No abstract

Highlights-Salt marsh vegetation can reduce near-bed orbital velocities during storm surges-Vegetation effect on orbital velocities varies with biophysical properties-Flexible low-growing plant canopies show high resilience to storm surge conditions-More rigid and tall grasses experience stem folding and breakage-The contribution of vegetation to wave dissipation is plant species specific

The three-dimensional structure of salt marsh plant canopies, amongst other marsh surface characteristics, is of critical importance to the functioning and persistence of coastal salt marshes. Together with plant flexibility it controls the contribution of vegetation to the tidal flow and wave energy dissipation potential of marshes. However detailed information on these two key biophysical properties of salt marsh canopies is scarce. In this paper we present biophysical properties of four plants commonly occurring in NW European salt marshes. We measured stem flexibility, diameter and height of the grasses Spartina anglica, Puccinellia maritima and Elymus athericus and above ground biomass and canopy height in stands of Elymus athericus and the dwarf shrub Atriplex portulacoides. Further we compared the performance of two methods for the non-destructive assessment of above ground biomass, such that they may be used during field assessments of marsh surface vegetation structure (i) Measurement of light availability within the canopy and (ii) side-on photography of vegetation. All data were collected on a salt marsh on the Dengie Peninsula, eastern England, UK, in summer (July). We found significant differences in stem flexibility both between species and between the different parts of their stems. P. maritima was found to be the species with the most flexible stems, and, as a result of their relatively large stem diameter, S. anglica the species with the stiffest stems. Above ground biomass and hence potential canopy resistance to water flow could be estimated more accurately by side-on photography of vegetation than from measurement of light availability within the canopy. Our results extend the existing knowledge base on plant properties with relevance to studies of habitat structure and ecosystem functioning as well as wave energy dissipation in salt marsh environments and can be used for the development of a more realistic representation of vegetation in numerical models and laboratory flume studies of plant-flow interactions. Highlights  Paper reports quantitative data on plant flexibility and above ground biomass (a proxy for vegetation structure), in salt marsh canopies. Both these biophysical properties of salt marsh canopies need to feed into flow and wave dissipation models, if the predictive capacity of such models is to be improved.  Stem flexibility of salt marsh plants differs significantly both between different species and between the different stem parts of specimens of one species.  Side-on photography of vegetation is an appropriate technique for non-destructive assessment of above ground biomass and vegetation structure in structurally complex salt marsh canopies.  Above ground biomass and its vertical distribution within the canopy can be estimated more accurately by side-on photography than by measurement of light availability in the canopy.

A full-scale controlled experiment was conducted on an excavated and re-assembled coastal wetland surface, typical of floristically diverse northwest European saltmarsh. The experiment was undertaken with true-to-scale water depths and waves in a large wave flume, in order to assess the impact of storm surge conditions on marsh surface soils, initially with three different plant species and then when this marsh canopy had been mowed. The data presented suggests a high bio-geomorphological resilience of salt marshes to vertical sediment removal, with less than 0.6 cm average vertical lowering in response to a sequence of simulated storm surge conditions. Both organic matter content and plant species exerted an important influence on both the variability and degree of soil surface stability, with surfaces covered by a flattened canopy of the salt marsh grass Puccinellia experiencing a lower and less variable elevation loss than those characterized by Elymus or Atriplex that exhibited considerable physical damage through stem folding and breakage.