Fine-scale fern ecological responses inform on riparian forest habitat conservation status

The recognition of the ecological quality of ecosystems and habitats therein is increasingly important in the Anthropocene. However, there are still scarcely explored ways of how and what to assess to obtain a sound ecological status of habitats. Ferns are an understudied plant group, especially given their usefulness as ecological indicators. Disentangling biotic and abiotic factors that drive fine-scale fern distribution could provide insight into the quality of their habitats. We investigated the environmental factors affecting the distribution of different largely distributed fern species in Europe. We studied their presence and abundance at different life stages in a forest habitat of European priority conservation concern. Our aim was to understand whether fern species can be used as an ecological indicator group in riparian alderwood habitat. We sampled 120 plots of 50 m2 in randomly selected transects along streams of a riparian forest habitat characterized by the presence of many fern species in the understory, controlling for the effects of geology and elevation. Within each plot, fern species were recorded, including vegetative and generative stages of each ramet (rosette of fronds). We modelled fern occurrence and abundance for the different fern life stages, and diversity indices of the fern community in relation to environmental predictors. We found that population- and community-level responses of ferns mainly depended on soil granulometry and, to a lesser extent, moss cover and stream orientation. We also found that the generative life stage compared to the vegetative adult stage benefits from different ecological characteristics for certain fern species. Alterations of the natural hydrology might lead to a general deterioration in habitat quality for ferns. We suggest that some fern species acting as early-warning species, and potentially their life stages, can be used as an ecological quality indicator for riparian forest habitats. This study deepened the understanding of the fine-scale ecology of an array of European ferns in riparian forests and provides valuable information to assist in the conservation of fern species and their populations.