The southeasternmost population of Joshua trees (Yucca brevifolia) in the USA is located off of US Highway 93, which connects Phoenix and Las Vegas. The best stands of these magnificent trees begin about 20 miles northwest of the town of Wickenburg and stop at the Santa Maria River drainage. In fact, US 93 has been officially named “The Joshua Tree Scenic Byway” by the state of Arizona due to the noteworthy density of this Dr. Seussian forest.

Distinct from the usual look of the species, these Joshua trees are characterized by fairly blue foliage and a highly offsetting habit whereby the trees form groves of as many as 50 stems spreading 40 to 50 feet across, all clonally reproduced from an original mother plant. This “Sonoran Blue” form of Yucca brevifolia may be separable into its own species if studies show that it is genetically distinct enough and reproductively isolated from the larger Mojave Desert populations that dominate elsewhere across the species’ range.

I believe that I once read about this subspecies being described under a different name from the Yucca brevifolia ssp jaegeriana one often applied to eastern Joshua tree populations, but I can’t recall what that name was. Whatever the taxonomy, there’s no doubt that it is a uniquely attractive plant. And note that the appellation “Sonoran Blue” is entirely colloquial and is placed in quotation marks for a reason – it is simply to recognize the distinctive appearance and growth habits, but does not necessarily reflect some sort of scientifically official status.

 

US Highway 93 wends its way across the rugged desert landscape in remote Yavapai County, Arizona in the middle of the image. Eventually this particular highway will be renamed Interstate 11, which is slated to be widened and completed circa 2035.

Here in this corridor of west-central Arizona, saguaro cacti (Carnegiea gigantea) grow sympatrically with Joshua trees in the southeastern portions of Joshua tree range. This biological pairing can only be seen in a narrow strip of Arizona, in Mohave and Yavapai Counties, and nowhere else on earth. These two species only co-occur in a discontinuous strip from Wickenburg to Wikieup to my hometown of Yucca. That diagonally-oriented strip is a total of maybe 100 miles long, and 10-20 miles wide, and it’s patchy and not evident all along the northwest/southeast distance covered.

This is actually a rather rare ecological coupling – usually it’s only one species or the other across their respective ranges. What makes it noteworthy is that saguaros and Joshua trees are the two largest native succulent plants in the USA. They both have a great deal of charisma, they are both used as primary indicators of the ecological boundaries of their respective deserts (Sonoran and Mojave), and they both have National Park units created to honor and protect them. That is why to have them growing together is important and valuable.

This photo shows the highly clonal nature of Sonoran Joshua trees. This is not Mojave Desert anymore, as indicated by the ecological prominence of classically Sonoran plants such as saguaros, palo verde trees, and palo cristi trees in the background. Farther north and west from here, the species composition starts to become more and more reflective of the Mojave Desert, until it entirely grades over to it with no truly Sonoran species left.

This photo depicts a particularly vigorous clumping stand of Joshua trees. Spanning more than 40 feet across and containing upwards of 40 individual stems, this grove of clonally-reproducing Joshuas must easily be several centuries old if not more.

A mixed-age clonal stand of “Sonoran Blue” Joshua trees. Obviously the largest trees are the oldest, and are the progenitors of the short single-stemmed offsets. Who knows how long ago the original seed that started this colony germinated, though?

Despite being so clonal, these “Sonoran Blue” plants are also capable of seed reproduction, and indeed there are individuals that are clearly seed-grown, being located in isolation from the next nearest tree(s). Pods with large numbers of viable black seeds are set abundantly in some benevolent years as well, so the entire 20-mile-long population along US Hwy 93 is genetically varied. And indeed some of the Joshuas here are singles even as adults, not being clonal whatsoever. But the large majority of the plants in this population start offsetting early and abundantly, and these are the most prevalently clonal populations I know of across a huge area, one that contains tens of thousands of plants. To be clear, there are clonal individuals in just about every Joshua tree stand across their far-flung range in four states, but in the central and western Mojave Desert of California they are infrequent and comprise maybe only 3% to 5% of the overall group. Here in Arizona’s eastern Mojave/northwestern Sonoran interface region, the clonal proportion is closer to 75%. (This is a non-scientific estimate, I admit.)

Other plants that grow with the Joshua tree/saguaro combination include buckhorn chollas (Cylindropuntia acanthocarpa) and yellow-flowering foothills palo verde trees (Parkinsonia microphylla).

There is a pattern to clonal Joshua tree distribution in western, central, and northeastern Mojave Desert Joshua trees that are primarily single in nature otherwise: Clonal individuals are most frequently located at the upper elevations and outermost limits of their natural desert ranges, where they often merge with chaparral and pinon-juniper (P-J) vegetation zones. Since these latter two biomes are quite fire-prone, Joshua trees that grow nearby would face a strong selective pressure to be clonal and capable of crown-sprouting after a fire. Individuals that cannot crown-sprout will be eliminated from the ecosystem when fires occur.

