going on and on about who-knows-what, and I’ll be totally tuned out, imagining what would happen if an ostrich were to pilot a nuclear submarine under the Arctic ice-pack, or how many coworkers I’d hit if I pulled that dead cat out of my laptop-case and started swinging it in a circle around me. Usually I’ll cover by staring at you intently with my brow slightly furrowed, as if you’re sharing some remarkable, life-changing insight that will fundamentally revolutionize our business, But really, you never are*.

*Because- and here is one of those remarkable footnoted nuggets of insight that just makes this blog so phenomenally worthwhile- anyone at any job in any company who has something worthwhile to say says it in under 90 seconds. After that, they’re just blathering.

But Thought Experiments aren’t just screwing around. Schrodinger’s Cat and the Twin Paradox are 2 well-known examples of Thought Experiments that are taken seriously by Real Actual Scientists. So pay attention and stick with me.

Imagine that you’re riding a mountain bike along a winding singletrack trail through a desert (i.e. no big trees, or shrubs taller than you are, and no big boulders or sharp rocks). You’re just riding along at maybe 15 MPH, when you miss a turn, go over the bars and go crashing into a typical patch of vegetation. How badly are you hurt?

Before you answer, I want you to run the experiment 3 different ways:

1- In the Great Basin Desert, say on Antelope Island in the Great Salt Lake

2- In the Mojave Desert, say on Barrel Roll trail, just outside of St. George, UT.

3- In the Sonoran Desert, say on one of the trails through the McDowell Mountain Preserve, just outside of Phoenix.

In the first example- as I can tell you from first-hand experience, you’re probably just fine. You may have landed in a patch of Sagebrush, Rabbitbrush or Bitterbrush, and while you may be a little scraped and bruised, you’re likely to dust off and be on your way promptly.

In the second example, you might be a bit more scratched up. You may well have crashed into a bunch of Blackbrush or maybe even- if you’re unlucky- a stand of yucca or even a patch of prickly-pear cactus. So you may need to spend a few minutes cleaning up, or even break out the tweezers and pull out a few thorns.

In the third example, man are you in a world of hurt. Seriously, if you fly off your bike and land on a big old Teddy Bear Cholla, Cylindropuntia bigelovii, I don’t know what you do. Start praying I guess, because you are in for a long, slow, excruciatingly painful process of disengagement.

This thought experiment may seem kind of “so-what”, but it highlights the question that always bugs me about the Sonoran: Why is it that the further South you go, and the hotter it gets, the desert gets spikier/sharper/spinier? Think about it. There’s just no comparison between the general spiny-ness of plants in open spaces around Salt Lake Valley, and those around Phoenix. Why is that?

Why So Spiky?

You would think that a question this basic would have a real easy, obvious answer. It doesn’t. I’ve googled every possible combination of “spines”, “thorns”, “Sonoran”, “desert”, “reason”, “why”, “evolution”, “plant”, “cactus” and about a dozen other terms. I’ve got a dozen+ great books on plants and deserts. Nowhere have I found a simple clear explanation for why the Sonoran is so darn spiky.

Typical “answers” read something like this: “Desert plants have adapted to their harsh environments with defensive measures such as spines and thorns to protect them from predators…” Yeah, so what? Predators of plants live in lots of environments- jungles, forests, grasslands, swamps- why aren’t those plants spiky? And even if there is some reason for specifically desert plants to be spiky, why are the Sonoran Desert plants so much spikier than those of other North American deserts?

Side Note: I’ve mainly ignored the Chihuahuan Desert in this post, only because I have the least familiarity with it, having hiked in it exactly once, for 2 days, 20 years ago. From what I know and remember, it is both hot and spiky, though not as spiky as the Sonoran.

All About Spines

People often use the words “spine” and “thorn” interchangeably, but they’re 2 different things. Cacti have spines, not thorns. A Cactus spine is a modified leaf that is dry, woody, and doesn’t contain chlorophyll. That much botanists agree on*. How cactus spines evolved is a much more complicated and as yet un-resolved question.

