Letter From The Desert: Vegetative Self-Awareness

An unobtrusive plant possesses a startling intelligence.

Apparently this is my year to think about bursage. I never have before, despite decades of being surrounded by it. Here’s why:

Ambrosia dumosa, shown here in a 2011 photo by Larry Blakely, is not what one would call charismatic. It’s leafless much of the time. What leaves it does bear are not particularly remarkable unless you look up close, and why would you? When you do, those leaves sometimes resemble those of dandelions, only rounder at the edges, covered in a grayish felt, and much, much smaller. Or like moose antlers, if the moose in question were two inches tall and able to photosynthesize.

That’s by Larry Blakely, again.

But this attractive leaf anatomy only shows up close. At typical observing height, with eyes say three to six feet off the ground, and with plenty of other more interesting things to see in the neighborhood besides, and you’re probably looking the other direction anyway and thinking about dinner or your boss or taxes, bursage does not command one’s attention.

Except, that is, when it blooms, at which point the species’ membership in the ragweed clan of aster family blooming plants becomes rather notable indeed. Ragweed pollen happens to be one of the few kinds of spring wind-driven pollen that I am not particularly allergic to — grasses are more my thing — but for those who are sensitive, bursage pollen can be a miserable addition to the neighborhood. Along with its closest relative in the Mojave, Ambrosia salsola a.k.a. cheesebush, bursage can make entire counties unhappy for three weeks out of the year, if conditions are right. And then the plant grows spiny little spherical fruit that make your dog unhappy when she steps on them, though bursage is just as happy to reproduce by cloning, sending off root shoots that eventually grow their own plants.

What bursage shares with its cheesy cousin, aside from noxious pollen, is an ability to colonize damaged desert landscapes well before other shrubs, then provide shelter for the tiny seedlings of those other shrubs. Bulldoze an acre of the Mojave and then leave, and within a decade cheesebushes a couple feet high will dot the area, with bursage clumps only a little less common. After a century, those pioneering cheesebushes will be long dead, but the original bursage clumps might well have survived to become clonal rings, and the creosotes and chollas and ephedras and sennas and galleta grasses and yuccas and Psorothamnuses that were lucky enough to germinate in the shelter of that first couple decades’ worth of colonizing ragweed might just be starting to look like a real desert again. And while bursage doesn’t seem to be eaten by any big animals other than imported cattle, it does support a remarkable diversity of insect browsers, many of which are ragweed specialists. And those insects are then consumed by other animals, which then in turn etc., and it is of relationships such as this that ecosystems are repaired.

Bursage is also a host plant for sandfood and its close parasitic cousins in genus Pholisma, so there’s that. Just one degree of trophic separation between bursage and hungry people.

Also, bursage is self-aware. Did I mention that yet?

A bursage seed hitches a ride from its parent plant on the coat of an animal using its eponymous burs. It lands in a likely spot, a bit of bare soil with a good mix of gravel and sand, and then waits. After a few cold, damp weeks in winter, the bursage sprouts with the spring’s growing warmth, sending up a rosette of fuzzy leaves.

But the real action is going on underground. Bursage roots lengthen cell by cell, about an inch every three days, casting a fine, thirsty net through the nearby desert soil. Eventually, later in the year, during summer monsoons or the first warm rains of fall, those roots will partner with mycorrhizal fungi, just like probably 80 percent the other plants in the desert do. But for now they just grow, seeking out moisture to keep the aboveground parts of the plant alive.

Eventually, those roots will encounter other roots. Competition! There may be a creosote nearby, or a mesquite, or some other plant whose roots are after the same water the bursage seedling wants. The bursage root keeps growing in an attempt to outcompete the other plant roots.

But if there are other bursages in the area, which is quite likely because let’s face it, you never get just one bursage seed stuck to your pant leg, so that furry animal probably dropped a dozen seeds in the vicinity, the plant adopts a different strategy. When bursage root encounters bursage root, each slows its growth, refraining from interloping into the other plant’s personal space.

That’s a sensible enough trait: it ensures that bursages don’t compete with each other for moisture. But it gets more complicated. Here’s the study if you want more info, but the elevator version: Bursages from the same populations seem to abide by the “no root encroachment” policy, whether they’re seedlings or clones. Bursages from geographically separated populations, if grown together, are less polite: A bursage from Tucson might as well be a tomato plant for all that a Joshua Tree bursage is concerned. Neither will slow its root growth to help the other.

And if roots from the same plant meet, they grow unrestrainedly.

It all makes sense from an evolutionary standpoint. Plants in the same population are more likely to share the same genes, so avoiding competition increases the possibility that your own genes will survive somehow. There’s no point in avoiding competition with your own roots, because whether one root or the other wins you get the water.

Here’s what broke my brain: roots from genetically identical clones will avoid competing, while roots from the same plant don’t. Which means the plant knows when it’s encountering itself.

How does it know?

I’ve spent the last few hours trying to puzzle out, without success, how a plant root could distinguish between another root to which it is connected and one to which it is not connected but is genetically identical, and I keep coming back to this: the plant is aware of itself. In us animals, a complex system of feedback in our nervous and musculoskeletal systems allows us to know more or less where our body parts are in relation to each other. It’s called proprioception or kinaesthesia, and it is why you can usually drink hot coffee blindfolded without major injury.

But plants don’t have nerves.

Self-awareness is a complicated thing, and a big problem in defining it is that we do so using it, which is like trying to drive a nail using the same nail as a hammer. Whatever kind of self-awareness a bursage has, it’s almost certainly a mistake to assume we know what it feels like. We might call it an unconscious self-awareness, but consciousness is what vertebrates do, and we’re probably being chauvinistic when we define consciousness as the be-all and end-all of self-awareness.

But at some deep sensory level, a bursage is aware of itself, how its roots are shaped and where they are. And it is aware at some level that it has family nearby.

And there’s this: scientists have only just started to look at what happens beneath the soil in the desert. Bursage probably doesn’t have a monopoly on underground self-awareness here.

But it’s a self-aware desert dweller that’s really good at repairing damage to the desert landscape, so someone probably ought to put it on a t-shirt.