Western and central Mojave Desert Joshua trees that are located in true, arid desert that is typically too sparse to burn for lack of fuel are most usually single, and seed-grown, away from the edges where they intergrade with chaparral or P-J forest.  There are notably clonal populations of Joshuas in borderline habitats in southwestern Utah, southeastern Nevada, and the eastern San Bernardino Mountains of California, to name a few places. These strongly clonal populations grow mostly in a desert/chaparral or desert/P-J forest interface. Meanwhile, only a few miles away and lower down the slopes where it dries out and vegetative density diminishes, most Joshua trees have become single and reliant upon seed reproduction. This is despite the proximity of clonal plants so relatively nearby.

The long, glaucous blue leaves of “Sonoran Blue” Joshuas are quite different from the shorter, greenish-yellow or medium-green of most Mojave Desert Joshuas. Many leaves here are a foot long, while they are mostly 4 to 6 inches long on most other Joshua populations. Since the Latin name of Yucca brevifolia actually means “short-leaved yucca”, this name is starting to become borderline inaccurate. 😉

The glaucous bluish leaf color of this extensive Joshua Forest Parkway stand of Joshuas is genetic, but it was probably selected for over time to protect against the Sonoran Desert sun. Many other yuccas also have glaucous blue leaves caused by a wax layer designed to retard water loss via evapotranspiration. But why don’t Joshua trees in the equally hot and sunny and even drier western Mojave Desert also have bluish leaves? Truth be told, I don’t know. Soil nutrition or climate probably has nothing to do with the blue coloration since these plants retain it when transplanted to different places. Plus their seedlings coming up in various climates and soil types also are blue, meaning it is genetically determined, not environmental in nature.

Despite the predominance of clonal Joshua trees in the southeastern part of their range, there are some individuals that are single, or maybe produce only one or two offsets, as opposed to expansive groves.

As part of the fire discussion and part of its relationship to encouraging cloning habits in Joshua trees periodically exposed to flames, I want to mention a well-known clonal population that used to exist in the far western Mojave Desert. This was what I and others have called the “Grapevine Population”, which grew near the junction of Interstate Highway 5 and California Highway 138 north of Los Angeles and south of the San Joaquin Valley above the town of Grapevine along the Tejon Pass corridor. It was a very unusual population of Joshua trees, nearly all single-trunked, and growing in groves numbering in the dozens or even as many as 100 stems. Most were quite uniform in height and few had any branches whatsoever. (Other similar-looking populations of Joshua tree either occur or have historically occurred along California Hwy 58 between the towns of Mojave and Tehachapi, and also along California Hwy 178 west of Ridgecrest on the way to Lake Isabella.)

Sadly, these unique stands of single-stemmed Joshua trees along the Grapevine Freeway no longer exist. The population was eliminated by fire. Ironically, fires not only originally created these stands via natural selection, but eventually also eliminated them. Why is this?

My guess is that this extinction was done by more than one fire, and that these fires occurred too closely together in terms of timing. Joshua trees are relatively slow growing, even if they can crown sprout, and if fires are at too frequent an interval – less than 15-20 years apart minimum – then the second fire could easily kill off the nascent resprouting Joshua trees. They only have so much of a starch and sugar reserve in their underground stolons, and they are digging deep into it when in recovery after a fire (or any other event that killed the top growth.) If burned a second time only a few years after the first fire, they probably won’t have the energy reserves to recover a third time. Thus they will die. 🙁  Apparently the same fate has befallen the Hwy 178 Ridgecrest/Lake Isabella Joshua trees, although I haven’t actually seen that population myself to be sure. At least the Mojave/Tehachapi groves still stand. For now.

Chaparral and P-J forest are not supposed to burn too often. And historically, pre-humanity, they did not. Original fire frequency before people arrived on the scene was sparse, perhaps only once every 30 to 50 years, or possibly even longer in some cases. But people changed that, starting with Native Americans, and accelerating with Europeans. Now many fire-tolerant ecosystems that used to burn infrequently (and by necessity) burn much more often, in some cases once every few years, which is very detrimental to numerous otherwise fire-resistant species.

This overly-frequent burn cycle is especially damaging to those plants entirely reliant upon seeds, and many species of chaparral plants are exterminated by fires spaced too close together. If subadult plants are killed before they can set a meaningfully large seed crop to replenish the soil seed bank, then they will be removed from the ecosystem in only two, or maybe three fire cycles if they are spaced too tightly. A toxic combination of invasive annual weeds, and human activities that ignite fires, are probably the primary reason for why overly-frequent wildfires are harming ecosystems everywhere. As stated, fire is good for chaparral (and pinon-juniper) ecosystems at relatively rare, widely-spaced intervals – not every 3 to 7 years as seems to occur in so many places nowadays. I suspect that the invasive weed/human ignition combo is what caused fires only a few years apart for both the Grapevine and Ridgecrest Joshua trees, and that was mortal for the Joshua trees who were unable to recover from a second burning that followed the first far too closely.

