Is saltmarsh restoration success constrained by matching natural environments or altered succession? A test using niche models

Abstract

Restored habitats, such as saltmarsh created through managed realignment, sometimes fail to meet targets for biological equivalence with natural reference sites. Understanding why this happens is important in order to improve restoration outcomes.Elevation in the tidal frame and sediment redox potential are major controls on the distribution of saltmarsh plants. We use niche models to characterize 10 species’ responses to these, and test whether differences in species occurrence between restored and natural saltmarshes in the UK result from failure to recreate adequate environmental conditions.Six species occurred less frequently in recently restored marshes than natural marshes. Failure of restored marshes to achieve the elevation and redox conditions of natural marshes partially explained the underrepresentation of five of these species, but did not explain patterns of occurrence on older (>50 years) restored marshes.For all species, an effect of marsh age remained after controlling for differences in environmental conditions. This could be due to differences in successional mechanism between restored and natural marshes. In recently restored marshes, high‐marsh species occurred lower in the tidal frame and low‐marsh species occurred higher in the tidal frame than in natural marshes. This supports the hypothesis that competition is initially weaker in restored marshes, because of the availability of bare sediment across the whole tidal frame. Species that establish outside their normal realized niche, such as Atriplex portulacoides, may inhibit subsequent colonization of other species that occurred less frequently than expected on older restored marshes. Synthesis and applications. Niche models can be used to test whether abiotic differences between restored sites and their natural counterparts are responsible for discrepancies in species occurrence. In saltmarshes, simply replicating environmental conditions will not result in equivalent species occurrence.

Manipulating saltmarsh microtopography modulates the effects of elevation on sediment redox potential and halophyte distribution

“…Plots were sampled in October 2013, four growing seasons after the experiment was set up. Species ordered by the median elevation (from lowest to highest) that they occurred at on natural marshes from Sullivan et al ()…”

“…Differing letters indicate significant differences between treatments (GLM p < 0.001, Tukey post hoc test). Species ordered by the median elevation (from lowest to highest) that they occurred at on natural marshes (from Sullivan et al, ); Armeria was not modelled by Sullivan et al (), so its average elevation was taken from our data…”

“…This result could be due to reduced competition in our plots, which were initially unvegetated, allowing species’ distributions to better reflect their fundamental niche (i.e. where they can survive), whereas on a natural marsh they may be outcompeted (Sullivan et al, ). Atriplex was the only species sufficiently common as a natural colonist to allow us to compare the responses of planted and naturally colonized individuals.…”

“…Generally, survival of planted species was high in raised plots, where redox potential was higher. However, in natural marshes, several of the planted forb species ( Limonium , Plantago , Triglochin ) are associated with pannes, depressed areas with low redox potential higher in the tidal frame (Ewanchuk & Bertness, ; Sullivan et al, ). These differences may reflect niche segregation induced by competition from the shrubby dominant, Atriplex portulacoides , which had very high survival in raised plots and is known to be associated with high redox potential (Crooks et al, ; Davy et al, ).…”

“…Previous work has shown that some, mainly annual, species colonize managed‐realignment marshes rapidly. However, many of the characteristic perennials may be dispersal‐limited or suffer poor establishment (Mossman, Brown, Davy, & Grant, ; Mossman, Davy, & Grant, ; Sullivan, Davy, Grant, & Mossman, ). For these species, investigation of the effects of local topography on performance must involve planting.…”