On the synonymization of Acaulon longifolium Herrnst. & Heyn with Acaulon fontiquerianum Casas & Sérgio (Pottiaceae)

During bryological work conducted in the Sharon plane region (Israel), a colony of Acaulon Müll. Hal. was discovered. The plants showed morphological affinities both to A. longifolium Herrnst. & Heyn and A. fontiquerianum Casas & Sérgio depending on the floras used for their identification which challenged their recognition as separate species. In order to solve this taxonomic dilemma, we conducted a detailed morphological characterization of the two species based on representative samples. The micromorphological study of vegetative, sporophytic and spore traits revealed the large overlap of range values between both species. Both species showed the characteristic combination of morphological characters, including the broadly incurved leaves with smooth cells and the 4-6 rows of inflated cells on the adaxial surface of nerve. Spore micromorphology revealed the same ornamentation pattern and the presence of 1-2 lamellar glomeruli on spore surface. Multivariate and ordination analyses based on quantitative traits did not reveal a significant separation among samples of the two species, thus opposing their recognition as separate species. As A. fontiquerianum has priority over A. longifolium, the latter is thus subsumed in the synonymy of A. fontiquerianum. The addition of new chorological data and amended taxonomy reveals that A. fontiquerianum has a distribution area extending from the Canary Islands to the eastern Mediterranean basin. However, the scattered pattern suggests its known distribution is largely incomplete because of the tiny size of the plants and their ephemeral nature.