*The other main theory is that the spines are modified bud-scales, but since bud-scales are themselves modified leaves, it’s kind of the same thing.

Cactus spines lack nearly all of the standard features common to leaves. They have no xylem, phloem or stomates- just woody tissue*. In a mature spine, all of the cells are dead, and even when growing, the living cells/new growth is only at the base**.

*Technically, they’re “woody tissue”, but not “wood”, because they don’t have all the features of wood either (example = no vessel elements.)

**This is really cool when you think about it. Cacti are dicots. In dicots new leaf growth occurs at the edges. In monocots, like grass, or yucca, new leaf growth occurs only at the base of the leaf. Have cacti re-invented the monocot trick? It’s even more interesting, because many cacti also grow “regular” leaves, and these leaves grow like normal dicot leaves- from the edges.

Somehow in the course of cactus spine evolution, the genes for leaf-type features got turned off, and the genes for woody-tissue-type features got turned on. How this change came about is still unknown.

Extra Detail: The common ancestor is thought to have been a woody New World shrub which some 50 million years ago evolved the areole*, the distinctive component out of which spines grow on all cacti. The evolution of the areole meant that cacti were able to grow large numbers of spines more quickly and efficiently than they would be able to grow them directly from the stem (like leaves.) The common ancestor is thought to have resembled a Lemon Vine, Pereskia aculeata.

*No, not the thing around your nipples- that’s the areola.

BTW in Part 1 I mentioned how Sonoran cacti are divided into Cereus (columnar) and Opuntia (segmented). These 2 types have different spines. Cereus have only big, visible spines. Opuntia have the big ones, as well as little teeny fine spines, called glochids. Some Beavertail/Prickly-Pear species appear to be spine-less, but actually lack just the large spines; they still have glochids. This is why you don’t want to grab an apparently “spine-less” Prickly-Pear pad with your bare hand.

OK, so we know what cactus spines are. Now, what are they for? Duh. Easy question right? Defense, of course. Well yes, but not only defense, and in fact it’s not clear if defense was the initial driver in the evolution of spines from leaves.

A common mistake people make when buying small cacti from nurseries is to place them in the direct sun. Nursery cacti are shaded, and need to adjust to strong sunlight. The most important way they adjust is by growing more spines, because spines shade the plant. On many cacti, the most important function of spines is to shield the plant from the full sun. This may sound counterintuitive at first; after all those spines are so skinny. But think again of our biking nemesis, the Teddy Bear Cholla.

If you ever walk up to one and try to touch a finger to the “body” of the plant, it’s virtually impossible; the plant is thoroughly covered with spines. And while each spine is a skinny little needle, the matrix-like cover provided by thousands of them creates substantial shade.

Why spines for shade? Why not just leaves? Because leaves are alive, and living tissue requires- among other things- water. Yes, Jojoba, Creosote and other plants manage to grow and sustain living leaves in dry conditions, but no living leaf is as water-efficient as a dead spine.

Cactus spines serve other functions as well, one of the more interesting of which is reproduction. Many species in the Opuntia group (explained in last post) including chollas and beavertails, reproduce asexually by dropping loosely-attached segments. These segment either hang off the plant or drop to the ground, where, in either case, their barbed spines latch on to any animal or shoe that happens to brush against them. On Saturday’s hike we picked up such “passengers” several times, which we carefully removed from our shoes, often dozens of yards from where we picked them up.

Tangent: BTW, speaking of getting around in the Sonoran, know what’s a great way to move fast in cholla-country? Stotting. Check out this Mule Deer moving.

If a segment lands in a suitable location, it can take root and develop into a new, genetically identical plant.

In fact many of the cholla species you see in the Sonoran reproduce primarily in this manner. The vast majority of Teddy Bear Cholla you come across in Southern Arizona are infertile chromosomal triploids that came about via dropped/carried segments.

