Scale-dependent Interactions and Community Structure along Environmental Gradients on a Coastal Salt Marsh

An emerging body of literature has recently emphasized the importance of scale-dependent feedback mechanisms that generate complex community structure. The literature commonly suggests that facilitation and competition operate at different spatial scales, but few attempts have been made to decipher how the extent and mode of such interactions for different species vary along environmental gradients. We evaluated this prediction by investigating the spatial structure of salt marsh vegetation in two 10 m x 10 m plots, each located on low- and mid-elevation areas at Skallingen, Denmark. We examined the coordinates of individual stems of Limonium vulgare and the centroid of patches of Triglochin maritima, Plantago maritima, and Halimione portulacoides to conduct second-order spatial analysis based on Ripley's K-function. Results showed that both positive and negative relationships are spatially manifested to a greater degree on the low marsh than on the mid marsh. This insight extends our current knowledge of scale-dependent interactions beyond pioneer zones to higher zones. On the low marsh, different types of bivariate point pattern (i.e. clustered, random, and regular) were observed for different combinations of species even at similar spatial scales. This finding implies that it is difficult to generalize at which scales competition and facilitation occur. This study proposes that the novel concept of scale-dependence in community structure may be advanced by including the dependence on environmental gradients and species.