Understanding processes of island development on an island braided river over timescales from days to decades

Abstract

Bar colonization by vegetation and subsequent island formation is a key bio‐geomorphological process in fluvial landscape evolution. Here we investigate morphological and ecological evolution of river islands over timescales from single floods to decades, focusing on islands initiated by deposited trees that sprout to form vegetated patches.

On a braided reach of the high‐energy Tagliamento River, Italy, we monitored 30 pioneer islands of 1 to 17 years age in comparison with unvegetated bar surfaces, open areas between islands, and established island surfaces. We integrated morphological, surface sediment and vegetation properties of islands initiated by different flood events, combining evidence from remotely‐sensed and ground observations, flow and climate time series.

At a decadal timescale, pioneer islands aggrade rapidly to the elevation of the mean annual flood, showing a steady increase in vegetation canopy height, fining of surface sediments from predominantly gravel to silty‐sand with a notable clay and organic fraction. The standing vegetation included over 130 species, with the largest number on island surfaces of intermediate elevation and flood disturbance. As islands age, standing vegetation becomes comprised mainly of competitor species with transient seed banks and typical of woodland, scrub, pasture and wetland habitats, whereas the winter seedbank is dominated on all surfaces by ruderal species with persistent seedbanks, mainly associated with aquatic, wetland, pasture, arable and wasteland habitats. At shorter timescales, the biogeomorphological trajectory of pioneer islands is initiated by large flood events that control the elevation of deposited trees, and subsequent flows that control tree survival and establishment. Island morphological evolution depends on the frequency‐magnitude of sediment and seed delivery and redistribution by flood and possibly wind events, whereas island ability to retain sediments reflects the degree of vegetation establishment, which in the short‐term may vary with seasonal to annual moisture supply, substrate characteristics and climatic growth conditions.