Environmental gradients and micro-heterogeneity shape fine-scale plant community assembly on coastal dunes

Questions: How do fine scale community assembly processes (e.g., environ-mental filtering, symmetric competition, hierarchical competition, facilitation)vary along a natural stress gradient on coastal sand dunes? How does local envi-ronmental heterogeneity affect these processes?Location: Mediterranean coastal sand dunes, central Italy (Montalto Marina,Lazio region).Methods: We quantified spatial (co-occurrence) and functional patterns(degree of divergence in the traits of co-occurring species) at a fine scale(0.5 m 9 0.5 m subplots) in herbaceous communities (2 m 9 2 m plots) ofcoastal habitats along the sea–inland vegetation zonation. We then studied howthe fine-scale co-occurrence patterns (aggregation vs segregation) varied acrosshabitats of the zonation. Finally, we fitted linear models assessing the relation-ship of the fine-scale functional patterns (convergence vs divergence) with (1)the average environmental conditions along the sea–inland environmentalstress gradient, and (2) the environmental heterogeneity within plots.Results: Spatial and functional patterns conveyed complementary information.Within-community spatial segregation was more common further from the sea,which suggests the dominance of competitive processes in the least stressedcommunities. Fine-scale convergence or divergence depended, for all functionaltraits, on the average environmental conditions of the community along the gra-dient, suggesting an effect of environmental stress on the prevailing assemblyprocesses. However, it also depended on the environmental heterogeneitywithin the community, suggesting that micro-abiotic filtering might play a moreimportant role than previously anticipated in determining fine-scale communityassembly.Conclusions: Our results suggest that contrasting assembly processes act simul-taneously on community assembly along environmental gradients, both as afunction of average environmental conditions and of local heterogeneity. Futurestudies assessing community assembly should therefore avoid neglecting therole of micro-abiotic filtering in shaping functional patterns. Moreover, only byintegrating multiple sources of information (trait patterns, spatial patterns andenvironmental variation) were we able to disentangle fine-scale communityassembly processes and reinforce our interpretation of community patterns.