Side Note: For long-time readers, if this reminds you of something, it should: Dandelions, of which here in North America the ones we come across are overwhelmingly also chromosomal asexual triploids. The analogy is imperfect though: Cholla reproduces by segmenting, while Dandelions create fertile seeds through apomixis*. Triploid dandelions also frequently produce viable (diploid) pollen, enabling them to fertilize the (haploid) ovules of any diploid dandelions they should happen to encounter, and thereby founding new triploid lines. I have no idea if the same is the case with Teddy Bear Cholla.

*Technically agamospermy in angiosperms.

Spines may serve other functions. It’s been postulated they help collect dew, and maybe even retain heat on cold desert nights. Regardless, it’s clear that for many species of cactus, spines serve important functions. It’s also clear that cacti, with their succulence, CAM photosynthesis, shallow rain-leveraging roots and other xerophytic adaptations, do well in the Sonoran, much better than many other plants. So maybe the reason the Sonoran is so spiky is simply because there are a lot of cacti, and cacti generally have spines.

But then there’s thorns.

All About Thorns

Many, many non-cacti in the Sonoran are also spiky. Ocotillo, Fouquieria splendens, is one example; Mesquite (genus = Prosopis), Catclaw Acacia (Acacia greggii), Ironwood (Olneya tesota) and Crucifixion Thorn (Canotia holacantha) are just a few of the many, many others. Unlike cacti, these plants don’t have spines; they have thorns.

Whereas spines are modified leaves, thorns are modified branches. They’re all over the place in the Sonoran. In general, if you plunge/crash into a thicket of non-cacti shrubs/shrees down in Southern Arizona, you’re going to get pretty scratched up, if not outright stuck. (Contrast that with a trailside crash in the Wasatch, say up around Pinebrook or Park City. Ninebark, Serviceberry, Chokecherry- it’s hard to think of anything thorny you’ll crash into, with the occasional exception of Wild Rose*…)

*Actually, just to complicate things, Rose-thorns are- botanically speaking- not thorns. They’re prickles. A thorn is a modified branch. A prickle is an extension of the stem’s cortex. Though they look similar and serve the same defensive function, they’re structurally 2 different things.

Side Note: For years I’ve had trouble telling the some of the most common Sonoran-thorny-things, such as Ironwood, Mesquite and Catclaw Acacia, apart. Part of that’s because I just don’t spend enough time down South, but in fairness, they’re all vaguely similar, with small, dull/light-green pinnately-compound* leaves. With a little patience and a decent plant guide they’re not too hard to tell apart, but last weekend I came up with a real quick & easy way to tell them apart- by their thorns. And since this is when you most often notice them- when they stick you- it’s a helpful distinction.

*See this post for explanation of “pinnately-compound.”

Mesquite thorns are always straight. They stick straight out/up, don’t curve, and hurt. But they don’t stick to you or your clothing, so you can extricate yourself quickly. Catclaw Acacia thorns on the other hand, are curved, just like- that’s right- a cat’s claw. And because they’re curved, they grab onto you, or your pack, or your hat or clothing, and they don’t let go.

The harder you struggle the more they dig in, and the more stuck you become. To extricate yourself, you need to stop, take a deep breath, and methodically, slowly, quietly focus on disconnecting one thorn at a time. (Better yet, ask a friend for help.)

Nested Side Note: Many desert rats also call CC Acacia “Wait-A-Minute Bush”, but Wait-A-Minute’s actually a different plant, Mimosa biuncifera.

Ironwood thorns are also curved, but not so much so. And on mature trees they only seem to grow on the younger shoots, so they’re less likely to snag feet or legs when passing by a mature tree. Ironwood, BTW, is a cool tree in general. It’s wood is super-dense- enough so to sink in water, like our own Curlleaf Mountain Mahogany back here in the Wasatch. Several sources claim it’s the best desert wood for grilling steaks- even better than mesquite- with slow-burning, long-lasting coals.

Another cool thing about Ironwood is that it has right around the same level of cold/frost tolerance as citrus trees, so its presence is a good indication as to whether citrus farming is viable at a given latitude and elevation.

