Site effect is stronger than species identity in driving demographic responses of Helianthemum (Cistaceae) shrubs in gypsum environments

Abstract

Regional climatic patterns result in the synchrony of biological processes along large spatial areas. These patterns may be critical for effective plant recruitment in (semi)arid environments. Nevertheless, recruitment patterns of plant species within their range of distribution are still poorly known. Moreover, this response may be species-dependent, particularly between coexisting congenerics, which are thought to vary in demographic responses to climatic conditions as a coexistence-promoting mechanism. In this context, we investigated whether two congeneric plant species show synchronic age structures at varying spatial scales and whether they display demographic differences. We sampled 600 Helianthemum squamatum and H. syriacum individuals at six localities in Spain. We used dendrochronological techniques to estimate age and radial growth. We compared the age structure among populations and between species and assessed the effect of site and species on different demographic parameters. Correlations between age structure and climatic factors were also determined. We detected a very low intraspecific synchrony across sites but a high interspecific coupling in age structure within localities. Secondary growth, mean age, and flowering showed large intersite differences and small interspecific differences. Summer rainfall was a good predictor of age structure. Fine-scale climatic variability plays a major role in determining age structure of the studied species. Climatic variability is more relevant than interspecific differences. Our results illustrate the relevance of including spatial variation in summer rainfall patterns when modeling the impact of climate change on Mediterranean plant demography.