Thelypteris palustris Schott (Thelypteridaceae), known as "marsh fern", is infrequent in the Mediterranean area. The occurrence of this species is known for almost all the Italian regions (except for Sardinia and Sicily), but with rare and declining populations. During floristic fieldwork on the Sicilian wetlands, a new unknown population was found. The aim of this paper is to analyze the morphological traits of the species, as well as its ecological features and the floristic composition of the plant communities where it lives. According to IUCN guidelines, here we provide the regional assessment (Sicily) of T. palustris. To analyze its morphological features, many living plants were examined, with particular attention to the spore structure. A total of 179 plots (110 species) and 34 pools were sampled. Our results highlight the relic character of the species which is at the southernmost border of its distribution range. The micro-morphological investigations on the spores show that the Sicilian population belongs to the subsp. palustris. The floristic analysis confirms the clear dominance of perennial temperate-cold zones Eurasian taxa. Finally, a new association, Thelypterido palustris-Caricetum paniculatae, within the Caricion gracilis alliance (Phragmito-Magnocaricetea class) is described.
Keywords: Mediterranean wetlands; conservation status; distribution; ecology; pteridophytes; relic fern; vegetation.