Rainfall variation shifts habitat suitability for seedling establishment associated with tidal inundation in salt marshes

Growing concerns over the ecological consequences of global changes in rainfall regimes have motivated exploration into how variable rainfall influences the persistence of plant species. Understanding the influence of rainfall events on seedling establishment is critical for predicting the consequences of potential changes to the rainfall regime in coastal salt marshes. However, it is not clear whether seedling establishment of various species responses to rainfall are strongly regulated by tidal inundation. We conducted a series of rainfall manipulation experiments in salt marshes and controlled greenhouse experiments to test the effect of post-dry season rainfall on seedling establishment of plant species among different tidal elevations and during drought and large rainfall events. Our results showed that post-dry season rainfall created an opportunity for seedling establishment by strongly influencing the suitability of abiotic factors for species niches, but plant soil rainfall interactions were nonlinear across existing soil conditions, which were controlled by tidal inundation. Seedling establishment was highly dependent on post-dry season rainfall amount. The seedlings of species that were sensitive to saline soil or had low tolerance to soil moisture only established successfully during large rainfall events. Consequently, changes in the rainfall amount caused shifts in native species composition and opened invasion opportunities for new species. Our study highlighted that small changes in rainfall amount or distribution can trigger comparatively large and abrupt changes in different species depending on their ecological niches and local abiotic gradients. Within tidal marshes, exploration of rainfall tide linkages advanced fundamental understanding of the effects of changing rainfall regimes and provided implications for coastal wetland management and restoration efforts to prepare for the future rainfall changes related to climate change.