A New Interpretation of Mexico’s Racemose Red Oaks

Ross A. McCauley

Published May 2021 in International Oaks No. 32: 17–24

Introduction

Quercus section Lobatae subsection Racemiflorae is a small section of Red Oaks united by the unique characteristic of producing acorns on racemose infructescences native to the Sierra Madre Occidental and southern Cordillera of Mexico. The group has been treated in a variety of ways since first recognized by Trelease in 1921. Species recognition is difficult in the absence of reproductive material that is often not collected, a fact that led to much misinterpretation of species and distributions. In 1996, Spellenberg and Bacon completed a systematic revision of the group, recognizing four species with distinct geographic ranges segregated by slight morphological differences and edaphic factors. These included three specialists on highly sterile, often mineralized soils: Q. radiata in the southern Sierra Madre Occidental, Q. tarahumara in the northern Sierra Madre Occidental, and Q. urbani in the southern Sierra Madre Occidental and in an area north of the Balsas Depression in central Mexico. The fourth species, Q. conzattii, was shown to occur on a variety of generalist soils in the mountains of Oaxaca and in the southern Sierra Madre Occidental. This treatment greatly increased our understanding of the group and also proposed unique bicentric distributions for both Q. conzattii and Q. urbani across the Trans-Mexican Volcanic belt with the ranges separated by up to 850 km.

References

McCauley, R.A., A.C. Cortés-Palomec and K. Oyama. 2019. Species diversification in a lineage of Mexican red oak (Quercus section Lobatae subsection Racemiflorae) – the interplay between distance, habitat, and hybridization. Tree Genetics and Genomes 15: 27.

McCauley, R.A. and K. Oyama. 2020. A re-evaluation of taxonomy in Quercus section Lobatae subsection Racemiflorae (Fagaceae), resurrection of the name Q. pennivenia and description of a new taxon, Q. huicholensis. Phytotaxa 471(3): 247-257.

Spellenberg, R. and J.R. Bacon. 1996. Taxonomy and distribution of a natural group of black oaks of Mexico (Quercus, section Lobatae, subsection Racemiflorae). Systematic Botany 21(1): 85-99.

Spellenberg R., J.R. Bacon, and D.E. Breedlove. 1995. A new species of Quercus (Fagaceae, sect. Lobatae, group Racemiflorae) from the Sierra Madre Occidental, Mexico. Madroño 42(1): 26-33.

Trelease, W. 1921. A natural group of unusual black oaks. Proceedings of the American Philosophical Society 60: 31-33 + 3 plates. Trelease, W. 1924. The American Oaks. Memoirs of the National Academy of Sciences 20: 1-255 + 420 plates.