Annuals, biennials, perennials, subshrubs, shrubs, or trees. Leaves usually cauline, sometimes mostly basal; alternate [opposite, whorled]; usually petiolate, sometimes sessile; margins entire or dentate to pinnatifid (often gland-dotted, especially in xerophytes). Heads homogamous (discoid; unisexual in Baccharis) or heterogamous (disciform or radiate), usually in corymbiform, paniculiform, racemiform, or spiciform arrays, sometimes borne singly. Calyculi 0. Phyllaries persistent or falling, usually in 3–5+ series (usually in spirals, sometimes in vertical ranks), distinct, unequal, and herbaceous to chartaceous or scarious or margins and/or apices notably scarious, sometimes in 1–2 series, distinct, subequal, and herbaceous with margins and/or apices barely to notably scarious. Receptacles usually flat to conic, sometimes concave, usually epaleate, rarely paleate (often foveolate, socket margins often fimbrillate; paleate in Eastwoodia and Rigiopappus). Ray florets 0 or in 1(–2+) series, usually pistillate, rarely neuter or styliferous and sterile; corollas usually yellow, cyanic, or white (laminae sometimes very reduced, e.g., Conyza spp.). Peripheral (pistillate) florets 0 or (in disciform heads) in 1–3+ series; corollas usually present, usually yellow, sometimes white, ochroleucous, or reddish to cyanic. Disc (inner) florets usually bisexual and fertile, rarely functionally staminate (e.g., Benitoa); corollas usually yellow, sometimes white, ochroleucous, or cyanic, actinomorphic, not 2-lipped, lobes (4–)5; anther bases obtuse or rounded, not tailed, apical appendages usually triangular to linear, sometimes none; styles abaxially glabrous and smooth or papillate (distally), branches ± linear, adaxially stigmatic in 2 lines from bases to apices or appendages, appendages usually deltate to lanceolate (abaxially papillate to hispidulous, adaxially smooth, glabrous). Cypselae monomorphic or dimorphic within heads, usually ± columnar to prismatic and 5-ribbed, sometimes compressed and 2-ribbed, rarely beaked, bodies smooth, muricate, ribbed, or rugulose (glabrous or hairy, hairs often glandular); pappi (rarely 0) usually persistent, usually of barbellulate to barbellate bristles in (1–)2–3+ series, sometimes of scales (scales sometimes aristate), rarely of both scales and bristles or of awns.

Genera 170, species 2800+ (77 genera, 719 species in the flora): almost worldwide, mostly temperate.

The classification and phylogeny of tribe Astereae has been the object of recent work, both morphologic (K. Bremer 1994; G. L. Nesom 1994, 2000) and molecular (notably the seminal paper by R. D. Noyes and L. H. Rieseberg 1999). Noyes and Rieseberg showed that most genera present in North America belong to a single monophyletic clade, called the North American clade. A series of subsequent studies done in various generic groups or subtribes led to a redefinition of many genera (see literature cited in each genus) and to the transfer of species between genera. The current treatment of the tribe reflects much of these novelties, many presented in a floristic work for the first time. Nesom and H. Robinson (unpubl.) present a worldwide overview of the classification of the tribe.

Large genera such as Erigeron, Symphyotrichum, and Solidago all originated on the continent and subsequently spread to Eurasia or South America. A few genera entered North America from neighboring continents, such as Aster in the strict sense (from Eurasia) or Baccharis (from Central America). A majority of Astereae genera in the flora of North America are endemic to the continent (more so if Mexico were included).

The generic order of the present treatment attempts to reflect the phylogenetic relationships established in recent molecular phylogenetic work.

SELECTED REFERENCES

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