Soil toxicity and species dominance rather than nutrient availability drive plant species richness in swamp forests of Central Europe

Aim: A resource-based conceptual model of plant diversity (RBCM) assumes
direct relationships between resource supply and the diversity of a local
plant assembly. However, the RBCM largely ignores variation imposed by
soil toxicity due to climatic effects. Both soil-limiting resources and
soil toxicity vary along climatic gradients but their net and interactive
effects on plant species diversity remain unknown. We asked how climatic
gradients shape resource availability, soil toxicity and dominance of
herb-layer graminoids, and how these predictors control local species
diversity of herbs and bryophytes. Location: Swamp forests,
Central Europe Taxon: Vascular plants, bryophytes
Methods: Alpha taxonomic diversity of vascular plants and
bryophytes was counted for 101 vegetation plots sampled in temperate swamp
forests distributed along an 800-km geographical gradient across the
Continental, Alpine and Pannonian biogeographical regions. Path analysis
(structural equation modelling) was used to quantify the direct and
indirect effects of climatic variables (potential evapotranspiration;
PET), limiting resources (soil N/P, Ca, C/N, proxies for light and water
availability), and soil toxicity (Mn) on graminoid dominance and community
diversity. Results: PET negatively influenced species richness of
both groups analysed either directly or indirectly through its positive
effect on the cover of graminoid species. Alpha diversity of herbs was
additionally reduced by soil toxicity (Mn). Limiting resources correlated
either with species dominance (canopy shading, soil Ca) or with PET (soil
N/P ratio), but they did not control species richness pattern. Main
Conclusions: Climate, soil toxicity and species dominance
determined alpha diversity instead of the expected importance of soil
limiting resources. These results are key to advancing the theoretical
framework of the RBCM. Increased soil toxicity (Mn) in well-watered
regions favours the dominance of plant competitors at the expense of less
tolerant species. This implies a potential threat to wetland diversity
under ongoing climate change.