QUERCUS LESUEURI, UN MIEMBRO MERIDIONAL DEL COMPLEJO DE Q. X UNDULATA (FAGACEAE, SUBGENERO QUERCUS).

Quercus gambelii Nutt. is a widespread white oak (Quercus, subgenus and section Quercus, subsection Gambelieae) in the southern Rocky Mountain region of the United States, extending to west-central Chihuahua in the Sierra Madre Occidental of Mexico. Quercus arizonica Sarg., rather distantly related (subsection Glaucoideae), has a more southern distribution in the United States, and extends through the Sierra Madre into Durango. In the southwestern United States hybrids between Q. gambelii and several white oaks, including Q. arizonica, are very common, often forming a major portion of the lower elevation oak vegetation, particularly in New Mexico. These hybrids are known in general as Q. x undulata Torr., pro sp. A plant very similar to Q. x undulata, Q. lesueuri C. H. Muller, was collected by Harde LeSueur at "El Salto de Babicora", in west-central Chihuahua, in 1937. It has not been collected since. An oak similar to Q. lesueuri was collected by the author in the nearby Sierra Catarina where it is sympatric with Q. gambelii and Q. arizonica. El Salto de Babicora was determined to be located north of the town of Madera. In this canyon Q. gambelii and Q. arizonica are common. Quercus lesueuri and the newly collected hybrid from the Sierra Catarina are shown to be strongly similar to Q. x undulata and intermediate between Q. arizonica and Q. gambelii through the use of the same six leaf characters (leaf width, relative depth of indentations of leaf margin, conspicuousness of intercostal reticulum, number of major secondary veins, number of rays per stellate hair, curliness of hairs) that Tucker used to assess intermediacy of certain collections to the putative parents Q. gambelii and Q. arizonica. In this paper Q. lesueuri is demonstrated to be part of this hybrid complex. Taxonomically it should be recognized as Q. x lesueuri C. H. Muller, pro sp., and to be considered as a synonym of Q. x undulata, sensu lato. [ABSTRACT FROM AUTHOR]