Artemisia (genus)

As of November 2020, Plants of the World Online accepted almost 470 species, including:[9]

Classification of Artemisia is difficult.[4] Divisions of Artemisia prior to 2000 into subgenera or sections have not been backed up by molecular data,[5] but much of the molecular data, as of 2006, are not especially strong.[4] The following identified groups do not include all the species in the genus.

Subgenera Artemisia and Absinthium, are sometimes, but not always, considered the same. Subgenus Artemisia (originally Abrotanum Besser) is characterized by a heterogamous flower head with female outer florets and hermaphrodite central florets, and a fertile, glabrous receptacle. Absinthium DC, though sometimes merged with subgenus Artemisia is characterized by heterogamous flower head with female outer florets and hermaphrodite central florets, and a fertile, hairy receptacle. Generally, previously proposed monotypic and non-monophyletic subgenera have been merged with the subgenus Artemesia due to molecular evidence. For example, in 2011 using ribosomal DNA analysis of their own and a review of molecular data (such as ITS sequence analysis) of others, S. Garcia and colleagues argued that it was logical to rename several Sphaeromeria and Picrothamnus (formerly designated sister genera to Artemisia) species as Artemisia, as well as to revert some Sphaeromeria species back to Artemisia, where they had been categorized previously.[12] Part of this was due to research by Watson and colleagues, who found that the four subgenera were not monophyletic except for Dracunculus, after analyzing and matching the internal transcribed spacers of nuclear ribosomal DNA from many Seriphidium and Artemisia species, and the related genera Arctanthemum and Dendranthema. The authors concluded that inflorescence morphology is not alone reliable for categorizing the genus or some subgenera, as qualities that previously demarcated them (such as homogamous, discoid, ray-less inflorescences) seemed to have undergone paralleled evolution up to seven times.[5] Picrothamnus Nutt. (“bud sage”), now considered Artemisia spinescens and Sphaeromeria Nutt. (“chicken sage”) are some examples, both endemic to North America.[12]

Section Tridentatae consists of eleven to thirteen species of coarse shrubs often known colloquially as "sagebrushes", which are very prominent parts of the flora in western North America.[13] In some classifications, they have previously been considered part of the genus or subgenus Seriphidium, although recent studies have contested this lineage to Old World species.[5] Tridentatae was first articulated as a section by Rydberg in 1916, and it was not until McArthur et al. in 1981 that Tridentatae was elevated to a separate subgenus from Seriphidium. The principal motive for their separation was geographical distribution, chemical makeup, and karyotype.[14] Much of the debate surrounding Tridentatae is phytogeographic, thus habitat and geography are frequently cited when understanding the evolution of this endemic North American subgenus. Evolutionary cycles of wet and dry climates encouraged “diploid and polyploid races which are morphologically similar if not indistinguishable” (McArthur 598).

Autopolyploidy among plants is not uncommon, however Tridentatae exhibits a remarkable amount of chromosomal differences at the population level, rather than the taxon level. This contributes to the difficulty in determining Tridentatae's phylogeny. The subgenus’ relative homogeneity within ploidies has enabled it to habitually hybridize and backcross, resulting in a high degree of genetic variation at the population level rather than the taxon level.[15] For instance, some articles suggest that to be monophyletic, section Tridentatae should exclude Artemisia bigelovii and Artemisia palmeri.[5][13] and include Artemisia pygmaea and Artemisia rigida.[16] These results were supported by extensive chloroplast DNA (cpDNA) and nrDNA sequencing which departed from prior morphological, anatomical, and behavioral data.

Traditional lineages within Tridentatae were proposed on the basis of leaf morphology, habitat preference, and the ability to leaf-sprout, among other morphological and behavioral characteristics.[13] For instance, sagebrush in the Artemisia tridentata lineage have tridentate leaves, live in especially arid habitats, and are unable to root-sprout.[13] This method of delimitation is problematic for species that do not fully adhere to the characteristics of a given lineage. The dry habitat and the presence of interxylary cork has often made the case for Tridentatae as a subgenus of its own,[17] and there is some ribosomal molecular evidence of a “Tridentatae core” group for the subgenus.[18] In 2011, Garcia and colleagues proposed enlarging Tridentatae and organized it into the sections Tridentatae, Nebulosae, and Filifoliae based on previous research establishing relationships via ribosomal and nuclear DNA.[19][12]

Intergrading forms are particularly common in recently radiated subgenera such as Tridentatae, given their frequent reversals and convergent evolution. Global reviews of Artemisia using ITS analysis support the hypothesis that Tridentatae has independent origins from Old World Seriphidium[5] These findings were compared with capitula morphology, challenging past assumptions based on floral characteristics. To better understand the rapid diversification and radiation relative to Old World Artemisia, a closer study of Beriginian or Arctic species may provide missing links.[20]

