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. © 2018 John Wiley & Sons, Ltd.

Physical engineering of an island‐braided river by two riparian tree species: Evidence from aerial images and airborne lidar

“…However, the four‐year window of opportunity between the 2004 and 2008 flood events probably also allowed newly recruited trees within the second phase of vegetation development to develop anchoring root systems and above‐ground biomass that could trap fluvial sediments during flow pulses and aggrade bar surfaces, so lowering their risk of flood inundation (Gurnell, 2016). Aggradation around vegetation of 0.5 m in the first 5 years of growth and 1.5 – 2 m over 25 years has been observed in the study reach (Gurnell et al, 2019). Hortobágyi et al (2018), following Eichel, Corenblit, & Dikau (2015), have described such periods of landform building as “biogeomorphic feedback windows”, providing trees with elevational protection from floods and finer, moisture‐retentive sediments within which to develop extensive root systems (Holloway et al, 2017; Holloway, Rillig, & Gurnell, 2017a, 2017b).…”

“…The spatially aggregated data for woody vegetation in general within areas I, II, and III, disguises the detail of interactions with the underlying river bed. Nevertheless, previous research at finer spatial scales within the study reach (Gurnell et al, 2019) and from comparisons of 1 km segments of a 20 km length of the river containing the study reach (Bertoldi, Gurnell, & Drake, 2011) have already clearly established a topographic signature of P . nigra ‐dominated landform development across time and space.…”

“…Furthermore, although the role of P . nigra as a physical ecosystem engineer, driving island development along the Tagliamento has been widely reported (e.g., Francis et al, 2009; Gurnell, 2016; Gurnell et al, 2005; Gurnell, Bertoldi, Francis, Gurnell, & Mardhiah, 2019; Gurnell & Petts, 2002, 2006; Zen, Gurnell, Zolezzi, & Surian, 2017), the role of A . incana is unknown.…”

Biogeomorphology, quo vadis? On processes, time, and space in biogeomorphology

“…(2018, 2019), Gurnell et al . (2019), and Corenblit et al . (2020) focus on the relationship between successional vegetation and sediment transport in fluvial systems.…”

Wood process domains and wood loads on floodplains

“…At the river-reach scale of 10 0 -10 3 km lengths of river corridor (river corridor here refers to the active channel and floodplain), large wood influences the formation of channel bars and islands, and their eventual accretion to the floodplain (e.g. Gurnell and Petts, 2006;Gurnell et al, 2016Gurnell et al, , 2019. The initial deposition of wood in the channel, followed by accretion to the floodplain, illustrates the close coupling of channel-floodplain wood dynamics.…”