Initial establishment of vegetative fragments within the active zone of a braided gravel-bed river (River Tagliamento, NE Italy)

Abstract

The sprouting of vegetative propagules disseminated hydrochorously throughout the active zones of braided rivers is a potentially important method of riparian tree establishment for some species. The hydrogeomorphological characteristics of the immediate local environment (patch), upon which the propagules are deposited, can have a significant influence on survivorship and shoot growth. We conducted a field experiment along the River Tagliamento, Italy during 2003, which involved the planting of Salix elaeagnos and Populus nigra cuttings (vegetative fragments) within exposed sediment patches differing in their relative elevations and sedimentary characteristics. Both species are found in riparian systems throughout Europe and are employed in riparian restoration/conservation schemes. The experiment was designed to examine the influence of environmental factors on the survivorship and shoot growth of these species immediately following propagule deposition. The study plots were comprised of a mix of silt, sand, and gravel. Small amounts of clay and organic matter (maximum 1 % and 1.3%, respectively, by weight) were present. Relation between survivorship and growth of the two species and environmental characteristics were explored using a combination of Principal Components and regression analyses. Probability of survival for both species was found to be greater in patches containing relatively greater amounts of clay and organic matter and low proportions of sand in relation to gravel. However, Salix elaeagnos had greater survival probability at lower elevations with relatively higher moisture levels, while the reverse was true for P. nigra. Both species were similar in their growth response to environmental variables, growth primarily being supported by relatively higher levels of clay, organic matter, and moisture, and lower elevations. Populus nigra survivorship was greater than that of S. elaeagnos, although shoot growth from S. elaeagnos exceeded that of P. nigra. The study highlights the influence of abiotic factors within a river’s active zone on vegetative sprouting and indicates the interspecific variations that can be observed in riparian tree species with similar life-history traits. The findings have implications for river management, restoration, and conservation efforts.