20. Quercus muehlenbergii Engelm. (chinkapin oak)

Quercus prinoides Willd. var. acuminata (Michx.) Gleason

Pl. 416 d, e; Map 1858

Plants trees (sometimes shrubby in exposed, rocky habitats) to 35 m tall. Bark light ashy gray, divided into loose plates, blocks or strips. Twigs 1.5–3.0 mm wide, brown or occasionally grayish brown, glabrous or with scattered, spreading hairs when young. Buds 3–5 mm long, reddish brown or grayish, the scales minutely pubescent or nearly glabrous, short-hairy along the margins. Petioles 14–33 mm long. Leaf blade 12–22 cm long, 5.0–14.5 cm wide, relatively thin and flexible, broadly obtuse, rounded, or truncate at the base; divided 10–30% of the width, the coarse scallops (8–)10–14 per side, equal or slightly shallower above midpoint; well-developed scallops rounded to acute, undivided; secondary veins 10–15 per side, each (except the basalmost) reaching the margin at the tip of a scallop; the upper surface shiny to rather dull, glabrous or nearly so (with a few inconspicuous, 3–6-rayed hairs), the undersurface white or sometimes pale green, with mostly 5–12-rayed, appressed, stellate hairs and sometimes also 2–6-rayed, spreading hairs, sometimes felty to the touch. Acorns sessile or the stalk to 8 mm long, the cup 5–12 mm long, 9–18 mm wide, covering 40–50% of the nut, bowl-shaped, the outer surface with the scales 2.0–2.5 mm long, those near the cup margin not differentiated. Nuts 11–19 mm long, 11–13 mm wide, ovoid or ellipsoidal. 2n=24. April–May.

Common nearly throughout the state (eastern U.S. west to Wisconsin, Nebraska, and New Mexico; Canada, Mexico). Mesic to dry upland forests, savannas, sand savannas, upland prairies, loess hill prairies, glades, banks of streams, margins of lakes; also pastures, railroads and roadsides, often on calcareous substrates.

This species is very similar to Q. prinoides, and plants of Q. muehlenbergii that are immature or dwarfed by growing in dry sites with shallow soil can resemble that species very closely. Quercus prinoides spreads by underground runners to form large, clonal colonies, and the shrubs may flower and fruit when they are only 3–5 years old and less than 0.5 m tall (Nixon and Muller, 1997), but Q. muehlenbergii usually is slower to flower. The two species hybridize when they come into contact with one another, and extensive intermediate populations have been reported from other states, but it is hard to assess the degree of hybridization accurately because the parental species are so similar morphologically. The unusual secondary hybrid between Q. bicolor and the Q. muehlenbergii/prinoides hybrid (Q. ×introgressa) was described from a small colony of trees in a pasture in Lafayette County using morphometric data, and the presence of three individuals of Q. muehlenbergii × Q. prinoides was mentioned, but apparently voucher specimens of these were not preserved (Thomson, 1977). Quercus muehlenbergii has a much wider range than does Q. prinoides, both in Missouri and outside the state. In the northeastern United States the two species apparently are well separated ecologically, with Q. prinoides on acidic bedrock and Q. muehlenbergii on limestone. The ecological separation seems less sharp in Missouri (perhaps because so much dolomitic limestone with a high chert content is found in the state). Hybridization seems to be commoner in areas with intermediate geology, as with many other oaks (Muller, 1952). The only other hybrids involving Q. muehlenbergii that have been documented from Missouri are those with Q. alba and Q. macrocarpa.

Plants having unusually broad leaves have been called f. alexanderi (Britton) Trel. (Q. prinoides f. alexanderi (Britton) Steyerm.).

M. Smith and Parker (2005) reported Q. montana Willd. (rock chestnut oak) from four sites in Wayne County. This species, which is very similar to Q. muehlenbergii morphologically, is widespread in the eastern U.S., west to the bluffs and upland woods immediately east of the Mississippi River from Union County, Illinois south to northern Mississippi. It will key to Q. muehlenbergii in the keys above, but differs in its bark, which is deeply divided into thick, persistent ridges; the stellate hairs of the leaf undersurface, which are always sparse and have only 2–4 rays (or sometimes more on hairs close to the midrib); and its large acorns with the usually hemispherical cup 9–15 mm deep and the ellipsoid nut 15–30 mm long. Regrettably, repeated subsequent searches for these plants uncovered only juvenile plants of Q. muehlenbergii and the incomplete specimens documenting the supposed Missouri populations cannot be matched to Q. montana unequivocally. Thus, although this species may be confirmed as a member of the Missouri flora in the future, for the present it has been excluded from the flora.