Biophysical properties of salt marsh canopies — Quantifying plant stem flexibility and above ground biomass

• Paper reports data on plant flexibility and evaluates methodologies to assess biomass and structure of salt marsh canopies
• Plant flexibility differs significantly between species and between the different stem parts of specimens of one species
• Side-on photography provides better estimates of biomass and vegetation structure than measurements of light-availability
• Plant flexibility and canopy bulk properties need to feed into flow/wave dissipation models to improve predictive capacity

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.