Windows of opportunity for smooth cordgrass landward invasion to tidal channel margins: The importance of hydrodynamic disturbance to seedling establishment.
Despite increasing concerns about the global threat of cordgrass (S. alterniflora) expansion and the interest in its invasion mechanisms, there is not yet a general understanding of the mechanistic processes underlying the interaction between cordgrass invasion and geomorphic structures such as tidal channels. This study elucidated the effects of the hydrodynamic disturbance of tidal channels on initial seedling establishment of cordgrass in the margins of two different types of tidal channels (i.e., main tidal channels and secondary tributaries). We performed field experiments that transplanted cordgrass seedlings to above-mentioned tidal channel margins with on-site controlled hydrodynamic conditions. The results showed that high hydrodynamic disturbance intensity (i.e., HDI) on the margin of main tidal channels (i.e., MMC) was not beneficial to cordgrass invasion, whereas low HDI created windows of opportunity for cordgrass invasion to the margin of secondary tributaries (i.e., MST) by facilitating the survival, growth, and stability of cordgrass seedlings. The presence of high HDI predominantly reduced the seedling survival and total biomass of cordgrass, whereas root biomass allocation of cordgrass increased significantly to resist dislodgment and toppling. Moreover, field investigations showed that soil salinity and moisture in the margin of tidal channels were not the limiting factors affecting the establishment of cordgrass seedlings. However, higher propagule pressure combined with suitable soil salinity-moisture conditions (i.e., low salinity and high moisture) laid a firm foundation favoring seedling establishment. Our results highlight the importance of hydrodynamic disturbance as a dominating driver regulating seedling establishment of cordgrass in tidal channel margins and the potential implications for controlling cordgrass landward invasions.