Elevated micro-topography boosts growth rates in Salicornia procumbens by amplifying a tidally driven oxygen pump: implications for natural recruitment and restoration

Abstract

Background and Aims The growth rate of pioneer species is known to be a critical component determining recruitment success of marsh seedlings on tidal flats. By accelerating growth, recruits can reach a larger size at an earlier date, which reduces the length of the disturbance-free window required for successful establishment. Therefore, the pursuit of natural mechanisms that accelerate growth rates at a local scale may lead to a better understanding of the circumstances under which new establishment occurs, and may suggest new insights with which to perform restoration. This study explores how and why changes in local sediment elevation modify the growth rate of recruiting salt marsh pioneers. Methods A mesocosm experiment was designed in which the annual salt marsh pioneer Salicornia procumbens was grown over a series of raised, flat and lowered sediment surfaces, under a variety of tidal inundation regimes and in vertically draining or un-draining sediment. Additional physical tests quantified the effects of these treatments on sediment water-logging and oxygen dynamics, including the use of a planar optode experiment. Key Results In this study, the elevation of sediment micro-topography by 2 cm was the overwhelming driver of plant growth rates. Seedlings grew on average 25 % faster on raised surfaces, which represented a significant increase when compared to other groups. Changes in growth aligned well with the amplifying effect of raised sediment beds on a tidally episodic oxygenation process wherein sediment pore spaces were refreshed by oxygen-rich water at the onset of high tide. Conclusions Overall, the present study suggests this tidally driven oxygen pump as an explanation for commonly observed natural patterns in salt marsh recruitment near drainage channels and atop raised sediment mounds and reveals a promising way forward to promote the establishment of pioneers in the field.

Need to Seed? Ecological, Genetic, and Evolutionary Keys to Seed-Based Wetland Restoration

“…Microtopography is small-scale vertical relief that provides a diversity of safe sites for seeds and seedlings (Urbanska, 1997;Peach and Zedler, 2006). Diverse microtopography can protect seedlings by providing areas with varying environmental conditions (i.e., differing temperatures, soil characteristics, redox levels, and water retention abilities) and has been shown to improve plant establishment, survival, and species richness in restorations (Urbanska, 1997;Peach and Zedler, 2006;Fivash et al, 2020;Mossman et al, 2020;Ott et al, 2020). Furthermore, the heterogeneity at a site can result in more complex plant communities and facilitate the development of ecosystem functions (Larkin et al, 2016).…”

Restoration of biogeomorphic systems by creating windows of opportunity to support natural establishment processes

“…One strong candidate abiotic variable with which to manipulate seedling establishment may be found in the pioneer zone of salt marshes. Here, raised sediment surfaces are reported to have growth‐amplifying effects on young salt‐marsh recruits (Fivash et al 2020 , Mossman et al 2020 ). This may be due to special conditions within raised sediment platforms, from which water can easily drain over the low‐tide interval (Fagherazzi and Mariotti 2012 ).…”

The onset of secondary seed dispersal is controlled by germination‐features: A neglected process in sudden saltmarsh establishment

“…These structures act as "barriers" or "shelters," which can effectively trap and retain (positively buoyant) seeds when they periodically contact with the substrate during the semi-diurnal tidal cycle (Wang et al 2018;Xie et al 2019;Qiu et al 2021). Interestingly, recent studies have reported that seedling establishment at new sites tends to occur on raised micro-topography covered by biostabilizers (such as diatom biofilms and fibrous algae Vaucheria; Fivash et al 2020;Van de Vijsel et al 2020), implying that biostabilizers may also provide a viable conduit for trapping and retaining the dispersed (germinated) seeds. Moreover, these biostabilizers can shape the local microhabitat by affecting sediment erodibility (Van de Vijsel et al 2020), water content, and dissolved oxygen content (Fivash et al 2020), and may therefore regulate the rooting rate of seeds, affecting their anchoring ability.…”