Microtopographic and Hydrological Filtering Constrain Natural Population Formation of the Endangered Fern Mankyua chejuense

Mankyua chejuense (Ophioglossaceae) is an endangered fern restricted to basaltic ephemeral wetlands on Jejudo-Island. However, its capacity to establish in transplantation habitats remains poorly understood. Its occurrence is shaped by rainfall-driven short-term inundation, rapid drying, basaltic microtopography, and seasonal light availability. We assessed its reintroduction potential through a two-year transplantation experiment using ramets collected from a natural population. Ramets were planted in two wetland-type recipient habitats: a small, microtopographically isolated wetland (WS) and a larger wetland with a steep inundation depth gradient (WL). During the 24-month monitoring period, transplanted ramets survived in both transplantation habitats, with establishment ratios of approximately 80% in WS and 55% in WL. Ramets maintained aboveground growth and reproductive structure formation, including stipe production, ramet height, vegetative pinna production, fertile segment formation, and reproductive ratio. Along the water-depth gradient in WL, deeper or fully inundated conditions tended to increase vegetative growth, particularly stipe production, whereas reproductive ratio was relatively lower under these conditions. These contrasting responses suggest a possible shift in growth–reproductive allocation along the inundation gradient. Overall, these findings indicate that M. chejuense can establish in environmentally suitable transplantation habitats when ramets are artificially introduced. Therefore, the absence of natural individuals in nearby unoccupied wetlands should not be interpreted as direct evidence of habitat unsuitability, but may instead reflect limitations in natural colonization processes, including dispersal arrival, germination, early establishment bottlenecks, and fine-scale microtopographic or hydrological filtering.