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

Summary 1. 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.