Syringa fauriei is an unresolved lilac taxon (Oleaceae) native to specific geographical areas and present in the rivers and valleys of Gangwon province in South Korea. It was first identified on Mt. Geumgang by Faurie in 1906 and recognized as an independent species. Nevertheless, S. fauriei has been controversial as a synonymous species for S. reticulata subsp. amurensis without clear evidence. Therefore, we conducted this study on the taxonomic position of S. fauriei by examining the morphological and molecular differences between S. fauriei and its related taxa, the S. reticulata complex. Phylogenetic relationships were also inferred using molecular markers of nuclear ribosomal DNA internal transcribed spacer (ITS1–5.8S–ITS2; ITS) and external transcribed spacer regions. Consequently, S. fauriei and its related taxa were classified into two distinct clades: S. fauriei and the S. reticulata complex. Morphological and molecular data revealed that S. fauriei was endemic to Korea. The species characteristics of S. fauriei were further described for taxonomic re-establishment.

Morphological dataset: Dried and liquid-preserved (fixed in 70 % ethanol) specimens for morphological analysis were collected during the flowering times (May to June 2020) from the four populations of S. fauriei and six populations of S. reticulata (synonym S. reticulata subsp. amurensis) in South Korea.

Molecular dataset: PCR was conducted to amplify the nrDNA ITS and ETS regions using target gene-specific primer pairs. The reaction mixture included 20–100 ng template DNA and Dr.MAX DNA Polymerase. The other conditions were adopted as those described by Li et al. (2001). PCR conditions for gene amplification were as follows: a pre-denaturation step at 95 °C for 5 min, followed by 35 thermal cycles of denaturation at 95 °C for 30 s, annealing at 52 °C for 30 s, and extension at 72 °C for 1 min, followed by a final extension at 72 °C for 10 min. The PCR product was purified. The purified PCR products were then sequenced using Sanger-sequencing. Three primers (ITS4, ITS3B, and ITS-Leu) were used to sequence the entire ITS region, including 5.8S gene (Li et al., 2001). We used the primer pair (STT-ETS and 18S-ETS) to sequence the segment at the 3' end of the ETS region (Li et al., 2001). Nucleotide sequences were determined on both strands of the PCR amplification products at the Macrogen sequencing facility. The sequences obtained for each sample in both the forward and reverse orientations were assembled. The ETS and ITS sequences were aligned separately and then concatenated and manually adjusted. Information of the sample sources was attached as 'source-table-sample'.

Data matrix of morphological characters was used to evaluate the morphological variations of Syringa fauriei and establish a relationship between S. fauriei and S. reticulata (synonym S. reticulata subsp. amurensis). There are some missing value, they has been filled using mean imputation.

Sequence data matrix of nrDNA ITS and ETS regions was used to evaluate the sequence divergence of ETS and ITS and a phylogenetic relationship between S. fauriei and S. reticulata (synonym S. reticulata subsp. amurensis) using their related taxa.