1. Understanding the mechanisms that underlie species assembly is a central concern in community ecology. Abiotic and biotic filters are probabilistic 'sieves' that allow species with certain functional traits to become a part of the community, or not. We manipulated natural plant assemblies in order to identify variations in the timings of biotic and abiotic filters that determine community trait assemblies.2. We extracted soil portions when the investigated annual plant community was in its seed phase ('community monolith'), thereby maintaining the structure and similar soil characteristics to the field conditions. Community monoliths were subjected to experimental manipulation in terms of the rainfall timing and amount, and perturbations of the biological soil crust (BSC; intact vs. perturbed). We surveyed the experimental community assembly over time based on the functional diversity by considering important functional traits in different life stages.3. We found that autumn droughts acted as abiotic filters by favouring the germination and establishment of species with greater investment in the root biomass.Under severe droughts (66% water reduction), the experimental assemblies were dominated by species with functional traits adapted to water shortage conditions: high leaf dry matter content, low specific leaf area, small individual size, low reproductive ratio and high root:shoot ratio. We identified two roles of BSCs in annual plant species assemblies: (a) as a biotic filter that limited the establishment of species based on seed size, and (b) as a buffer against water stress conditions by reducing soil evapotranspiration. Synthesis.We demonstrated the importance of the timing and amount of rainfall for shaping annual plant communities, and identified germination filters as the main process that determined community assemblies. Our results suggest that the phenotypic integration of functional traits facilitates resistance to drought during the life cycle. The BSC-annual plant relationship shifted from negative, by acting as a germination filter, to positive, by acting as a buffer in later stages.Climatic fluctuations and fine scale biotic determinants of spatial heterogeneity emerged as sources of changes in the community assembly in time and space to

Soil seed bank assemblies in our semi-arid plant community were the result of above-ground vegetation dynamics and of the direct filtering processes on seed fate operated by the spatially heterogeneous BSCs. Cover of BSCs was negatively correlated with seed abundance and species richness, and affected seed species composition in the soil. Changes in species composition and enrichment when the BSC cover is low suggest that BSCs promote a fine scale niche differentiation in the soil seed bank and thereby potentially enhance species coexistence and high species diversity in these communities.

Rainfall and biocrusts are important sources of temporal and spatial environmental heterogeneity and niche differentiation for annual plants, a major component of diversity in drylands. Therefore, global change processes comprising shifts in rainfall timing and drought exacerbation, together with biocrust disturbance may affect species coexistence and result in disrupted diversity patterns. In this study, we experimentally evaluated the effects of the rainfall amount and timing as well as physical biocrust disturbance and their interaction on the taxonomic, phylogenetic and functional diversity of annual plant communities on gypsum soil drylands. All diversity estimates were determined at different times during community development in each experimental unit (α), as the contribution of each experimental unit to the total diversity in each treatment (β) and as the total diversity in each treatment (γ). Rainfall timings led to changes in all diversity dimensions, with higher diversity under the typical timing. The community was quite resilient to moderate reductions in rainfall, but extreme droughts decreased the alpha and beta taxonomic, functional and phylogenetic diversities. In addition, the simultaneous occurrence of biocrust disturbance and extreme drought led to consistent collapses in all diversity dimensions, probably because the effects of water shortage were exacerbated. Observations of the community at different times during its development highlighted the importance of regenerative strategies for niche differentiation and species coexistence, and their strong dependence on global change drivers. Indeed, our experimental study demonstrated that rainfall patterns and biocrusts are key factors related to the maintenance of diversity in semiarid annual plant communities. In particular, our results highlight the key role of biocrusts in modulating the effects of drought on plant diversity and the need for integrative approaches that consider both plants and biocrusts in order to elucidate the influence of climate change on the diversity of drylands.

Background and Aims Standing vegetation and soil seed banks are tightly related compartments in annual plant communities. However, little is known about how this relationship changes spatially and temporally and their dependence on abiotic and biotic factors. We investigated the similarity between both compartments under contrasting water availability conditions, perennials and biocrusts. Methods We established a field experiment in a Mediterranean annual plant community growing on gypsum soils. We evaluated the relationships between aboveground vegetation and soil seed bankover three years and along three spatial scales and tested the effects of water availability (irrigation), perennial vegetation and biocrust on both compartments. Results The soil seed bank was more similar to the standing vegetation closer to the third year, when seed bank was collected. The similarity between both compartments was higher at the large spatial scale (20 ´ 20 cm) but the modulatory effects of irrigation, perennial tussocks and biocrust were more evident at medium (10 ´ 10 cm) and fine scale (5 ´ 5 cm). The similarity increased with irrigation and was higher under perennial vegetation. The biocrust acted mainly as a barrier to the incorporation of seeds into the permanent seed bank. Conclusion Our findings suggest that abiotic and biotic factors had hierarchical effects on the similarity between the standing vegetation and soil seed bank compartments. Interannual shifts such as rainfall amount and frequency, were the main determinants but tussocks, biocrust, and irrigation modulated similarity, thereby showing that annual plant communities are driven by complex processes.