Adventures of salt marsh recruits: Mechanistically understanding the journey to dispersal and establishment

Abstract

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.