A taxonomic revision and conservation assessments of Thesium Section Frisea (Subgenus Frisea, Santalaceae)

Abstract

Thesium L. (Santalaceae) is the largest genus in Santalales, with a global distribution, except for Antarctica. The genus exhibits significant morphological variation and plasticity, and is characterized by its hemi-parasitic nature, linear or scale-like leaves, and dry nut-like fruits. Within Thesium is sect. Frisea, a monophyletic group endemic to the Greater Cape Floristic Region (GCFR) of South Africa which includes up to 20 species, known for their pronounced morphological variability, especially in leaf and inflorescence characteristics. Previous taxonomic treatments and recent molecular phylogenetic studies highlight the taxonomic challenges within this section. This study aims to revise the circumscription of these species, employing detailed morphological analyses and geographical distribution data. Nearly 800 herbarium specimens, field collections and high-resolution images were used for morphological investigations. Univariate and multivariate analyses identified key morphological traits distinguishing 18 distinct species clusters and highlighted relationships among certain taxa, supporting synonymization hypotheses for four taxa. Further, the results indicate significant morphological diversity and highlight the need for in-depth taxonomic revision to improve species identification and conservation planning. Conservation assessments of all species were revised, revealing categories from Least Concern (LC) to Critically Endangered (CR). Notably, six taxa previously listed as Data Deficient due to taxonomic uncertainties (DDT) have been resolved. Data remain deficient for three species, warranting a continued classification of data deficient (DDD). The study provides updated taxonomic treatments, identification keys, and threat status assessments for each species.