Fagus

Denk, Thomas, Grimm, Guido W., Cardoni, Simone, Csilléry, Katalin, Schulze, Mirjam Kurz Ernst-Detlef, Simeone, Marco Cosimo & Worth, James R. P., 2024, A subgeneric classification of Fagus (Fagaceae) and revised taxonomy of western Eurasian beeches, Willdenowia 54, pp. 151-181 : 160

Fagus L. subg. Fagus – Type (designated by Green 1929: 189): Fagus sylvatica L., Sp. Pl. 2: 998. 1753.

Molecular diagnosis — The subgenus differs consistently from all species of Fagus subg. Englerianae in any sufficiently divergent nuclear marker sequenced so far ( Denk & al. 2002, 2005; Renner & al. 2016; Cardoni & al. 2022). Its ITS variants belong to Lineages II–IV as defined in Denk & al. (2005); the sequenced part of the Crabs Claw (CRC) gene (~1650 bps) and the 2 nd intron of the Leafy gene (LFY, up to ~1300 bps) include 14 subgenus-sorted SNPs (CRC pos. 212 [C vs T in F. subg. Englerianae], 714 [C↔T], 904 [C↔A], 1148 [A↔T/Y] and 1329 [T↔G]; LFY pos. 241 [A↔C], 398, 402, 465 [all G↔A], 507 [T↔A], 529 [C↔T], 1031 [T↔G], 1206 [A↔G] and 1246 [T↔A]; according reference matrices are included in the SDA, file RefMatrixCRCLFYSpCons.nex). Additional subgenus-diagnostic SNPs can be found in 21 of the 28 nuclear loci sequenced by Jiang & al. (2022; cf. supplement to Cardoni & al. 2022). Plastomes are divergent but geographically sorted and reflect two independent origins: Lineage I in North America; sibling lineages Lineage IV in East Asia and Lineage V in western Eurasia; see Fig. 1) The exception are populations of the Japanese species F. crenata comprising individuals that may carry near-private haplotypes of Lineage II (supplementary content, file Genotypification.xlsx, sheet PlstmDissim; including information from upcoming complete plastome data; Worth & al. 2021, work in progress).

Morphological diagnosis — Trees; buds sessile; leaves thick-chartaceous, abaxial leaf surface commonly smooth or papillate ( Fagus longipetiolata ), wax ornamentation on abaxial leaf surface missing or present ( F. longipetiolata ), size of stomata usually large, small in F. hayatae Palib. , F. pashanica C. C. Yang , and F. grandifolia , subsidiary cells of stomata usually actinocytic to cyclocytic, anomocytic in F. grandifolia , leaf margin smooth or serrate; cupule peduncle short to long; pollen usually large, intermediate in F. hayatae , colpi usually short with more or less acute apex, or long and narrow with rectangular apex in F. grandifolia and occasionally in F. longipetiolata .

Species — Twelve: Fagus sylvatica , F. orientalis , F. hohenackeriana , F. caspica sp. nov. in western Eurasia (west to east); F. chienii W. C. Cheng (†?), F. crenata , F. hayatae , F. longipetiolata , F. pashanica , F. lucida in East Asia; F. grandifolia , F. mexicana MartÍnez in North America.

Remarks — Members of Fagus subg. Fagus can be traced in the fossil record based on their pollen and leaf-anatomical similarities with one or several modern-day species ( Denk & Grimm 2009; Renner & al. 2016; Worth & al., work in progress). The oldest fossils representing this modern subgeneric lineage are cupules and leaves from the Eocene-Oligocene boundary, Northeast Asia ( Pavlyutkin & al. 2014; see also Denk & Grimm 2009). Any molecular-phylogenetic tree analysis (e.g. Denk & al. 2005; Jiang & al. 2022) relying on sufficiently variable nuclear data will produce a prominent split with high (BS ≥ 70, PP ≥ 0.9) to unambiguous (BS = 100, PP = 1.0) support between the subgenera irrespective of the optimality criterion used for tree-inference. Newly sequenced individuals can be easily placed in either subgenus using e.g. the evolutionary placement algorithm implemented in RAxML 8 and its successor RAxML-ng. In contrast, any plastome data requires in-depth analysis and, in some cases, may fail to elucidate the subgeneric affinity.