Soils on Newly-Constructed Coastal Berms for Reforestation of Coastal Forests Damaged by the 2011 Mega-Tsunami

Abstract

The mega-tsunami following the Great Eastern Japan Earthquake of March 2011 greatly damaged coastal forests along the Pacific Ocean coastline from the Aomori to Chiba Prefectures. According to reports on tsunami-damaged forests, groundwater levels in areas where trees were uprooted were less than ca. 1 m below the surface, which implies that the vertical root depths of most uprooted trees were too shallow to resist the tsunami. Increase of effective soil depths is required for planting trees on berms to allow the development of deeper root systems when reconstructing the coastal forests. To prevent further damage in the affected area, it is necessary to deepen our understanding in regard to which physical characteristics of soils are affected by constructing berms. Soils in the surveyed berms located along the Pacific coast in the Miyagi Prefecture have characteristics of high density and low water permeability, due to the use of heavy machines to construct the berms. These soils can be classified as Linic Spolic Technosol according to the world reference base for soil resources 2014 (WRB 2014; IUSS Working Group WRB. World Reference Base for Soil Resources 2014, update 2015. International soil classification system for naming soils and creating legends for soil maps. World soil resources reports no. 106. FAO, Rome (2015)) mainly due to both the existence of quite low permeability horizons and the high content of artefacts. Soils consisting of relatively high permeability horizons can be classified as Spolic Technosol (IUSS Working Group WRB, World reference base for soil resources 2014, update 2015. International soil classification system for naming soils and creating legends for soil maps, World soil resources reports no. 106. FAO, Rome, 2015) due to the high content of artefacts. Tillage after berm construction can effectively eliminate the compacted soil layer and can induce high water permeability in berms. Most of soils have no structure containing gravels and boulders in some soil profiles. Soils with weak sub-angular blocky structure have been observed only at surface horizons. Tillage after the berm construction is the only way to enhance water permeability and vertical root extension.