Testing the effect-response framework: key response and effect traits determining above-ground biomass of salt marshes

Question How do species traits respond to environmental conditions and what is their effect on ecosystem properties? Location Salt marshes, Northwest Germany. Methods On 113 plots along the German mainland coast and on one island, we measured environmental parameters (soil nutrient content, inundation frequency, groundwater level and salinity), collected traits from 242 individuals (specific leaf area [SLA], whole plant C:N ratio, and dry weights of plant organs) and sampled above-ground biomass as an ecosystem property. We constructed a path model combining environmental parameters, functional traits at community level and above-ground biomass, which was tested against a dependence model using path analysis; model fit was evaluated by structural equation modelling (SEM). Results The final model showed good consistency with the data and highlights the major role of groundwater level, salinity and nutrient availability as the most important factors influencing biomass allocation in salt marshes. Above-ground living biomass was mostly determined by stem biomass, which was mediated through an allometric allocation of biomass to all other plant organs, including leaf mass. C:N ratio and SLA were the major drivers for dead biomass. Conclusion We emphasize an indirect link between standing biomass and environmental conditions and recognize stem biomass, plant C:N ratio and SLA as keystone markers of species functioning in determining the relationship between environment and ecosystem properties.