Evidence of functional species sorting by rainfall and biotic interactions: A community monoliths experimental approach
1. Different abiotic and biotic sources of environmental heterogeneity have been proposed as determinants of community assembly, but few experimental studies have identified the processes and functional traits through which they operate. Thus, we aimed to experimentally determine the relationships between variations in functional structure according to spatiotemporal environmental heterogeneity to understand how biotic and abiotic drivers affect the highly dynamic nature of annual plant communities. 2. Entire annual plant communities were subjected to experimental manipulation of rain regimes (timing and amount) and perturbation of the biological soil crust (BSC; intact vs. perturbed) in a community common garden. We surveyed the experimental assemblages at different life stages (seedlings to reproductive adults) and characterized their functional structure (community weighted means and Rao) based on key functional traits. 3. We found an important filtering effect of rain timing and amount on the composition and functional structure. Drought conditions limited the community weighted means and diversity to better functionally equipped species in terms of stress resistance (high leaf dry matter content and low specific leaf area) and resource economy (short species with low reproductive ratios and higher root:shoot ratios), although the timing of drought could determine the intensity of the filtering effect. BSC acted as a filter on germination via seed mass and as a ‘biotic buffer’ in later stages by minimizing the stressful conditions during severe droughts decreasing evapotranspiration. 4. Synthesis. We found clear cause–effect relationships among abiotic and biotic factors and their interaction, and the experimental communities in terms of species composition and functional structure. We demonstrated the importance of rain regimes for shaping annual plant communities and identified germination as the main process determining the community assembly, suggesting phenotypic integration of a complete set of functional traits for resisting drought throughout the whole life cycle. The BSC–annual plant relationship shifted from negative by acting as a germination filter to positive by acting as a biotic buffer in later stages. We demonstrate the importance of evaluating these relationships at multiple 37 plant stages. The temporal and spatial heterogeneity related to the factors considered are key sources of niche differentiation that promote species coexistence.
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