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

Abstract

1. 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.2. 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.3. 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. 4. Synthesis. The contrasting responses found for dominant species from calcareous and siliceous soils support the trade-off between stress tolerance and competitive | 4133

Assessing the accuracy of paired and random sampling for quantifying plant–plant interactions in natural communities

“…At the Spanish site, Euphorbia increased in dominance with increasing stress, but the converse was observed for Astydamia , which is consistent with the former being present in drier conditions than the latter. In agreement with the functional tradeoffs described in several studies between physical stress- and shade-tolerance (Liancourt et al 2005b, Nemer et al 2021), Euphorbia was outcompeted at the three stress levels, whereas Astydamia was increasingly facilitated with increasing stress. At the Italian site, Smyrnium was strongly facilitated by Olea , whereas Cynara was more abundant in the open than below the trees, thus, highlighting that the former was less tolerant to high irradiance and the latter more negatively affected by shade.…”

“…studies between physical stress-and shade-tolerance (Liancourt et al 2005b, Nemer et al 2021, Euphorbia was outcompeted at the three stress levels, whereas Astydamia was increasingly facilitated with increasing stress. At the Italian site, Smyrnium was strongly facilitated by Olea, whereas Cynara was more abundant in the open than below the trees, thus, highlighting that the former was less tolerant to high irradiance and the latter more negatively affected by shade.…”

Higher facilitation for stress‐intolerant ecotypes along a metal pollution gradient are due to a decrease in performance in absence of neighbours

“…However, our result support previous studies that found that plants from benign habitats with low stress tolerance are more facilitated than plants from harsh environments with high stress tolerance (Liancourt et al 2005, Wang et al 2008, Gross et al 2010). Similarly, Nemer et al (2021) also found that the higher drought‐tolerance of species from calcareous habitats compared to species from siliceous habitats reduced their competitive ability, and was therefore responsible for their exclusion from siliceous sites by competitive, high‐biomass siliceous species. However, the very low species richness occurring in the most polluted habitat 1 impeded us to replicate this treatment to straightforwardly conclude on this species effect.…”

“…Most studies and experiments conducted over the past two decades have focused on the use of these metallicolous species as a potential phytoremediation tool to clean up or stabilize polluted soils (Ghosh 2005, Jadia and Fulekar 2008, Marques et al 2009, Meeinkuirt et al 2013, Losfeld et al 2015, Rodríguez‐Seijo et al 2016, Stanovych et al 2019). Fewer have investigated the nature and outcome of biotic interactions in plant communities growing on metal‐polluted soils, although environmental stresses and disturbances are known to drive the direction of plant–plant interactions and ultimately plant diversity (Grime 1973, 1974, Nemer et al 2021). According to the stress‐gradient‐hypothesis (SGH) (Bertness and Callaway 1994), competition dominates plant–plant interactions in benign environments, whereas facilitation predominates in stressed or disturbed environmental conditions (Bertness and Callaway 1994, Liancourt et al 2005, Sthultz et al 2007, He et al 2013, Pugnaire et al 2015, Cavieres et al 2018).…”

“…Fewer have investigated the nature and outcome of biotic interactions in plant communities growing on metal-polluted soils, although environmental stresses and disturbances are known to drive the direction of plant-plant interactions and ultimately plant diversity (Grime 1973, 1974, Nemer et al 2021. According to the stress-gradient-hypothesis (SGH) (Bertness and Callaway 1994), competition dominates plantplant interactions in benign environments, whereas facilitation predominates in stressed or disturbed environmental conditions (Bertness and Callaway 1994, Liancourt et al 2005, Sthultz et al 2007, He et al 2013, Pugnaire et al 2015, Cavieres et al 2018.…”