Large Wood Dynamics of Complex Alpine River Floodplains

Abstract

Despite a considerable amount of literature on large wood (LW) in freshwater ecosystems, its dynamic nature in large rivers has hardly been investigated. Our study focused on the mass and turnover of LW in braided floodplains of the Tagliamento River (northeastern Italy), the last morphologically intact large river flowing out of the Alps. LW masses and turnover were quantified by establishing 165 permanent plots (100 m² each) and then revisiting them after 4 floods of differing magnitude. The following hypotheses were tested: 1) presence of vegetated islands increases LW densities; 2) masses of LW remain constant through time; 3) species composition of LW matches the species composition of woody plants on vegetated islands; 4) lateral erosion is the most significant source of LW; and 5) the probability that a LW deposit survives a flood increases with the presence of islands, the size of the deposit, and its location within vegetation, but decreases with flood magnitude. During the study, LW mass was high and constant in the island-braided reach, reaching values (100-150 t/ha) comparable to those reported for pristine mountain streams. In the bar-braided reach, LW mass was significantly lower (15-70 t/ha) and more variable. Although the total quantities of LW on the floodplain remained relatively constant, turnover rates of LW were very high (up to 95% during one major flood). An analysis of the species composition of LW showed that, although it was similar to that of woody plants on vegetated islands, at least 30% of the wood originated from upstream. Analysis of deviance from stepwise forward logistic regression models showed that the probability that a LW deposit survives a flood depends on flood magnitude, deposit volume, and position of the LW within the channel.