4. Frangula betulifolia (Greene) Grubov, Trudy Bot. Inst. Akad. Nauk S.S.S.R., Ser. 1, Fl. Sist. Vyssh. Rast. 8: 268. 1949.
Birchleaf buckthorn Birchleaf buckthorn
Rhamnus betulifolia Greene, Pittonia 3: 16. 1896 (as betulaefolia)
Shrubs or small trees, 1–4 m. Stems brown to gray-brown, glabrous or pubescent. Leaves deciduous; petiole (2–)5–16 mm; blade yellowish green abaxially, green adaxially, elliptic to oblong, elliptic-ovate or narrowly ovate, (4–)4.5–10 x (2–)2.5–5.5 cm, 1.6–2.6(–2.9) times longer than wide, ± herbaceous, base obtuse to truncate or rounded, margins serrate to subcrenate, apex usually acute to obtuse, sometimes slightly acuminate, both surfaces hirtellous, glabrescent; secondary veins (8–)9–13 pairs. Inflorescences umbels, pedunculate, 2–20(–38)-flowered. Pedicels 3–7 mm. Stigmas 3-lobed. Drupes black, globose, 5–10 mm; stones (2–)3(–4).
Flowering Apr–Jun. Moist canyons, stream banks, rocky slopes, cliff bases, ledges, ridges, roadsides, deciduous, coniferous, and mixed woodlands; 900–2800 m; Ariz., N.Mex., Tex.; Mexico (Chihuahua, Coahuila, Durango, Nuevo León, Sonora, Tamaulipas).
In the flora area, Frangula betulifolia is found in southeastern Arizona, the southern two-thirds of New Mexico, and trans-Pecos Texas. It and F. obovata are allopatric and morphologically distinct. C. B. Wolf (1938) considered a collection from Cochise County in southeastern Arizona, named as Rhamnus blumeri (Frangula ×blumeri), to be a hybrid between Frangula betulifolia and F. californica var. ursina; see the discussion of the latter taxon for more information.