Relationships Between Plant Assemblages and Water Flow Across a Boreal Forest Landscape: A Comparison of Liverworts,Mosses, and Vascular Plants

The distribution of water across landscapes affects the diversity and composition of ecological communities, as demonstrated by studies on variation in vascular plant communities along river networks and in relation to groundwater. However, non-vascular plants have been neglected in this regard. Bryophytes are dominant components of boreal flora, performing many ecosystem functions and affecting ecosystem processes, but how their diversity and species composition vary across catchments is poorly known. We asked how terrestrial assemblages of mosses and liverworts respond to variation in (i) catchment size, going from upland-forest to riparian settings along increasingly large streams and (ii) groundwater discharge conditions. We compared the patterns found for liverworts and mosses to vascular plants in the same set of study plots. Species richness of vascular plants and mosses increased with catchment size, whereas liverworts peaked along streams of intermediate size. All three taxonomic groups responded to groundwater discharge in riparian zones by maintaining high species richness further from the stream channel. Groundwater discharge thus provided riparian-like habitat further away from the streams and also in upland-forest sites compared to the non-discharge counterparts. In addition, soil chemistry (C:N ratio, pH) and light availability were important predictors of vascular plant species richness. Mosses and liverworts responded to the availability of specific substrates (stones and topographic hollows), but were also affected by soil C:N. Overall, assemblages of mosses and vascular plants exhibited many similarities in how they responded to hydrological gradients, whereas the patterns of liverworts differed from the other two groups.