Biodiversity effects on dune and salt marsh biogeomorphology – a trait-based approach Von der Fakultät für Mathematik und Naturwissenschaften der Carl von Ossietzky Universität Oldenburg zur Erlangung des Grades und Titels eines Doktors der Naturwissenschaften (Dr. rer. nat.) angenommene Dissertation von von Julia Bass geboren am 03. Februar 1989 in Bremen Oldenburg 2019 Gutachter Prof. Dr. Michael Kleyer Zweitgutachter Prof. Dr. Gerhard Zotz Tag der Disputation 13.11.2019 Contents Summary .........................................................................................................................1 Zusammenfassung ..........................................................................................................4 1 Introduction ............................................................................................................8 1.1 The concept of biogeomorphology ..................................................................8 1.2 The trait-based perspective in ecology ..........................................................13 1.3 Combining the biogeomorphic succession model and the functional trait approach .........................................................................................................18 1.4 Barrier islands and Halligen in the Wadden Sea ...........................................25 1.5 Research objectives .......................................................................................35 2 Morphological plasticity of dune pioneer plants in response to timing and magnitude of sediment disturbance ....................................................................38 2.1 Abstract ..........................................................................................................38 2.2 Introduction....................................................................................................39 2.3 Methods .........................................................................................................41 2.4 Results ...........................................................................................................46 2.5 Discussion ......................................................................................................56 3 Storm resistance and resilience of natural and artificial coastal foredune communities ..........................................................................................................64 3.1 Abstract ..........................................................................................................64 3.2 Introduction....................................................................................................65 3.3 Methods .........................................................................................................68 3.4 Results ...........................................................................................................74 3.5 Discussion ......................................................................................................80 i 4 Plant traits affect surface elevation change in salt marshes: an example of biogeomorphic ecosystems .................................................................................. 86 4.1 Abstract ......................................................................................................... 86 4.2 Introduction ................................................................................................... 87 4.3 Methods ......................................................................................................... 90 4.4 Results ........................................................................................................... 96 4.5 Discussion ................................................................................................... 101 5 Synthesis.............................................................................................................. 114 5.1 Summary of findings ................................................................................... 114 5.2 The “window of adaption” in dune pioneer species .................................... 115 5.3 Is the community weighted mean a suitable index for biogeomorphic ecosystems? ................................................................................................. 118 5.4 Insurance effect vs. keystone species .......................................................... 122 5.5 Implications for management and restoration – lessons learned for trait- based management of coastal dunes ............................................................ 124 5.6 Towards a new research horizon ................................................................. 130 Appendix .................................................................................................................... 133 References .................................................................................................................. 138 List of figures ............................................................................................................. 167 List of tables ............................................................................................................... 171 Danksagung ................................................................................................................ 174 Curriculum vitae ......................................................................................................... 176 Authors’ contributions ................................................................................................ 179 Erklärung .................................................................................................................... 180 ii Summary Summary Current climate crisis is associated with rising sea level and increasing frequency and strength of storm surges. This raises the concern of losing coastal ecosystems such as dunes and salt marshes to erosion and drowning. This is particularly serious as these ecosystems fulfill multiple ecosystem services including providing habitats for specialized plants and animals, attenuating wave energy, reducing erosion, and protecting areas of major socio-economic interest from flooding. Coastal dunes and salt marshes are biogeomorphic ecosystems where the form and function depend on the two-way interaction between geomorphic and biological processes. The primary geomorphic processes in coastal dunes are aeolian processes while salt marshes are shaped primarily by fluvial processes. These abiotic processes influence the vegetation and lead to responses on the community and the species level. In return, plants influence matter and energy flow and thus shape landscape formation. Despite strong research interest and a general understanding of biogeomorphic ecosystems, there is still a lack of knowledge in a number of aspects of vegetation responses and effects in these ecosystems. Morphologic plasticity of plants is known to help species survive in dynamic and changing environments such as biogeomorphic ecosystems. However, specific trait responses of pioneer species to different physical disturbances are not yet fully known. In biodiversity and ecosystem functioning research it is generally believed that diversity increases ecosystem’s resistance and resilience by means of the insurance effect. However, biogeomorphic ecosystems are an exception as it is often the resistance and resilience of single keystone species that are the main drivers for ecosystem stability. Controlled experiments on this matter, however, are lacking. The importance of vegetation in biogeomorphic ecosystems is generally accepted. Yet, there is an ongoing debate whether vegetation has an effect on sedimentation processes in salt marshes. To fill these research gabs, this thesis presents three different studies. To gain a better understanding of the responses of vegetation to abiotic factors, a controlled greenhouse experiment studying the morphological plasticity of two dune 1 Summary species is presented. Ammophila arenaria and Cakile maritima individuals were exposed to artificial sediment accretion and erosion events of varied magnitudes and times after germination. Mortality rate was lower for A. arenaria than for C. maritima. Growth was mostly increased through burial and erosion in C. maritima and decreased in A. arenaria. Further, a mesocosm experiment studying the effects of species and trait diversity on dune resistance and resilience is presented. The comparison of artificial dunes vegetated with A. arenaria only or with mixed-species communities showed that single-species dunes were more resistant to wind erosion than were mixed-culture dunes. Surviving species showed no responses in their trait expression after the disturbance event, i.e. no difference in resilience. Lastly, a study looks at the effects of plant functional traits on sedimentation and erosion in salt marshes. Three salt marsh systems in the Wadden Sea were analyzed: natural marshes on a barrier island, moderately anthropogenic influenced mainland marshes, and highly anthropogenic influenced marsh island. Mean surface elevation change was found to be not sufficient to keep up with current sea-level rise. However, there are pronounced local differences in surface elevation change, a part of which can be explained by distance to marsh edge and plant functional traits. Higher vegetation roughness increases sedimentation; stronger anchoring capacity can reduce erosion. These three studies