Elevated carbon dioxide and reduced salinity enhance mangrove seedling establishment in an artificial saltmarsh community

The global phenomenon of mangrove encroachment into saltmarshes has been observed across five continents. It has been proposed that this encroachment is driven in part by rising atmospheric CO₂ concentration and reduced salinity in saltmarshes resulting from rising sea levels enhancing the establishment success of mangrove seedlings. However, this theory is yet to be empirically tested at the community-level. In this study, we examined the effect of CO₂ and salinity on seedling growth of two mangrove species, Aegiceras corniculatum and Avicennia marina, grown individually and in a model saltmarsh community in a glasshouse experiment. We found that the shoot (210%) and root (91%) biomass of the saltmarsh species was significantly greater under elevated CO₂. As a result, both mangrove species experienced a stronger competitive effect from the saltmarsh species under elevated CO₂. Nevertheless, A. marina seedlings produced on average 48% more biomass under elevated CO₂ when grown in competition with the saltmarsh species. The seedlings tended to allocate this additional biomass to growing taller suggesting they were light limited. In contrast, A. corniculatum growth did not significantly differ between CO₂ treatments. However, it had on average 36% greater growth under seawater salinity compared to hypersaline conditions. Avicennia marina seedlings were not affected by salinity. From these results, we suggest that although CO₂ and salinity are not universal drivers determining saltmarsh–mangrove boundaries, it is likely that rising atmospheric CO₂ concentration and reduced salinity associated with sea level rise will enhance the establishment success of mangrove seedlings in saltmarshes, which may facilitate mangrove encroachment in the future.