Quercus robusta

Trees , deciduous, to 13 m. Bark brown or black, roughly furrowed. Twigs dark reddish brown, 1.5-2.5 mm diam., densely pubescent or glabrate. Terminal buds glossy light brown, acutely ovoid, 4-7 mm, glabrous or pubescent on apical 1/2. Leaves: petiole 5-20 mm, pubescent or glabrate. Leaf blade acutely ovate to elliptic, widest at or proximal to middle, 55-120 × 20-50 mm, base cuneate to rounded or subcordate, margins with 6-8 teeth or shallow lobes with rounded sinuses, rarely entire, 1-10 awns, apex acute or attenuate; surfaces abaxially glabrous except for small axillary tufts of tomentum or pubescent along midrib, adaxially glabrous or persistently pubescent near base and along midrib. Acorns biennial; cup deeply cup-shaped, 6-9 mm high × 8-12 mm wide, covering 1/4-1/3 nut, outer surface puberulent or glabrate, inner surface uniformly pubescent, scales tightly appressed, acute or attenuate; nut oblong to broadly ellipsoid, 10-22 × 7-10 mm, glabrate, scar diam. 3.5-4.5 mm.

Flowering spring. Moist wooded canyons in Chisos Mountains; of conservation concern; 1500 m; Tex.

After describing this species, C. H. Muller later (1951, 1970) concluded that it represented a hybrid between Quercus emoryi and Q . gravesii . The extreme forms (e.g., the type specimens) of Q . robusta and Q . gravesii are easily differentiated, but these two taxa appear to occupy the ends of a morphologic continuum. Muller's recent view, however, is that Q . robusta deserves species status. The origin of this taxon is still worthy of study.