Abstract
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.
