Controls on the sexual and asexual regeneration of Salicaceae along a highly dynamic, braided river system

Abstract

Salicaceae are key pioneer riparian tree species that have the ability to reproduce sexually and asexually. Recent research has suggested that Salicaceae act as ecosystem engineers, modifying hydrological and geomorphological processes, resulting in the stabilisation and growth of landforms. Understanding these interactions requires knowledge of the controls on Salicaceae regeneration. This paper describes a study of Salicaceae establishment and growth along a reach of a highly dynamic, island-braided river. The sexual and asexual regeneration of three species were investigated using experimental planting of cuttings and observation of seedlings. Plots were located at a range of elevations, in different habitats associated with the established riparian vegetation and in contrasting sediment types. Survival and growth were monitored over two growing seasons. Asexual regeneration was more successful than sexual regeneration, with cuttings demonstrating faster growth rates and tolerance of broader environmental conditions than seedlings. Cutting survival and growth was highest in sediments with a relatively high organic content and in plots located between patchy Salicaceae stands or in the lee of islands. Seedling mortality was extremely high due to fluvial disturbance, although seedlings in habitats that were protected from fluvial disturbance survived. Seedling growth showed preferences for particular sedimentary conditions, which varied between species. The major control on regeneration was the upstream presence of established Salicaceae, particularly on islands, which provided open sites that were protected from fluvial disturbance and suitable for regeneration. Thus, asexual regeneration facilitated sexual regeneration by rapidly colonising sites that provided habitats protected from fluvial disturbance for seedling establishment. This supports previous work suggesting that Salicaceae can act as ecosystem engineers.

The role of large wood in retaining fine sediment, organic matter and plant propagules in a small, single-thread forest river

“…Furthermore, Pettit et al (2006) surveyed some of the 200,000 wood piles deposited along a 105-km length of the Sabie River's bedrock macrochannel following a very large flood and found that vegetation regeneration from seedlings was occurring in 28% of wood piles and that wood was sprouting in 36% of the piles. The substantial vegetated patches that are produced by these processes not only influence further vegetation colonization and associated landform development but the resulting vegetated landforms can provide sufficient shelter from disturbance for areas of exposed sediment downstream and that seedlings can successfully initiate vegetation colonization of these sheltered areas (Moggridge and Gurnell, 2009). However, to our knowledge, no direct assessment of seed bank development associated with wood jams has been undertaken to date.…”

Fluvial islands: First stage of development from nonmigrating (forced) bars and woody-vegetation interactions

“…As shown by Osterkamp (1998) and Gurnell et al (2001), established fluvial islands can result from several processes among which excision of floodplain deposits or building involving dead or living woody vegetation present on channel bars (Osterkamp, 1998;Kollmann et al, 1999;Gurnell et al, 2001Gurnell et al, , 2012Nakayama et al, 2002;Bertoldi et al, 2011;Mikuś et al, 2013). For this latter case, the regeneration process of vegetation on flow resistance and sediment deposition influences the island creation: sprouting from living driftwood exerts a rapid and strong feedback on physical parameters whereas seedling impact is reduced during their first stage of development (Moggridge and Gurnell, 2009). Woody pioneer vegetation can recruit or sprout on two general types of alluvial bars, namely free (or migrating) and forced (nonmigrating) bars (see review in Rodrigues et al, 2015).…”

Are Vegetative Reproduction Capacities the Cause of Widespread Invasion of Eurasian Salicaceae in Patagonian River Landscapes?

“…S. cinerea, S. babylonica and S. fragilis spreading aggressively [11], [8], [12]. Salicaceae are known to alter fluvial dynamics and to facilitate the development and growth of sand bars and islands [13] and thus have traits that could alter the ecosystem profoundly if they are invasive. Other possible consequences of invasive willows are the displacement of native vegetation resulting in a loss in biodiversity, the obstruction and diversion of streams and consequent erosion [8].…”