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Plant Focus

Quercus peninsularis
A Red Oak (Section Lobatae) endemic to inland ranges of northern Baja California, Mexico

Agrifoliae: the California Red Oaks

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Al Keuter and Paul S. Manos

Published May 2019 in International Oaks No. 30: 191–202


Like water from an incredibly slowly leaking faucet the recognition of the four species (Fig. 1) of the California Floristic Province (CA-FP) native Red Oaks (Quercus, section Lobatae, series Agrifoliae) has dripped out one at a time over nearly two hundred years. The eleven currently accepted taxa – four species including eight varieties plus three named hybrids – were each described by different authors, most prior to 1890. A recent study (Hauser et al. 2017) combining both morphological and molecular analysis has finally examined all eleven taxa simultaneously. Morphometric and DNA sequence (RAD-Seq) results are in general agreement, confirming the four California Red Oak species to be distinct while also identifying two samples as F1 hybrids. Here we describe and interpret the taxonomic history of the Agrifoliae and provide a detailed synthesis of the findings which have consequences to biodiversity and conservation concerns. We conclude that: 1) DNA analysis establishes the Agrifoliae to be a discrete clade separate from and sister to the combined Eastern North America and Mexican Red Oaks, splitting from them roughly 30 million years ago; 2) Q. wislizeni is not composed of two varieties as described by Engelmann; 3) Q. parvula var. tamalpaisensis is of hybrid origin, not a variety of Q. parvula; 4) DNA analysis reveals an unnamed F1 hybrid (Q. kelloggii × Q. parvula var. shrevei); and 5) both morphological and DNA analyses suggest Q. parvula var. parvula is a hybrid, but sample sizes are too small to confirm this.


hybridization, introgression, Lobatae, morphometrics, phylogeny, Quercus hybrids, RAD-Seq, taxonomy


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