Coastal forests have been restored since the Great East Japan earthquake and tsunami; thick growth bases have been built with embankments to allow tree roots to grow deeper into the soil to prevent the trees from toppling. Because the growth bases were built by heavy machinery, their soils tend to be harder than those of sand dunes where the former coastal forests grew. As a result, there has been concern that the hard soils in the growth bases may prevent roots from extending deep into the soil. The present study aimed to investigate the root development of the coastal forest into hard soils. For this purpose, a soil column, with a 4-cm set layer whose soil hardness could be adjusted to three different levels, was constructed. The container seedlings of five tree species, Pinus thunbergii, Zelkova serrata, Quercus dentata, Quercus crispula and Quercus serrata, were planted above the set layer. After one growing season, the cross-section of the roots above and below the set layer was measured for each plant. In case of all five tree species, hardly any roots could penetrate the hardest set layer, whereas some roots could penetrate the second hardest set layer. Whereas the total cross-section of roots penetrated the second hardest set layer was little for Q. dentata, that of roots penetrated the second hardest set layer was about 30% in comparison with that of roots penetrated the softest set layer for the other four species respectively. These findings suggest the roots of Z. serrata, Q. crispula and Q. serrata can achieve the same level of penetration of hard soil as those of P. thunbergii.

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In the restoration of coastal forests that suffered from the Great East Japan Earthquake Tsunami, thick growth bases have been built with embankments that let roots grow deep into soil. The growth bases are apt to have soil that is harder than the sand dunes where the former coastal forests stood. There has been some concern that the hardness of the growth bases prevents roots from growing deep into the soil. In order to elucidate the effects of soil hardness on the root development of Pinus thunbergii, which is the primary species in the coastal forests of Japan, we set up a deep (about 1.5 m depth) tilled area and an untilled area on a growth base with embankment, and investigated the root development of P. thunbergii planted on each area for 30 months. Since soils of the tilled areas were much softer than those of the untilled areas, it was confirmed that deep tillage is an effective method to soften hard soils. The roots in the untilled areas were hardly out of the planting hole after 30 months, while the roots in the deep tilled areas reached about 80 cm deep after 5 months, and about 110 cm deep after 17 months. These results confirm that the hard soil prevents roots from developing. Moreover, the vertical soil hardness profiles where the roots penetrated suggest that soil hardness limits to root penetration of P. thunbergii.

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Currently there is limited information on the resilience of uprooted broadleaved trees after tsunami disasters in coastal forests. Therefore, studies must investigate the root distribution of broadleaved trees in coastal sand dunes. In this study, we measured the root morphology and three-dimensional distribution of Pinus thunbergii and several broadleaved tree species by excavation survey. The gathered data were then compared with previous root morphology studies at mountainous sites. Except for some excavated root samples, the roots of P. thunbergii, Quercus dentata, Zelkova serrata, and Tilia japonica generally showed similar morphology to mountainous tree roots as reported by Karizumi (1979). The roots of young Q. dentata and Z. serrata trees were concentrated in shallow soil layers (0–50 cm), probably due to a higher stand density with adjacent trees and buried plastic waste. We also hypothesized that root growth was not inhibited by groundwater and hard soil layers at coastal forests on sand dunes because no high groundwater layers in soil profiles were observed when root samples were excavated. At some excavation points, we observed hard soil pan layers, but roots had penetrated into these layers. Sandy soils exhibited considerable permeability due to their porous nature and were not tightened. These observations suggested that the root distribution of broadleaved trees in coastal sand dunes with low groundwater levels, sandy soils, no higher stand density with adjacent tree, and no buried objects were similar to the root distribution of broadleaved trees in mountainous sites.

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There are many low-lying lands with back swamps near the coastal areas of Kujukurihama and Cape Futtsu in Chiba Prefecture, Japan. Since the 1990’s, before afforestation of these areas occurred, growth bases with embankments were constructed to prevent planted tree roots from rotting due to waterlogged soil conditions. It was often observed, however, that the consolidated soil layers of growth bases built with heavy machinery suffered from obstructions to the deep development of their tree roots. In order to identify tree species whose roots can develop deep into the anthropogenic soil of these growth bases, we investigated the relationship between soil hardness and root development of Pinus thunbergii, which is the major tree species in the coastal forests of Japan, and some broadleaf tree species, which have the possibility to take on the role of P. thumbergii in case the species’ pine wilt disease spreads, at 2 sites of coastal forests (Futtsu and Komatsu site) in Chiba Prefecture. At the Futtsu site where ground water levels were about 0.8 m in depth, consolidated soil layers and groundwater levels seemed to behave as barriers against the vertical root developments of P. thunbergii and Machilus thunbergii. At the Komatsu sites where ground water level could not been observed, the consolidated soil pans were observed in shallow areas (about 30 cm deep) of the growth bases. Irrespective of species, many thin roots (< 1.0 cm in diameter) of the planted trees were observed to develop to the deep layer under the consolidated pans. The findings of this study suggest that some broadleaf species, including M. thunbergii, could be introduced to the moderately softened, anthoropogenic soils of the growth bases with embankment in inland coastal forests, as well as P. thunbergii.

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Broadleaf trees in coastal forests are expected to reduce tsunami damage, act as windbreaks, and reduce soil erosion. Trees with strong and deep root systems are desirable for providing these ecosystem services, but root development can be affected by soil properties, especially when trees are planted on consolidated soils. We assessed the trunk and root growth of planted broadleaf trees in relation to soil properties in four plots in Akita and Chiba Prefectures, Japan. Two plots in Akita prefecture, Mukaihama and Hamayama, contained deciduous broadleaf trees (including Quercus dentata and Zelkova serrata) planted on sand dunes. The remaining two plots in Chiba prefecture, Futtsu and Komatsu, contained evergreen broadleaf trees (including Machilus thunbergii and Quercus phillyraeoides) planted on embankments of consolidated soil. Pinus thunbergii was planted in all plots. Diameter at breast height, tree height, and soil hardness were assessed and the roots of two to four trees of each species were excavated in each plot. The lengths of vertical and lateral roots >1 cm in diameter were measured and we compared the depth of the consolidated soil layer to root length. In Hamayama, the vertical roots of P. thunbergii and some roots of Q. dentata and Z. serrata extended below the consolidated soil layer, which was 1 m in depth. Although the soil was extremely soft at Mukaihama, tree growth was poor due to oligotrophic conditions and strong winds. In Komatsu, where trees had been planted a decade previously, the consolidated soil layer was found at a depth of approximately 30 cm, whereas at Futtsu it was found at depth of 70 cm because of aquifer was found at 80 to 100 cm in depth. We therefore concluded that root growth in coastal forests is affected by soil hardness as well as tree attributes and other soil properties.

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In recent years, anthropogenic soils in forested lands have been of great interest. This is because the intensity of human influences for natural soil with forested lands is continuously increasing due to better land management, urban and industrial developments, and recovery works from several disasters with devastating damages. Human impacts have been considered a great factor for the formation of soil. Regarding the soil classification systems of natural soil with forested lands in Japan, there are two categories of soils, namely Immature Soils (Immature Soil) and Immature Soils (Eroded Soil). In this paper, we investigate and present the survey results of anthropogenic soils in the coastal disaster prevention forests and seaside parks in north-eastern Japan. We realized that the impacts of human influence like soil materials, construction methods, and contamination of some exotic artefacts for anthropogenic soil morphology were varied. The soil morphological information is useful for evaluating soil function and considering soil geneses in heavily anthropogenic environments.

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