A Tangled Past: An Uncertain Future
by Arch McCulloch
Most residents and visitors to the high desert consider themselves familiar with Joshua trees. Ubiquitous in some areas, they are arguably the signature species of the Mojave Desert. It may be surprising then, to review past and present uncertainties surrounding these trees.
Yucca brevifolia var brevifolia at JTNP on snowy day. The Western Joshua proposed for listing.
Photo by Nelda McCulloch
The Mojave is the ancestral homeland of several Native American tribes; all were familiar with the species, and had names for it. The Serrano of the southwestern Mojave called it hunuvat chiy'a, utilized its leaves for fiber and ate its blossoms. Spanish and American travelers in the region undoubtedly coined many names for it, ultimately bequeathing us its current common name. Explorer John C. Fremont first encountered it near Tehachapi Pass on his second expedition, and memorably described it as "the most repulsive tree in the Vegetable Kingdom."
Fremont was the first (in 1844) to collect and describe the tree scientifically, but the specimens collected were unfortunately damaged during a river crossing on the trip back to the United States. It was not until 1854 that new specimens were collected and formally described. Botanist John Torrey classified the tree as Yucca draconis var. arborescens, that is, as a new variety of a plant common in southeast coastal areas of North America. Over the next half century, the species was reclassified by several taxonomists, as Yucca brevifolia, Y. arborescens, Clistoyucca arborescens, C. brevifolia, and finally back to Y. brevifolia.
In 1932, Harvard botanist Susan McKelvey spoke with a Riverside Junior College instructor named Edmund Jaeger, who told her of Joshua trees in the northeastern Mojave that looked different (were morphologically different) from those in the southwest. Later, Jaeger told her the variety "reaches its greatest density in the vicinity of the New York Mts.” He was referring to the forest on Cima Dome. Encountering the trees northeast of Baker, McKelvey took measurements and collected specimens with shorter leaves and a more shrublike growth habit than the described species. In 1935, in a short account of her findings, she proposed a new variety of Joshua tree, designated as Y. brevifolia var. jaegeriana. Her view was accepted by other botanists at the time, and Munz's (1958) Southern California Flora listed both vars. brevifolia and jaegeriana.
In 1977, taxonomist James Reveal, a major contributor to the massive Intermountain Flora: Vascular Plants of the Intermountain West, while writing the Yucca section in volume 6, took issue with McKelvey's conclusions, noting that the original specimen in Torrey's 1854 collection also had short leaves, as did some specimens McKelvey collected outside of her proposed range of var. jaegeriana. He concluded, "it seems best to consider all of these elements as representing only one kind of plant, and call them Y. brevifolia. Those plants from the mountain ranges surrounding the Mojave Desert do have longer leaves . . . , and may deserve some formal recognition." Is this asserting that Jaeger and McKelvey's short leaved species should properly be called "Yucca brevifolia,” and the longer leaved southern and western population possibly recognized as a new variety?
From this point begins a divergence of opinion among taxonomists. The comprehensive Flora of North America North of Mexico agreed with Reveal's determination, as did Kew Gardens' World Checklist of Selected Plant Families (2011), both listing only Y. brevifolia, with no varieties. The 2nd edition of The Jepson Manual (2012), follows suit, but without explicitly citing Reveal.
Reveal's conclusion was not universally accepted, though. In 2001, Hochstatter proposed that jaegeriana be considered a subspecies. Lenz (2007) asserted that Y. brevifolia and Y. jaegeriana should be considered separate species, based mostly on differences in flower structure, and due to their different species of primary pollinators. More recently, the Joshua Tree Genome Project and others have analyzed genetic data that provide some evidence of differences at the species level.
As of today, both Kew's 2022 database and Jepson's current eFlora list var. jaegeriana as a synonym of Y. brevifolia, effectively opining that even varietal separation is questionable. On the other hand, correspondence with several desert botanists indicates a growing consensus that the two are different varieties, with disagreements remaining about separation at species level.
So what is the difference between a variety and a species?
The classical, mostly animal-based, Darwinian, definition of species is based on the organism's ability to successfully produce offspring when mating occurs. A population of organisms is considered to be of the same species if individuals of appropriate sexes can successfully breed and produce viable offspring ("viable" means the offspring must also be able to breed.) Subspecies are recognizably different groups within a species that are geographically isolated from each other. Varieties are recognizable groups that are not geographically isolated. (Varieties are only recognized in plants.)
The boundaries between species, subspecies, and varieties are now understood to be a great deal less distinct than the "classical" definitions convey. What happens when two geographically disjunct subspecies are brought together?
First, a slight digression. All evolutionary change is driven by mutations. Organisms within a population constantly collect random genetic mutations. Favorable mutations, including those that increase reproductive success, often spread over time within a population, because individuals carrying those mutations will have more offspring than individuals without. Mutations that decrease reproductive success will (either quickly or eventually) disappear. (Mutations that are neither favorable nor unfavorable can also spread within a population. The slow accumulation of such mutations in separated populations is called genetic drift.)
Species differentiation can occur when two populations of the same species are separated (say, by an ocean or a hot desert valley). Over time, each population varies through accumulation of genetic mutations, both from genetic drift and as it responds to the pressures of its environment. Since the environmental pressures and the mutations are different for each population, differences arise between the populations. If the differences are noticeable, (to botanists and possibly to other humans), the two populations may be referred to as separate subspecies.
