Biogeomorphological processes and structures facilitate seedling establishment and distribution of annual plants: Implications for coastal restoration

Sediment water content drives movement of intertidal crab Helice tientsinensis more strongly than salinity variations

Intertidal wetlands undergo dynamic water and salinity variations, creating both promising and challenging habitats for diverse organisms. Crabs respond strongly to these variations by means such as altering their movements, thereby restructuring their spatial distribution and influencing coastal ecosystem resilience. However, the movements of crabs under varying environmental conditions require further elucidation. We conducted a systematic mesocosm experiment using the ubiquitous intertidal crab species Helice tientsinensis with four amount levels and six salinity levels of sprayed water applied through a custom apparatus, with a primary focus on crab movement. Crab movement from the experimental side of the apparatus (with altered conditions) to the control side (resembling field conditions of the intertidal wetlands of China's Yellow River Delta) and vice versa was recorded. The results revealed significant differences in moving out of the experimental side and moving in among the different water and salinity conditions, both separately for the two factors and simultaneously. Decreases in water content had a more pronounced effect on crab movement, leading to an increased number of crabs moving out of the experimental side of the apparatus. Conversely, as the experimental side became wetter, crabs tended to move towards it, and this movement was intensified by increases or decreases in water salinity. A structural equation model revealed that the moving-out and moving-in played fundamental roles in determining the number of resident crabs at the end of each experiment. While crabs preferred moist sediment with lower salinity, changes in salinity alone had minimal direct effect compared to sediment water contents. Our results clarify crab movements under varying water and salinity conditions, offering valuable insights to support adaptive interventions for crab populations and inform adaptive conservation and management strategies in intertidal wetlands.

Seed settling and trapping during submerged secondary dispersal: Implications for saltmarsh recruitment and restoration

Given the decline of global salt marshes, there is a pressing need to pinpoint the key processes that limit and facilitate seed-based pioneer recruitment. Secondary seed dispersal, in the form of short-distance submerged movement, is a prerequisite for initiating pioneer establishment in adjacent tidal flats but has not been fully appreciated and understood. In this study, using a settling tube and race-track flume, seeds of four global occurring saltmarsh species were studied in terms of their settlement speed and trapping opportunity to understand how seed traits and physical settings affect submerged dispersal behavior and thus seed-based saltmarsh recruitment. Present study led to the following novel insights: 1) Seeds have density-dependent settling speeds, which are comparable to that of fine sand, but much faster than that of very fine sand and silt. Since the latter is the type of sediment commonly found in many estuaries worldwide (such as the Scheldt), seeds will typically settle faster than local sediments. A sufficiently long hydrodynamic-calm period allows slowly settling sediment to bury settled seeds, otherwise, seeds will remain uncovered if the period is short. 2) Seed trapping ratio increased linearly with surface roughness (a proxy for local topographic complexity), but this effect becomes smaller with increasing hydrodynamic intensity. Seed drag coefficient was identified as the key biotic factor contributing to interspecies variability in trapping ratio. Overall, present results suggest that submerged seed dispersal may form a primary bottleneck for salt marsh recruitment by limiting seed availability via two mechanisms: i) reduced chance of seed burial through asynchronous settling of seeds and sediment particles; ii) reduced probability of seed trapping due to encountering smooth tidal flat surfaces. This study provide mechanistic and data basis for the targeted application of biophysical models in predicting outcomes of saltmarsh recruitment and long-term maintenance, thereby informing seed-based conservation and restoration.

Invasion patterns of Spartina alterniflora: Response of clones and seedlings to flooding and salinity-A case study in the Yellow River Delta, China

The invasion of Spartina alterniflora has caused severe damage to the coastal wetland ecosystem of the Yellow River Delta, China. Flooding and salinity are key factors influencing the growth and reproduction of S. alterniflora. However, the differences in response of S. alterniflora seedlings and clonal ramets to these factors remain unclear, and it is not known how these differences affect invasion patterns. In this paper, clonal ramets and seedlings were studied separately. Through literature data integration analysis, field investigation, greenhouse experiments, and situational simulation, we demonstrated significant differences in the responses of clonal ramets and seedlings to flooding and salinity changes. Clonal ramets have no theoretical inundation duration threshold with a salinity threshold of 57 ppt (part per thousand); Seedlings have an inundation duration threshold of about 11 h/day and a salinity threshold of 43 ppt. The sensitivity of belowground indicators of two propagules-types to flooding and salinity changes was stronger than that of aboveground indicators, and it is significant for clones (P < 0.05). Clonal ramets have a larger potentially invadable area than seedlings in the Yellow River Delta. However, the actual invasion area of S. alterniflora is often limited by the responses of seedlings to flooding and salinity. In a future sea-level rise scenario, the difference in responses to flooding and salinity will cause S. alterniflora to further compress native species habitats. Our research findings can improve the efficiency and accuracy of S. alterniflora control. Management of hydrological connectivity and strict restrictions on nitrogen input to wetlands, for example, are potential new initiatives to control S. alterniflora invasion.

Unraveling the wheel of recruitment for salt-marsh seedlings: Resistance to and recovery after dislodgement

Elucidating bottlenecks at critical life stages and quantifying associated resilience (including resistance and recovery) to physical processes are central in inform restoration and attain sustainable development of coastal biogeomorphic ecosystems. Seedling establishment is a key life stage determines saltmarsh restoration potentials. However, the resilience of these recruits, especially through recovery, remains poorly understood. Here, two contrasting globally occurring saltmarsh species, namely Salicornia europaea and Spartina anglica, were employed to generate insights in i) seedling resistance against dislodgement, and ii) seedling recovery potential after dislodgement. Regarding resistance, we found that 1) root-shoot antagonism characterizes the growth rate of seedling resistance to dislodgement through hydraulic disturbance, 2) the root length determines seedling resistance to dislodgement through sheet erosion; 3) a 5 mm sedimentary setting amplifies seedling resistance without inhibiting their morphological evolution. Regarding recovery, we found that 4) dislodged seedlings have a high probability for achieving long-distance dispersal; 5) seedling age and the inundation-free period regulate the re-establishment potential of dislodged seedlings. Overall, S. anglica showed stronger resilience than S. europaea, characterized by stronger seedling resistance against dislodgement and higher re-establishment potential. Our results on seedling resilience suggest that seedling dislodgement is not an end-of-life cycle but a new spin on the "Wheel of Recruitment", a proposed short-term cyclic behavior with alternating phases of seedling dislodgement, dispersal, and (re-)establishment. The Wheel of Recruitment concept is important for forecasting resilience and persistence of biogeomorphic systems such as salt marshes under global change and for guiding life cycle informed restoration.