Weedy invasive species in chaparral and other semiarid biomes are usually what both start the excessively common fire regime, and are what eventually end up replacing the native ecosystem as well. Invasive annual weeds tolerate fires extremely well in the form of seeds that survive frequent scorching protected by layers of soil, while many long-lived and slow-growing woody plants cannot. Basically, regular fires favor rapid and wholesale conversion of native aridland ecosystems to a much impoverished weedy savanna at the expense of the diverse woodland or shrub community that used to exist there. We see it all over the world: Diverse native ecosystems are ruined by fires that occur too regularly, in some parts of the world burning virtually annually. Weeds and human-caused fires are a toxic ecological combination. Joshua trees, even the clonal ones, have suffered from this tragic dynamic.

The “Sonoran Blue” Joshua trees and associated cactus and succulent species are relatively safe for now.

Deserts do not usually burn, because ironically they are too dry to do so. What I mean is that normal desert vegetation in both the intact Sonoran and Mojave ecosystems is too sparse to carry fire, at least if no invasive weeds are present in dangerous, fire-transporting densities. As a result, most succulents evolved in the absence of burning, and show little resistance to even small fires. The thatch of dead leaves in the crowns of Joshua trees make them extremely prone to vigorous burning, and nearly 100% of Joshua trees die when set aflame. Meanwhile cacti have their spines singed off, which both gives third degree burns to the photosynthetic green skin and denudes the plants of their physical armature that protects them from threats such as being eaten or sunburn in summertime. Many burned cacti simply rot and decay from the trauma of their burn wounds within a few months to a year after the fire passes. They simply have developed no real resistance to flames in an ecosystem that should never burn.

Overall, fires are quite detrimental to succulents of most sorts. Their populations become seriously reduced in both overall abundance as well as individual vigor of survivors upon being burnt. There are a handful of exceptions, but on balance fire damages succulent populations terribly and the plants tend to survive mainly in places where fires cannot reach, like extremely rocky hills, outcrops, and cliff faces.

The fires of 2005-2007 in the northeastern Mojave Desert of southern Nevada and southwestern Utah are a good example of this. There was once an extensive and beautiful woodland of Joshua trees and a dozen other succulent species that grew across wide expanses of northeastern Mojave Desert up until the mid-2000s. But invasive weed growth spurred by several abnormally wet winters resulted in fires that burned nearly a million acres in May and June 2005. More fires occurred in 2006 and 2007, finishing off what survived 2005. Millions of Joshua trees and other succulents burned, with only a handful or survivors that randomly escaped the flames. The dense, scenic, and biologically diverse yucca forest that once existed there is nearly 90% gone, never to return. I loved that area the way it was in the late 1990s and early 2000s and I still mourn its passing from this earth. I’ve gone back to see it in the past 15 years and it is simply wrecked. It’s never coming back. 🙁

I’ll close this discussion by posing the following question: Does the “Sonoran Blue” Joshua tree forest, and the associated saguaros and other fascinating native plant community, face a threat from fires? As near as I can tell, I think not for the time being. The region is not adjacent to any fire-prone ecosystems like the aforementioned chaparral or P-J forest zones, and is true Arizona Uplands Sonoran Desert with a full array of expected species along with a few surprises. There are invasive weeds present within the Joshua tree/saguaro woodland, but they appear to be kept in check by active grazing from cattle. Ironically, it is probably cattle ranching that originally brought in and spread the invasive weeds in the first place, albeit accidentally; but it appears that cows are suppressing the weed growth that might otherwise threaten the forest with unaccustomed fires.

I don’t know why in the absence of fires the “Sonoran Blue” Joshua trees are as clonal as they are. It is admittedly a significant break in pattern from the rest of the Mojave Desert range aside from the limited fringes of chaparral and P-J forest intergrading that happens elsewhere. I don’t know if there is some survival advantage to being clonal in the Arizona climate, which varies somewhat but not drastically from the California and Nevada climates. It could also be simply be that genetic isolation and geographical separation is taking the “Sonoran Blue” Joshua trees in a different ecological direction for no particular reason at all. Whatever the case may be, let’s hope that as long as we can keep excessive invasive weeds and terrible fires out of the Sonoran Desert, these beautiful blue yucca trees will survive long into the future.

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Addendum: For some interesting information on Joshua tree research and conservation effort via a combination of ecological work and laboratory techniques, you can visit this website – https://joshuatreegenome.org/