Coexistence of Ambrosia chamissonis and Ammophila arenaria in coastal dunes in the Nuble Region, Chile

Coastal dunes are unique ecosystems distributed worldwide. They share coastline-to-inland abiotic stress gra-dients determining their plant ' s spatial distribution and biological interactions. Ambrosia chamissonis and Ammophila arenaria dominate coastal dunes in Chile, but there is scarce ecological information about their in-teractions and spatial distribution. The aim of this study was to characterize the coexistence of these two species and to evaluate the effect of stress gradients on their spatial distribution and performance. The study was carried out in three coastal dunes of the N similar to uble Region, Chile; two dunes in which each species dominates and one in which they coexist. In each dune, sampling sites were defined along study transects perpendicular to the coast, where both soil characteristics (pH, salinity, contents of water, organic matter, nitrogen, phosphorus, and po-tassium) and biological variables of each species (cover, height, water content, content of foliar macronutrients) were measured. Variation of each abiotic variable with the distance from the high tide level was correlated to determine environmental stress gradients. A Canonical Correspondence Analysis (CCA) was performed to determine soils characteristics that better explained changes in plant abundance. The Relative Interaction In-tensity index was calculated from biological variables and compared intraspecifically to determine dominant interactions along the dunes. Our results showed two soil stress gradients (soil salinity and available potassium) which decreased inland and may define in part, the spatial distribution of the species, as shown by the CCA. Our results support the idea that dune plants do not always follow the stress gradient hypothesis suggesting a modification of the hypothesis.