Propagule deposition along river margins: linking hydrology and ecology

Abstract

Summary 1.Research has highlighted the importance of buoyancy for hydrochorous propagule dispersal, but recently the river bed has been identified as a significant store of viable propagules. 2. Over four consecutive 4-month periods, deposited propagules (predominantly seeds) and sediment were sampled at 78 river bed, bank face and bank top sites within three river reaches in two catchments. Species of deposited propagules were compared with the propagule bank and standing vegetation to identify 'new' species. 3. Forty-four percent of deposited propagule species were not present in the vegetation and the largest proportions were deposited in winter. New propagules showed a higher proportion of species that establish long-term seed banks and produce light seeds, a higher proportion of Rstrategists, and a lower proportion of C-strategists than species in the vegetation. 4. Ordination of propagule species abundance data revealed differences in samples between reaches; bed and bank locations; and sampling periods. Within depositional samples there were significant positive correlations between average C scores and the proportion of species producing relatively heavy, short-lived seeds and scores on an ordination axis describing a gradient from channel bed to bank top. 5. Correlations and multiple regression models between species richness and abundance of deposited propagules (total and new), and the quantity and calibre of river-deposited sediment, demonstrated a direct link between propagule deposition and hydraulic conditions during inundation. 6. Synthesis . Riparian connectivity of river flows, varying hydraulic conditions and temporary storage of propagules are all complex components of hydrochorous propagule dispersal in the study reaches. Flood flows can transfer sediment particles and non-buoyant, viable propagules from the river into the riparian zone. Propagules can pass in and out of storage within the channel bed and margins through a variety of dispersal processes and can then settle out of the water in suitable depositional environments generating distinct spatial and temporal patterns in total propagule deposition. New propagules show greatest floristic similarity to propagule bank and bed samples. They are preferentially smaller, lighter and form longer term seed banks, making them particularly suitable for long distance transport over prolonged time periods and for widespread dispersal across river margins.

Modelling of floating seed dispersal in a fluvial environment

“…In this paper we have focused on one aspect of seed transport, the dispersal of floating seeds. However, to understand the movement of all fluvially dispersed seed, further modelling needs to be carried out on propagules that either move suspended in the water column or saltate along the bottom of the channel, similar to suspended sediment and bedload movement (Groves et al, 2007;Gurnell, 2007;Gurnell et al, 2008) Further development and calibration of the model is required to establish the reliability of the coefficients across a wider range of flow regimes and landscapes. Specifically we suggest additional research should be done on (1) microscale impacts that influence seed transportation and deposition; (2) variance in the dispersal kernel with different seed and propagule sizes; and (3) the impact of overbank flow on fluvial seed transport.…”

Invasion legacy effects versus sediment deposition as drivers of riparian vegetation

“…Over-winter sediment deposition following floods is widely regarded as an important mechanism for plant dispersal and colonisation in riparian habitats (Andersson and Nilsson 2002;Gurnell et al 2008). Large quantities of plant propagules have been recorded in freshly deposited sediment which, by creating gaps, may provide recruitment opportunities in aboveground vegetation, yet the potential link between IAPs and sediment deposition has received limited attention (Baattrup-Pedersen et al 2013).…”

Understanding reference processes: linkages between river flows, sediment dynamics and vegetated landforms along the Tagliamento River, Italy

“…Surface hydrological connectivity between landscape elements supports the exchange of matter, energy and organisms across the river landscape (e.g. Ward et al, 1999;Pringle, 2003;Paetzold et al, 2006;Gurnell et al, 2008;Indermaur et al, 2009), maintains hydraulic refugia as river flows fluctuate (e.g. Tockner et al, 2000;Van der Nat et al, 2002) and drives surface-subsurface exchanges of water, sediment and nutrients as well as moderating water temperatures within different water bodies and channel segments (e.g.…”