Positive interactions between species increasingly are recognized for their importance in plant communities; however, questions remain regarding the relationship between facilitation and environmental stress gradients, and the contributions of positive species interactions to the species composition and diversity of systems. While the long-standing hump-backed model of species richness emphasizes the role of competitive interactions in limiting species densities under high community biomass and low environmental stress or disturbance, recent work suggests expanding this model to include the role of facilitation in maintaining higher species densities under conditions of lower biomass and greater environmental stress. We explored the applicability of this expanded model to rich fens, which are peatlands characterized by low-to-moderate community biomass, high forb species densities, and high environmental stress due to soil saturation and nutrient limitation. We hypothesized that mosses covering the peat surface—and previously shown to alleviate reducing conditions and phosphorus limitation in shallow soils—would facilitate seedling establishment and increase species density of vascular plants. We used a field survey and a moss removal experiment to investigate how rich fen mosses influenced forb seedling emergence, survival, and establishment, and how these influences were reflected in plant species composition and species density.

Results/Conclusions

As expected, in the field survey and experimentally, forb species density was greater in the presence of mosses, while graminoids showed no response. These results reflected moss effects on forb cover, abundance, and establishment: total forb cover was greater and abundance of forb individuals was 1.5-3 times greater where mosses occurred, and this difference persisted when individuals grew into larger (>5 cm) size classes. The 15 most common forb species were more abundant or equally abundant where mosses occurred, suggesting that mosses improved forb success at the community level. Additionally, the presence of mosses resulted in greater survival for seedlings of four tracked species, including two diagnostic rich fen species (Solidago patula, Packera aurea) and particularly two wetland generalists (Eupatorium maculatum, Mentha x piperita). This work provides additional evidence for the importance of positive plant species interactions in maintaining high species densities in environmentally stressful habitats, and suggests the possibility (following recent theoretical work) that fen mosses promote forb diversity by enabling specialist and generalist wetland species to become established under stressful reducing or low-phosphorus conditions. Conservation and restoration of high forb species density in rich fens may require particular attention to retention or establishment of the moss layer.