Abstract
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.
