Tidal channel-mediated gradients facilitate Spartina alterniflora invasion in coastal ecosystems: implications for invasive species management.
: Understanding how geomorphic landscape features affect physical habitat conditions is essential for predicting if such geomorphic landscape features may act as conduits facilitating invasive species expansion in coastal ecosystems. Although the effects of various abiotic and biotic factors on plant invasion in salt marshes have been well studied, little is known about the impact of tidal channels on plant invasion. Here, we examined the effects of bottom-up determinants on the lateral expansion of Spartina alterniflora in tidal channel margins of different elevational marsh zones: the low, middle and high marsh, in the Yellow River Delta, China. Field observations and experiments showed that the microhabitats of tidal channel margins are characterized by relatively low soil salinity and high inundation, which significantly facilitated the establishment, growth, colonization and sexual reproduction of S. alterniflora. Moreover, the facilitating effect of tidal channel margins on plant landward invasion appeared more sensitive in middle and high marshes compared with the low marshes. High propagule pressure combined with suitable physical conditions of the structural microhabitats formed within tidal channels potentially promoted the rapid expansion of S. alterniflora along the tidal channel margins. Based on these results, a conceptual model was built illustrating the lateral expansion mechanisms of the invasive plant mediated by tidal channels. These results highlight the importance of tidal channel-mediated gradients in bottom-up abiotic and biotic-dispersal factors in facilitating the expansion of coastal invasive plants. For wetlands management, our results imply that geomorphic landscape features should be incorporated into ecological management and risk assessment of invasive plants.