Species stress tolerance and community competitive effects drive differences in species composition between calcareous and siliceous plant communities

Both chemical and physical properties of soils have been used for two centuries for explaining differences in species composition and diversity between plant communities from calcareous and siliceous soils. Here, we examine the prediction from the 'physical hypothesis', which proposes that species from siliceous soils are absent from calcareous soils because of high drought stress, while species from calcareous soils are absent from siliceous soils because of high competition due to higher water availability. This hypothesis therefore predicts a trade-off between stress tolerance and competitive response that allows calcareous and siliceous species to dominate their respective communities. There has been no attempt to experimentally assess these predictions. We cross-transplanted with and without neighbours using a removal experiment, two dominant species from calcareous soils (Bromus erectus and Brachypodium sylvaticum) and two species from siliceous soils (Pseudarrhenatherum longifolium and Deschampsia flexuosa) in herbaceous communities from the two soils types. We repeated the experiment 4 years, taking advantage of inter-annual climatic variability to test the interplay between year quality (dry vs. wet years) and soil types on survival responses with and without neighbours. We quantified species stress tolerances, their competitive responses and the competitive effect of the two community types. In the absence of neighbours, species from siliceous soils had much lower survival than species from calcareous soils during dry years, in particular on calcareous soils. It indicates a lower stress tolerance of siliceous species and a higher level of stress occurring on calcareous soils. Competition strongly reduced the survival of calcareous but not siliceous species, indicating a greater competitive response of the latter and competition was more important on siliceous soils than on calcareous soils during wet years. However, we found that species stress tolerances were more important than competitive effect of neighbours in driving differences in species composition. Synthesis. The contrasting responses found for dominant species from calcareous and siliceous soils support the trade-off between stress tolerance and competitive response proposed by the physical hypothesis. Our results bring additional evidence that differences in water availability and stress between the two soil types are driving species responses to bedrock types.

Automated summary

The study found that dominant species from siliceous and calcareous soils exhibit different responses to competition and stress, supporting the trade-off between stress tolerance and competitive response proposed by the physical hypothesis. Siliceous species showed lower stress tolerance and higher water pressure on calcareous soils, while competition had a greater impact on survival of calcareous species. Differences in stress tolerance among species played a more important role than the competitive effect of neighbors in driving species composition.