Riparian tree establishment and river island formation within the active zone of the River Tagliamento, Northeast Italy

Within the active zone of rivers, riparian vegetation dynamics are controlled by ecological and hydro geomorphological processes and their interactions. Within Europe, understanding of natural riparian tree establishment has been limited by a lack of unregulated, natural river systems. This thesis examines key biotic and abiotic factors influencing the establishment of three riparian tree species, Populus nigra, Salix elaeagnos and Alnus incana, that grow within the active zone of the River Tagliamento, a morphologically-intact alpine/mediterranean river in Northeast Italy. Field observation, field experimentation and greenhouse experiments indicate: (i) Interspecific and propagule form variations in survival and growth response to hydrogeomorphic conditions: P. nigra cuttings performed best in low free-draining sites with a small proportion of fine sediment but which were not subject to prolonged inundation, while seedlings preferred fine sediments and higher elevations. Salix elaeagnos cuttings performed best at lower elevations, with a preference for mixed sediments, and proved tolerant to drought and inundation. S. elaeagnos seedlings grew best at higher elevations and in coarse/mixed substrates. Alnus incana cuttings only grew in fine sediments under stable water levels in greenhouse experiments; mortality was total in field experiments. Deposited trees of an species survived better at lower elevations. (ii) Differences in growth rates according to propagule type: cuttings grew twice as fast as seedlings in the first year but not in the second, and whole deposited trees produced far higher levels of shoot growth and biomass. (iii) Timing of deposition in the growing season and size of propagule also significantly affected performance. These results suggest that species and propagule form directly influence the rate of vegetation establishment within the active zone of rivers and that plant physiology alongside hydrogeomorphology is critical to riparian vegetation dynamics. These results are explored in the context of the development of river islands, an endangered but ecologically important type of landform once common within natural, braided European rivers.