If the populations then come back together (by removal of the barrier or by migration), hybridization can occur. If hybrids are at any disadvantage in reproductive capacity (that is, if hybrids produce fewer offspring than non-hybrids), then evolutionary pressure will favor the non-hybrids. The mechanics of this differentiation are reproductive isolating mechanisms. Structural or behavioral differences (caused by mutations) that make mating difficult or that prevent fertilization will be favored. Varieties with such differences cannot hybridize, and are therefore considered separate species. (Again, this is the Darwinian model.)
This process is still believed to be substantially correct. What has changed is an increased recognition that successful hybridization is far more common than was once believed, especially in plants. Oaks, for example, are known to hybridize extensively. Every oak in a grove is exposed to pollen from all surrounding oak trees, and it is to each tree's reproductive advantage not to be too choosy.
Joshua tree pollination is performed by moths. One of the best arguments that brevifolia and jaegeriana are separate species is that each has its own species of moth; brevifolia is pollinated by Tegeticula synthetica, and jaegeriana by T. antithetica. This would seem to be a perfect example of a reproductive isolating mechanism.
Both brevifolia and jaegeriana (and their associated moths) occur in the remote Tikaboo Valley in Nevada, so a natural experiment of sorts is taking place there. Will the two types of trees freely hybridize, or will reproductive isolating mechanisms kick in to further separate them into species?
As it turns out, T. antithetica (jaegeriana's pollinator) is physically unable to lay eggs in brevifolia flowers. This looks very much like it could be an isolating mechanism. However, T. synthetica (brevifolia’s natural pollinator) can successfully lay eggs in jaegeriana flowers, though not as successfully as in its preferred brevifolia flowers. So the isolating mechanism (if that's what it is) only operates in one direction. The Joshua Tree Genome Project's data indicate that brevifolia and jaegeriana do hybridize, though less commonly than if hybridization were unrestricted. So hybrids may be somehow disadvantaged, perhaps by being less attractive to either pollinator.
The "natural experiment” in Tikaboo Valley is, then, not just about the evolution of Joshua trees, but about co-evolution of the trees with their pollinator insects. The moths produce a new generation every year, but Joshua trees may take decades to reach sexual maturity, so observing changes that might resolve the trees' taxonomic status could take many years. Of course, natural experiments, by their very nature, have many variables. One variable in the Tikaboo experiment is climate change. It's unclear how the trees, or their pollinators, will react to warming temperatures. Will earlier spring warmth induce the trees to bloom earlier? If so, will it also cause the moths to emerge earlier? What happens if they don't?
Scientific research, especially in the field, can take a long time, and the results may reveal more new questions than answers. The answers we want can be elusive. Species are considered "rare" based on number of occurrences, and on immediate threats to their survival. For example, the California Native Plant Society generally considers species with less than about 80 occurrences to be “rare." This number is within guidelines used by the California Fish and Game Commission (CFG).
Regardless of whether there are one or two species of Joshua tree, there are many more than 80 occurrences. So it comes down to how we assess "immediate threats,” in this case, the existential threat of climate change, including the increase in destructive fires driven by climate change. It could take 50 years or more for climate change to reach catastrophic levels for Joshua trees. Of course, 50 years was sufficient to wipe out passenger pigeons, and to nearly extirpate the buffalo. But climate change equally threatens a huge number of species; arguably most desert species. Is it ethical to use rare plant legislation to choose one charismatic species for special protection? Is it ethical not to? Should other species that will gain protection from saving Joshua tree habitat also be considered? The answers seem obvious to those who believe catastrophe is coming, and who value the particular species. But questions like these, rather than tentative scientific conclusions, are the main criteria CFG will have to use to make their decision. The issues involved are ultimately more complex than the convoluted taxonomy of Joshua trees.
Are Joshua Trees Threatened?
Joshua trees are generally but not universally recognized to have western and eastern varieties. The western trees, such as those at Joshua Tree National Park (JTNP), have been proposed for a California State listing of "Threatened" (which is less serious than "Endangered"). The threat is climate change. Warming temperatures and related effects are believed to be the cause of low reproductive rates of trees in JTNP. Outside of JTNP, most of the western variety are in areas currently unprotected from development.
Yucca brevifolia var. jaegeriana on Cima Dome. Photo by Nelda McCulloch
The western variety was granted provisional protection while the issue is decided. In April, the California Department of Fish and Wildlife recommended against the species receiving Threatened status. Final determination will be made by the California Fish and Game Commission in June. The decision could hinge either on insufficient differences between eastern and western varieties, or (more likely) on the threat not being considered sufficiently imminent.
The eastern variety, such as those in Mojave National Preserve, while believed to be in better shape, are themselves threatened (as are the western trees) by fires driven by invasive grasses. Joshua trees (and many other desert plants) are not adapted to fire. The Dome Fire of August, 2020 destroyed about a quarter of the contiguous Joshua tree woodland (over 40,000 acres) on Cima Dome in the Preserve, killing 1.3 million trees.
I corresponded with a number of biologists in preparing this article, both to educate me in how evolutionary theory has changed in the nearly 50 years since my college coursework, and to provide details specific to Joshua trees. James Andre, Cameron Barrows (both of University of California, Riverside), Nick Jensen, Aaron Sims (both of California Native Plant Society), and Jeremy Yoder (California State University, Northridge) all patiently shared their time and answered my questions. Any errors in summarizing or synthesizing their views are mine alone
Arch McCulloch majored in Biology, but was seduced by the dark side and became a software and data security engineer. Now retired, he ponders life's mysteries from the shade of a mesquite. He is a Director of Morongo Basin Conservation Association, and is rumored to be reviving the Mojave Desert Chapter of California Native Plant Society.