The Endangered Fern <i>Mankyua Chejuense</i> Exhibits Enhanced Genetic Diversity and Population Size in Basaltic Vernal Pools With Greater Light Availability

ABSTRACT Mankyua chejuense (Ophioglossaceae) is a small, evergreen, endangered fern restricted to isolated wetlands occurring on basaltic terrain in Jeju Island, Republic of Korea. These wetlands are surrounded by evergreen broadleaf forests, but only sparse deciduous trees are found within the pools. The species' survival is strongly influenced by canopy openness and light availability, particularly during its winter growing season. This basic study aimed to identify the key factors influencing population size in M. chejuense , with a focus on genetic diversity, leaf vigor, and habitat characteristics. Using ecological data collected over a period of 2 to 10 years, we performed a path analysis to evaluate the relative influence of seven variables: population size, chloroplast DNA diversity, trophophyll and sporophyll development, photosynthetically active radiation, litter depth, and wetland area. Results showed that larger populations were associated with significantly higher genetic diversity, greater development of trophophylls and sporophylls, and occupancy of more extensive wetlands in terms of area. Such broader wetlands offered more open canopies and increased light input, particularly in winter when leaf shedding by deciduous trees enhances light penetration. In contrast, dense evergreen cover reduced light availability and was associated with lower population size and reduced reproductive frond development. These findings highlight that population viability in M. chejuense is dependent on open‐canopy structures and sufficient wetland area. Conservation strategies should include limiting the expansion of evergreen broadleaf trees and maintaining open, light‐accessible habitats to support long‐term population persistence.