Hamatocaulis vernicosus and Calliergonella cuspidata commonly co-occur in base-rich fens, reflecting overlapping ecological niches. While C. cuspidata is a widespread and ecologically plastic fen species often associated with eutrophicated wetlands, H. vernicosus is a habitat-specialist species of conservation concern. This study investigated the competitive interactions between these two moss species and the role of microhabitat conditions in their coexistence. A reciprocal transplant experiment was conducted in a natural, rich fen in southeastern Lithuania using replicated experimental plots across different microtopographic and hydrological conditions. Species cover and spread were monitored to assess competitive performance following transplantation. The results showed that under wet conditions, H. vernicosus was able to expand into surrounding areas and successfully compete with C. cuspidata. In contrast, C. cuspidata showed limited spread within H. vernicosus patches under wet conditions and was gradually displaced. An advantage of C. cuspidata was observed only in hummocky microtopographic settings. These findings indicate that stable hydrological conditions maintaining microhabitat heterogeneity promote the coexistence of both species. Alterations in the water regime may reduce the competitive ability and long-term persistence of H. vernicosus, highlighting the importance of hydrology-focused management for its conservation.
Kalvaitienė et al. (Thu,) studied this question.