Section Tridentatae includes above species with exception of Artemisia longiloba, which is treated as a subspecies of Artemisia arbuscula. Section Nebulae includes Artemisia californica, Artemisia nesiotica, and Artemisia filifolia.[21]

The Old World species which different classifications put into the genus or subgenus Seriphidium consist of about 125 species native to Europe and temperate Asia, with the largest number of species in Central Asia.[22] Some classifications, such as that of the Flora of North America, exclude any New World plants from Seriphidium.[4] They are herbaceous plants or small shrubs.[22]

Seriphidium Besser was morphologically categorized by a homogamous flower head with all hermaphrodite florets and fertile and glabrous receptacle. Tridentatae was originally categorized as within Seriphidium due to floral, inflorescence, and leaf morphological similarities, until McArthur et al.’s analysis in 1981, which explained these similarities as convergent evolution.[23] Old World Seriphidium, with 125 species native to Europe and temperate Asia, was a previous classification of Seriphidium. North American or "New World" Seriphidium and Old World Seriphidium. North American Seriphidium were later placed into Tridentatae Rydb due to geographical distribution, growth habit, and karyotypic and chemotaxonomic similarities (such as presence of certain terpenols).[24]

One group which is well-supported by molecular data is subgenus Dracunculus. It consists of 80 species found in both North America and Eurasia,[4] of which the best-known is perhaps Artemisia dracunculus, the spice tarragon.

Dracunculus Besser. has historically been characterized morphologically by a heterogamous flower head with female outer florets and hermaphrodite central florets, but with a female-sterile, glabrous receptacle. Dracunculus is the most supported and resolved subgenus of Artemisia, which includes Artemisia dracunculus L., known as the cooking spice tarragon. Chloroplast and ribosomal DNA sequence analysis in 2011 supported monophyly with two clades, one of which includes some North American endemic species as well as most species of Europe and Asia, while the second clade includes just A. salsoloides and A. Tanaitica, found in Eastern Europe and Siberia to the Western Himalayas.[25] This study places Dracunculus as one of the more recent subgenera within Artemisia, situating A. Salisoides more basally on the tree, with North American endemic groups such as the sagebrushes having derived on the other end of a split from a common ancestor with Dracunculus. Formerly proposed genera Mausolea, Neopallasia and Turaniphytum are now argued to be within the subgenus Dracunculus due to ribosomal and chloroplast DNA evidence, with further species resolved as sister groups to Dracunculus due to phytochemical relationships.[25][26]

In North America, several species of Artemisia have become important parts of local environments, with wide adaptability. Artemisia papposa described by S.F.Blake & Arthur Cronquist can grow in the harsh, dry expanses of alkali flats, but also adapts to meadowlands.[40]

Sagebrushes like A. papposa (of the Tridenteae subgenus) in general are found in the north and southwest areas of the North American continent. In the Intermountain West, in a habitat known as Sagebrush Steppe, A. tridentata, A. tripartite, and A. arbuscula grow alongside various grasses and species of bitter bush, creating an important environment for mule deer, pygmy rabbits, antelopes, and the sage grouse.[41] Understanding the phylogenetic relationships among the sagebrushes has been helpful in understanding the relationships among these plants and their environments, as well as learning more about how these plants formed these communities over long stretches of time.[12] Sagebrushes, which include A. ludoviciana and A. Tridentata among others, can often also be found growing near junipers, particularly in the Elkhorn Mountain region, where the Juniper Woodlands form an ecosystem which provide cover for many animal species in both summer and winter months and storms. Because the habitat should burn only every 400–600 years,[42] with sagebrush shrubs living as long as 200 years (though potentially typically 88), this particular combination of Artemisia with other flora form an enduring habitat.[24] As it often goes, however, governments and farming businesses have often cleared sagebrush-juniper communities to create land for cow and domestic animal feedcrops, and Artemisia species may be declining due to this and invasive species such as cheatgrass.[43] Destabilization of the vegetation creates higher risk of fires, causing concern among the local conservation and wildlife groups.[41][43]

Due to their often extensive rhizome systems and other potential characteristics, however, some Artemisia species are often resilient to mowing or pulling, giving some species of Artemisia the ability to easily become invasive if introduced to comfortable, though non-native habitats. A. annua (native to Eurasia) is found in wetland habitats, and though it has been naturalized in much of North America it is considered weedy or invasive by some localities, such as Kentucky.[44] This is particularly true of Artemisia vulgaris, known as “common mugwort,” in North America, where it was introduced by European colonists and settlers in the 1600s, when Jesuit priests and other colonizers may have first brought the herb for ointments and teas and likely also let into port cities via ballast dumping.[45] Artemisia vulgaris will grow in dense groups and out-compete other plants in an area, in part due to its ability to grow on poorly enriched soils. Disturbed habitats, cities and roadsides or parking lots can easily become a field of A. vulgaris, which is the Artemisia species designated as invasive by New York State.[46]