Annual plant community assembly in edaphically heterogeneous environments

Questions Does edaphic heterogeneity (i.e. switches between gypsum and calcareous soils) affect annual plant community assembly in highly restrictive soils? Our main hypothesis is that soil filtering is the main determinant of species assembly, subsequently modulated by climate and lastly by biotic interactions, such as Stipa tenacissima and the biological soil crust (BSC) at fine spatial scales. Our study system is highly suitable to test the importance of edaphic heterogeneity to the assembly of annual plant communities, since calcareous and gypsum soils are in close contact and freely receive propagules. Location Annual plant communities of semi-arid steppes in central Spain. Methods We built a soil affinity index (SAI) for each species to measure mean soil affinity (MSA; analogous to the community-weighted mean – CWM) in each local assemblage. Results Most species were able to establish in both substrates, but gypsum soils clearly exerted a stronger filtering effect. Stipa favoured an expansion of the niche space in calcareous soils but not in gypsum, and BSCs not only reduced annual species richness, diversity and cover, but also the range of SAI values of local assemblages. Conclusions This study highlights the importance of the probabilistic filtering (sensu From plant traits to vegetation structure, 2010, Cambridge University Press, Cambridge, UK) of soil characteristics to the assembly of annual plant communities, as opposed to an ‘all-or-nothing’ filter.