Abstract
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.
