Quercus opaca is a species of evergreen shrub in the genus Quercus (oak), first described by William Trelease in 1924. It is native to northeastern and central Mexico.
Description
Quercus opaca typically does not exceed 2 meters in height, with an average height of around 1–1.5 meters. Its leaves are evergreen, oblong to lanceolate or elliptic in shape, measuring approximately 2.5 to 4.5 cm in length and 1 to 1.5 cm in width. The leaf apex may be acute, obtuse, or rounded, often with a mucronate tip. The leaf base is typically obtuse to cordate, sometimes rounded. The leaf margin is thickened and may appear flat or slightly rolled back; most leaves are entire, though some may possess 1–3 pairs of sharp mucronate teeth near the apex. The upper (adaxial) leaf surface is glaucous and dull, with small star-shaped hairs at the base of the midvein. The lower (abaxial) surface is also glaucous and not lustrous, with a notably whitish papillose epidermis. Leaf veins consist of 6–10 pairs that are flat above and slightly raised beneath. The petiole is 3–7 mm long, pubescent, and reddish-brown in color.
The male catkins are 3–4 cm long and contain many flowers. The fruit is an ovoid acorn approximately 9 mm long, glabrous, partly enclosed by a half-rounded, slightly tomentose cupule. Acorns mature within the first year, typically in September, and appear solitary or in pairs on a glabrescent peduncle about 0.5 cm long.
The bark is grey and scaly. Twigs are 1.5–2 mm in diameter, sulcate, waxy, and initially covered in short pubescence of sessile multiradiate trichomes and glandular hairs, becoming glabrescent over time. Buds are subglobose, 1–2 mm wide, brown, with pubescent scales along the margins.
Distribution and habitat
Quercus opaca is found in the Mexican states of Hidalgo, Nuevo León, San Luis Potosí, and Tamaulipas. According to Plants of the World Online, its range extends from Tamaulipas south to Oaxaca. It inhabits arid tropical scrub habitats on the Mexican Plateau, typically at elevations of 1,200–2,000 meters above sea level.
Taxonomy
Quercus opaca belongs to sub-genus Quercus, section Quercus, series Leucomexicanae, group Opacae. It is closely related to Quercus pringlei. It can be distinguished from Quercus sebifera by its oblong, elliptic, or lanceolate leaves and non-waxy abaxial blade surface; Q. sebifera has obovate leaves and a waxy abaxial surface. Quercus pringlei differs in having narrower, oblong or narrowly-elliptic, bicolored leaves with an olive-green and shiny adaxial surface.
Conservation
The International Union for Conservation of Nature (IUCN) classifies Quercus opaca as Data Deficient (2018 assessment). There is a lack of data regarding population size, population trends, and potential threats. The absence of population data and threat assessments, along with undersampling, hinders determination of whether the species is at risk.
