Spatial-temporal dynamics of islands in a sandy, braided river: the role of riparian vegetation

Thirty-six islands in the Tiétar River (Spain) were monitored through morphological and vegetation-related traits measured remotely and in the field. For the former, we used aerial photographs from 1956 to 2019. For the latter, two sampling campaigns were carried out in 2016 and 2022. Additionally, we characterized the flow regime of the last decades. The results show that islands are formed along incised areas where willow seeds accumulate, germinate, and get established, and where moisture remains during summer. Willow seedlings increase the roughness, favoring accumulation of sand, and when this process is maintained in time, a vegetated island may arise. Island formation starts with the establishment of a single pioneer species (Salix salviifolia), and if the island resists subsequent floods, it continues with the colonization of other species (Fraxinus angustifolia, Celtis australis, Sambucus nigra) as part of an ecological succession. Island size increases correspond to the growth of colonizing woody plants, while size reductions are due to dead individuals or if uprooting occurs. Older islands have greater heights and support greater biodiversity, while younger islands are only covered by willows. Overall, we found that woody vegetation coverage is continuously increasing in both studied segments, and that some exotic plant species are progressively invading mature islands.