Quercus prinoides (Fagaceae) hybrids in the Capitan Mountains of New Mexico, U.S.A.

The chinquapin oaks, Quercus prinoides Willd. and Quercus muehlenbergii Engl., are known to hybridize with each other as well as other members of the white oak subgenus throughout their occurrence in the eastern United States (Tirmenstein 1991). However, Tucker (1961) postulated that Q. muehlenbergii and five other oak species that hybridized with Q. gambelii Nutt. in the southwestern United States were the sources of variation in a group of variable populations, of apparent hybrid origin, that he called the Q. undulata Torr. complex. Quercus muehlenbergii is unique in the complex as the only member whose main occurrence is in the widely separated forests of the eastern and central United States. While Tucker studied the effects of hybridization on Q. undulata, it is possible that hybridization has affected the chinquapin oaks as well, both morphologically and in such features as habit or soil preference. Today, taxonomists treat the chinquapin oaks as two taxa but disagree on the appropriate rank; a single species with two varieties (Burger 1975; Gleason 1963; Mohlenbrock and Thomson 2009; Steyermark 1963) or two species, as in Flora of North America (Nixon and Muller 1997), with Quercus prinoides a shrub and Q. muehlenbergii a tree. In this paper I refer to the chinquapin oaks as a single species, Q. prinoides. Disjunct populations of Quercus prinoides occurring in New Mexico and western Texas were examined as part of a study of the species status of the chinquapin oaks (Thomson 1978). During that study, evidence of hybridization between Q. prinoides and two members of the Q. undulata complex was discovered in the Capitan Mountains of New Mexico. Results of that study are presented here.