Soil conditions in seawater-inundated coastal forests damaged by the 2011 mega-tsunami following the Tohoku earthquake along the eastern Pacific coast of Japan

Abstract

The 2011 Heisei Sanriku mega-tsunami destroyed most of the forests along the eastern Pacific coastline of Japan. The Forest Agency of Japan (FAJ) has played a positive role in the restoration of tsunami-damaged coastal forests by constructing tide embankments and planting trees. Seawater inundation and sea-sand deposition in and on topsoil generally lead to excessive Na^+ absorption and accumulation in plants, competitive inhibition of nutrient uptake by Na^+, and a decrease in suction potential caused by low osmotic pressure between the soil matrix and plant roots. During the disaster, excessive Na^+ absorption in topsoil, incremental changes in soil pH (H_2O), and deterioration in ground water drainage due to ground subsidence strongly affected rhizosphere microenvironments. These changes in turn caused severe damage, including needle discoloration, debilitation, droop, and finally death, to broad areas of the eastern Pacific coastal forests. Rehabilitation of tsunami-created saline soils in these forested lands requires the removal of sea salt, a difficult task as most Japanese forests have poor irrigation facilities. Efficient removal of sea salt from the soils must therefore rely on high-precipitation events, such as typhoons and the rainy season in Japan. Reforestation of seawater-inundated forests along the Pacific coastal areas affected by the tsunami will require continued soil chemistry monitoring and rehabilitation.