81. Quercus engelmannii Greene, Ill. W. Amer. Oaks. 1: 33, plate 17. 1889.
Trees , subevergreen, to 10 m. Bark gray or whitish, closely furrowed. Twigs light brown, 1-1.5 mm diam., densely or sparsely stellate-tomentose, soon glabrate. Buds reddish brown, subspheric to broadly ovoid, 1-2 mm, glabrous or basal scales pubescent; stipules persistent about terminal buds. Leaves: petiole (2-)3-4(-6) mm. Leaf blade oblong to elliptic, occasionally lanceolate or ovate, (20-)30-60(-80) × (5-)10-20(-25) mm, base cuneate to cordate, margins entire, undulate, sometimes irregularly toothed, especially toward apex, secondary veins 7-8(-10) on each side, branched, apex acute or broadly rounded; surfaces abaxially blue-green or pale green, densely and loosely glandular-tomentose, quickly glabrate or persistently floccose, especially about base of midrib, at maturity strongly glaucous, adaxially gray-green or pale green, bluish green or glaucous. Acorns solitary or paired, subsessile or on peduncle to 5-6 mm; cup cup-shaped or shallowly cup-shaped, 8-l0 mm deep × 10-15 mm wide, enclosing 1/3 nut, scales 1.5-3 mm wide, strongly and regularly tuberculate near base of cup, gray-pubescent; nut light brown, ovoid or oblong, 15-25 × 12-14 mm, glabrate or puberulent about apex. Cotyledons connate. 2 n = 24.
Flowering in spring. Oak woodlands, margins of chaparral, arroyos, slopes and bajadas; 50-1200 m; Calif.; Mexico (Baja California).
Quercus engelmannii is closely related to and possibly conspecific with Q . oblongifolia . The cups of Q . engelmannii are larger, deeper, and generally more tuberculate than those of Q . oblongifolia , and the scales are usually larger. Based on available samples, the nuts of Q . engelmannii are consistently larger than those of Q . oblongifolia , apparently with little, if any, overlap in diameter. Considerably more variation occurs within Q . engelmannii in leaf form, possibly reflecting introgression from other white oak species such as Q . cornelius-mulleri , Q . dumosa , and Q . durata (see treatment).
On Catalina Island, Quercus engelmannii is known only from a small grove of trees. Putative hybrids between Q . engelmannii and Q . cornelius-mulleri are common in areas of contact between the two species in Riverside and San Diego counties in southern California. Such a population was the basis for Q . acutidens Torrey [ Q . dumosa var. acutidens (Torrey) Wenzig]. Other names applied to those populations are Q . macdonaldii var. elegantula Greene and Q . dumosa var. elegantula (Greene) Jepson. Variable in leaf form and stature, those intermediates form extensive populations and are probably best disposed of under the name Q . × acutidens .