Spatial distribution of bryophytes along a moisture gradient: an approach using photosynthetic pigments as indicators of stress

Height above water table in aquatic habitats has long been recognized as the key factor for determining the spatial structure of bryophyte assemblages. Using photosynthetic pigments as indicators of stress along a wet-to-dry gradient, I tested whether different habitat preferences were revealed. A field experiment was set up to obtain a semi-natural hydrological gradient. The spatial distribution of species was quantified by determining the pigment content in natural assemblages and by applying the regression equations derived from the experimental manipulation. The species were able to move water in quantities that were statistically significantly different from each other, which was in agreement with the species characteristics and the colony architectures. Photosynthetic pigments responded in a complex way to the water gradient and, for each species, one or more stress indicators were reliable. A wide overlapping of spatial distribution exists in natural conditions, especially in complex habitats like springs. Although in this study a restricted selection of species has been examined, the method proposed to evaluate the theoretical height above water table is coherent with the broad spatial pattern observed. This approach may be particularly useful for understanding both the coexistence of similar species and the spatial distributions of bryophytes in spring habitats in relation to water stress tolerance.