Colonization of a newly developing salt marsh: disentangling independent effects of elevation and redox potential on halophytes

Abstract

Summary1. Many characteristics of the salt marsh environment covary with elevation. It has therefore proved difficult to determine which environmental characteristics limit the distributions of particular species in the field. Oxygen supply to the rhizosphere may be particularly important, as it is determined by the duration and frequency of flooding. 2. The re-activation of a salt marsh by managed coastal realignment provided an opportunity to investigate the large-scale manipulation of environmental effects on halophyte distribution in a situation where the usual relationships between environmental characteristics, elevation and succession had been partially uncoupled. 3. Most locations sampled lay between mean neap and mean spring tidal levels. As expected, anoxic conditions occurred at lower elevation, redox potential increased generally with elevation and sediments were oxic on the upper parts. However, sediment oxygenation at any given elevation was variable, particularly at intermediate levels in the tidal range. This imperfect correlation between elevation and sediment redox allowed quantification of their independent effects on species distributions using the statistical technique of Hierarchical Partitioning. 4. Effects of elevation and sediment redox potential were distinguishable from each other. Salicornia europaea occurred predominantly at lower elevation but was not influenced by redox potential. Puccinellia maritima favoured low redox potentials independently of elevation. In contrast, Suaeda maritima tolerated a wide range of elevations but was absent from areas with low redox potential. Atriplex portulacoides was apparently more averse to low redox potential than to low elevation. Elytrigia atherica was restricted to both high redox potential and high elevation. Smaller independent effects of sediment depth, salinity, water content, nitrate concentration, shear strength and loss on ignition were apparent for some species. 5. Synthesis. Although much of the elevational zonation of species on salt marshes is mediated by differential tolerance of the consequences of co-linearly varying variables, particularly sediment anoxia and elevation, these variables have independent effects that are quantifiable in the field. Hierarchical Partitioning provides a valuable tool for distinguishing the mechanisms underlying species zonations on environmental gradients, especially where large-scale environmental manipulations have partially decoupled the usual co-linear variation.

Salinization during salt-marsh restoration after managed realignment

“…The strong correlation between elevation and chloride concentration found in this study corresponds with observations both in natural salt marshes (e.g. Callaway et al 1990;Bockelmann et al 2002) and de-embanked sites (Davy et al 2011). A striking observation was the initially lower chloride concentration at the same elevation at the de-embanked site compared to the more seaward-located reference site.…”

“…de-embankment) involves reintroducing tidal inundation to formerly reclaimed land, through the breaching of coastal embankments. Several studies have evaluated the effects of de-embankment on vegetation composition (Wolters et al 2005;Garbutt and Wolters 2008;Hughes et al 2009;Mossman et al 2012a) and environmental characteristics such as inundation frequency and soil redox potential (Pétillon et al 2010;Davy et al 2011;Mossman et al 2012b). However, the process of salinization after de-embankment has rarely been studied.…”

“…This explains the generally negative relationship between elevation and salinity. However, marsh topography also strongly affects the frequency and duration of water retention (Davy et al 2011;Veeneklaas 2013). At the meso-scale within a salt marsh, the vegetation pattern may be strongly influenced by fluvial geomorphic processes, such as drainage patterns in relation to tidal creeks (Zedler et al 1999).…”

Global change effects on decomposition processes in tidal wetlands: implications from a global survey using standardized litter

“…This finding and its implication may occur counterintuitive with respect to the often sharp 360 redox gradients along tidal wetland zonations and with flooding (Davy et al, 2011;Langley et al, 2013), and the mechanism by which S is decreased in the more flooded zones is unknown. Because we did not observe consistent salinity effects on S and k in our data (Figs.…”

Invertebrate Community Composition Across Inundation Regimes and Its Potential to Reduce Plant Stress

“…Nearly 50% of global salt marshes have been lost or severely degraded due to coastal development and land reclamation (Davy et al 2011, Simenstad et al 2011.…”