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Salt marshes are among the most valuable ecosystems, but currently suffering from massive habitat shrinkage. Conservation and restoration practices oriented toward salt marsh ecosystems are in full swing around the world. However, outcomes of such practices were often elusive because of the erratic demographic loss or mortality during the early recruitment stages. Mechanistic insights into the underlying processes that enable or disable vegetation recruitment are thus indispensable cornerstones for decision-making in salt marsh conservation and restoration. Relative to asexual reproduction through tillering and cloning, significant advantages exist in rapidly establishing foundational vegetation and building population genetic diversity through seed-based recruitment strategies. In this study, we provide integrated experimental insights in the relative importance of biotic and abiotic factors in dominating seed-based salt marsh recruitment. After detaching from parent plant, the dispersal potential of seeds is primarily constrained by buoyancy period. In wind- and wave-dominated environments, seed dispersal speed would vary greatly depending on the wave magnitude, wind direction, as well as dispersal units’ type and morphology. The floating seeds would end up in microsites due to loss of buoyancy, interception of landscape elements and burial of sediments, with their subsequent re-movements typically tied to high-energy hydrodynamic disturbance and associated erosion events rather than biotic traits. In spring, however, seeds would adjust their lift-off threshold, driven by regaining buoyancy and increasing the surface-drag forces during germination, thereby greatly increasing the probability of seed entrainment during calm hydrodynamic events. Seedling dislodgement would occur at any point during rooting, depending on species-specific and time-varying entanglement between stochastic disturbance pulses versus seedlings’ resistance, underscoring the significance of Windows of Opportunity (WoO; i.e., disturbance-free/low periods) for recruitment outcomes. In the early stages of establishment, root-shoot antagonism characterizes the growth rate of seedlings’ resistance to dislodgement by hydraulic disturbances. Subsequently, root length seizes the dominance and determines seedlings’ resistance to dislodgement by sediment erosion. Shallow burial by sediments during rooting would disproportionately strengthen the overall resistance of seedlings and amplify the effectiveness of WoO. Seedling displacement due to insufficient resistance does not necessarily purport the end of recruitment. Since dislodged seedlings are positively buoyant and able to sustain growth in water columns, they have the potential to drift with tidal currents over long distances. After arriving at a new settlement site, these dislodged seedlings may repeat similar processes, including retention, (re-)rooting, and survival, under analogous regulatory mechanisms as the initial establishment from seeds. Of particular note, the effectiveness of WoO in promoting rooting here is further regulated by the age of seedlings being dislodged, with seedlings dislodged at a younger age more likely to achieve re-establishment. Overall, this study contributes to the holistic understanding of both opportunities and bottlenecks involved in seed-based salt marsh recruitment. This mechanistic process study can be translated into practical guidelines for effective conservation and restoration of salt marshes through seed-based or seedling-based approaches, thereby contributing to incorporating salt marshes in nature-based applications such as coastal defense, climate stability, or carbon sequestration.

Salt marshes are among the most valuable ecosystems, but currently suffering from massive habitat shrinkage. Conservation and restoration practices oriented toward salt marsh ecosystems are in full swing around the world. However, outcomes of such practices were often elusive because of the erratic demographic loss or mortality during the early recruitment stages. Mechanistic insights into the underlying processes that enable or disable vegetation recruitment are thus indispensable cornerstones for decision-making in salt marsh conservation and restoration. Relative to asexual reproduction through tillering and cloning, significant advantages exist in rapidly establishing foundational vegetation and building population genetic diversity through seed-based recruitment strategies. In this study, we provide integrated experimental insights in the relative importance of biotic and abiotic factors in dominating seed-based salt marsh recruitment. After detaching from parent plant, the dispersal potential of seeds is primarily constrained by buoyancy period. In wind- and wave-dominated environments, seed dispersal speed would vary greatly depending on the wave magnitude, wind direction, as well as dispersal units’ type and morphology. The floating seeds would end up in microsites due to loss of buoyancy, interception of landscape elements and burial of sediments, with their subsequent re-movements typically tied to high-energy hydrodynamic disturbance and associated erosion events rather than biotic traits. In spring, however, seeds would adjust their lift-off threshold, driven by regaining buoyancy and increasing the surface-drag forces during germination, thereby greatly increasing the probability of seed entrainment during calm hydrodynamic events. Seedling dislodgement would occur at any point during rooting, depending on species-specific and time-varying entanglement between stochastic disturbance pulses versus seedlings’ resistance, underscoring the significance of Windows of Opportunity (WoO; i.e., disturbance-free/low periods) for recruitment outcomes. In the early stages of establishment, root-shoot antagonism characterizes the growth rate of seedlings’ resistance to dislodgement by hydraulic disturbances. Subsequently, root length seizes the dominance and determines seedlings’ resistance to dislodgement by sediment erosion. Shallow burial by sediments during rooting would disproportionately strengthen the overall resistance of seedlings and amplify the effectiveness of WoO. Seedling displacement due to insufficient resistance does not necessarily purport the end of recruitment. Since dislodged seedlings are positively buoyant and able to sustain growth in water columns, they have the potential to drift with tidal currents over long distances. After arriving at a new settlement site, these dislodged seedlings may repeat similar processes, including retention, (re-)rooting, and survival, under analogous regulatory mechanisms as the initial establishment from seeds. Of particular note, the effectiveness of WoO in promoting rooting here is further regulated by the age of seedlings being dislodged, with seedlings dislodged at a younger age more likely to achieve re-establishment. Overall, this study contributes to the holistic understanding of both opportunities and bottlenecks involved in seed-based salt marsh recruitment. This mechanistic process study can be translated into practical guidelines for effective conservation and restoration of salt marshes through seed-based or seedling-based approaches, thereby contributing to incorporating salt marshes in nature-based applications such as coastal defense, climate stability, or carbon sequestration.

Managed realignment is the landward relocation of flood infrastructure to re-establish tidal exchange on formerly reclaimed land. Managed realignment can be seen as a nature-based flood defence system that combines flood protection by the realigned dike (artificial) and restored saltmarshes (nature-based). So far, research on coastal managed realignment is primarily directed to saltmarsh restoration on formerly reclaimed land. This study focuses on the realigned dikes. The aim of this research is to characterize realigned dikes and to indicate the characteristics that offer opportunities for nature-based flood protection. We categorized 90 European coastal managed realignment projects into two realigned dike groups: (1) Newly built landward dikes and (2) Existing landward dikes of former multiple dike systems. The second group has two subcategories: (2a) Former hinterland dikes and (2b) Realignments within summer polders. For each group we present the realigned dike characteristics of a representative case study. We consider that the use of existing landward dikes or local construction material make realignment more sustainable. From a nature-based flood protection perspective, the presence of an artificial dike is ambiguous. Our results show that targeted and expected saltmarsh restoration at managed realignment does not necessarily result in a greener realigned dike design that suits for combined flood protection with restored saltmarshes. We recommend coastal managers to explicitly take combined flood protection into account in the realigned dike design and steer the topography of the realignment site to facilitate nature-based flood protection and promote surface elevation increase seaward of the realigned dike in response to sea level rise. This makes managed realignment a nature-based flood defence zone for now and for the future.

Lit.opg. en verklarende woordenlijst, bijl.

Lit.opg. en verklarende woordenlijst, bijl.