Breeding bird communities of riparian and upland woodlands respond differently to an Atlantic-Mediterranean climatic gradient in Mainland Spain

Riparian habitats are key landscape elements promoting regional biodiversity, particularly in areas where water availability is scarce. In Mediterranean Iberia, the microclimatic conditions that occur near rivers support the development of humid riparian woodlands, which differ markedly from those growing in equivalent upland areas but share structural and compositional similarities with higher-latitude (Atlantic) formations. However, the effects of this variation on the animal species assemblages inhabiting each woodland type have not been well-documented. We characterized the vegetation of riparian and upland woodlands (general structure and composition) and their avian breeding communities (diversity, climatic and habitat preferences) in 38 sites across mainland Spain to explore how these characteristics changed across a climatic gradient of increasing summer drought. Our results revealed that the prevalence of deciduous woody plants decreased with increasing summer drought. However, this reduction was less steep in riparian than in upland woodlands, reinforcing the idea that Mediterranean riparian areas act as microclimatic refugia where more Atlantic (deciduous) woody species can persist. We also found a lower similarity between riparian and upland bird assemblages in areas with higher indices of summer drought, making riparian woodlands comparatively more diverse than upland woodlands in the Mediterranean. This pattern was partly attributed to the presence of deciduous-tree-related bird species that could not persist in equivalent upland woodlands. These findings suggest that restoring habitat of riparian woodlands would provide refuges with suitable conditions for many woodland species whose persistence on the limit of their distribution range in southern Iberia is threatened by global warming.