Unlike spines, thorns don’t seem to serve much purpose other than defense. And even without spines or thorns, it seems as though Sonoran plants still manage to find ways of being sharp and spiky. Shrub Live Oak, which isn’t common in the Superstitions, but was all over the place when AS and I hiked in the Ajo Mountains* to the Southwest of Phoenix a couple years back, has tough, stiff leaves whose tips have become fairly sharp points.

*The oak in the Ajos is real interesting, BTW. It’s the same Shrub Live Oak, Quercus turbinella, we get in Southwest Utah, but slightly different. For a while it was considered a distinct, endemic species- Q. ajoensis- but is now considered a subspecies of Q. turbinella. (Somewhere I have some great photos from that trip of the distinctive leaves, but can’t locate them just now…)

And even the stem tips/ends of Palo Verde are disturbingly pointy. It’s almost like everything growing in the Sonoran is sharp and pointed at you.

Side Note: Two notable exceptions are Creosote and Jojoba, both of which as it turns out are formidable chemical warriors. Creosote we’ve looked at previously. Jojoba distinct wax/oil makes its nuts indigestible to animals, with the notable exception of Bailey’s Pocket Mouse (pic right, not mine), which has evolved the ability to digest it.

So why all the anger? What is it about the Sonoran that makes it so exceptionally spiky? As I said earlier, I couldn’t get a clear easy answer. And as longtime readers know, whenever I can’t find an answer, I come up with a Half-Baked Theory. So here we go:

First off, there has to be a reason. Growing spines or thorns has a benefit and a cost. The benefit is that you may live longer, but the cost is that the energy and resources spent growing spines/thorns is energy/resources that could be spent growing something else- like flowers and seeds. (Think about a Dandelion: thorn-less, spine-less, good to eat, but it’s done great by growing lots of seeds.) In the Sonoran, some condition(s) has tipped the balance more toward the benefit side of spines/thorns than in other North American desert; things with thorns/spines have reproduced more successfully.

The Sonoran is dry, but it isn’t the driest desert North America. It is the hottest and the most botanically diverse desert in North America. (I should say the most consistently hottest; the Mojave can experience higher extremes, but the average temperatures in the Sonoran are greater.)

Side Note: Why it’s more botanically diverse is part of a broader, unsettled question of why biodiversity- in all biomes- tends to increase as one gets closer to the equator, and decreases the farther from it one gets. Quick trivia factoid: Mexico has 698 native cactus species. The US has only 191. Guess how many Canada has? First right answer = WatcherSTICKER*.

*What, you already have a WatcherSTICKER? Then you don’t get a prize, but have the consolation of being Way Cool.

When you walk across the open Sonoran Desert, the shrub spacing is greater- on average- than in the Great Basin or Mojave*, and as we saw last post, that spacing is determined by competition for water. So while not the driest, it’s not unreasonable to assume that the Sonoran routinely experiences the greatest competitive water stress, in part due to the heat and maybe(?) due to the (possibly related) greater botanical diversity as well.

*With the exception of obvious localized extremes, such as playas, mud-flats and other unusually barren and/or saline areas.

The competitive water stress, as we saw in the last post, determines to a large which plant grows in what spot. And when the stress is high, the number of possible “new spots” is reduced. The most vulnerable period of a plant’s life is early on, when it first germinates and starts growing as a young shoot. And when the number of potential “new spots” is so limited, the chances of any existing, established plant managing to reproduce in a given year is similarly reduced.

With a lower chance per year/season of successfully reproducing, the plants likeliest to successfully reproduce would be those who lived long enough to attempt reproduction many times over many seasons. So in such an environment, if you managed to germinate and get established, it would pay to stay alive and established for as many seasons as possible. Or in other words, in the poor reproductive environment created by intense competitive water stress, long life and modest fecundity pay off better than short life and high fecundity.

That’s my half-baked theory anyway. It was a great trip. Can’t wait to get back.