A long-term experiment showed how coastal groundwater salinization impaired tree carbon–water balance

In this study, we address the pressing problem of coastal forest decline with a long-term experiment with a factorial design of sedimentary nutrient fertilization and tree density. Here we show, following the first decade of enhancement by eCO2 and nutrient fertilization, tree growth strikingly declined toward higher mortality risks in coastal forests experiencing accelerated SLR. In the first decade (1995-2005), tree basal area increment (BAI) positively responded to eCO2 and nutrient supply, with iWUE (using tree-ring isotopes) slightly increased. However, progressive increases in groundwater salinity in the following years (2005-2015) caused severe stomatal closure and photoinhibition, which remarkably decreased Leaf internal CO2 concentration and increased iWUE. These physiological changes consequently translated into decreased BAI and enhanced tree mortality, which was further exacerbated by tree competition. These results help explain ghost forest formation in many coastal areas and support a transient stimulatory effect of CO2 uptake in these areas.