This thesis focuses on functional traits of salt marsh plants. Plant traits most responsive to environmental constraints were identified and tested for their applicability to trait-environmental concepts. Trait-trait relationships were investigated and ultimately, the effects of plant traits on ecosystem properties were assessed and the role of biodiversity in the maintenance of those ecosystem properties was highlighted. The study was conducted in salt marshes along the mainland coast of Northwest Germany and on the island of Mellum. The most important findings of this study were that nutrient availability and water-related environmental conditions most strongly influence trait expressions of salt marsh plants. Morphology-based traits were partly different from those of other terrestrial plants and trait-trait relationships of element-based traits were consistent to the findings of other studies. Different plant traits were found to affect different ecosystem properties. Multifunctionality of salt marshes can be ensured by conserving functional diversity via species diversity, and its loss might adversely affect adjacent ecosystems, like the Wadden Sea system.