LAUSR

Co-ordinating Editor: Alicia Teresa Rosario Acosta Abstract Questions: Complex distal factors such as climate and topography can interact to drive direct proximal ecological gradients and vegetation zonation in geographical and ecological space, in particular salinity and soil moisture in arid climates. We pose the following questions: (a) Does increasing climate aridity alter direct water and salinity gradients along topography gradients in saline depressions in arid climates? (b) Do these effects of increasing climate aridity in turn alter vegetation lifeform zonation? (c) Does increasing climate aridity alter community composition along the zonation? Study site: Continental saline depressions of the Mediterranean arid climate of central and southern Tunisia. Methods: We sampled vegetation structure, diversity, and composition and environmental variables at five positions along the topography gradient of continental saline depressions in wet and dry conditions. Results: Climate aridity altered salinity gradients and vegetation structure but not diversity and community composition. Salinity increased with increasing aridity at the lowest positions of the depressions but decreased with increasing aridity at their highest positions. Total vegetation cover and relative abundance of therophytes decreased, whereas relative abundance of chamaephytes increased with increasing climate aridity at the upper habitats. Conclusions: Ecological factors acting at a regional scale can alter the spatial variation of other ecological factors acting at smaller spatial scales, ultimately explaining vegetation zonation in continental saline depressions. Salinity collapses with increasing aridity.