Hydraulic traits and carbon dynamic responses of Picea sitchensis to seawater exposure in the Pacific Northwest, USA

Rising sea-level strengthens the salinity level in soils of coastal areas due to increased tidal flooding, thereby strongly affecting coastal ecosystem functioning globally. Plant mortality has been observed due to salinity stress by influencing water relations and carbon & cations status, subsequently effecting plant growth and survival. However, the hydraulic and carbon based mechanisms that underlie potential survival strategies for coastal ecosystems still remain unclear. We investigated tree hydraulic traits, intrinsic water use efficiency, gas exchange, tree growth, and nonstructural carbohydrates concentrations in a coastal watershed in the Pacific Northwest USA where a dike was breached in 2014, allowing tidal fluctuations to intrude into a previously freshwater dominated forest. Growth of Picea sitchensis (Sitka-spruce), the dominant tree species, declined rapidly after 2014 due to the dike removal. High osmotic pressure under salinity stress contributed to greater vulnerability to xylem embolism and reduced hydraulic conductivity. Stomatal closure to maintain osmotic pressure with higher salinity induced a decline in photosynthesis rate, which in turn decreased carbohydrates production. Combined with decreased cations concentration (eg. K⁺, Ca⁺) in tree tissues due to increased Na⁺ concentration, salinity stress decreased Sitka-spruce growth and ultimately led to mortality. In parallel, stored carbohydrates declined prior to to Sitka spruce death. This study offers an assessment of coastal trees responses to salinity stress, and will improve our understanding of underlying physiological responses to high salinity under projected sea-level rise.