Mechanical properties of Japanese black pine ( Pinus thunbergii Parl.) planted on coastal sand dunes: resistance to uprooting and stem breakage by tsunamis

Given that Japanese black pine trees ( Pinus thunbergii Parl.) are predominant in the coastal forests of Japan and are part of the defence structure against tsunamis, the quantification of their resistance to tree damage is necessary. The resistance of Japanese black pine to uprooting and stem breakage and its bending properties were estimated by a tree-pulling test and bending test of green logs in conjunction with the published literature. A general equation to estimate the critical turning moment for uprooting was developed using diameter at breast height and tree height as predictor variables. For moduli of elasticity and rupture of stems (MOE and MOR), medians [5th and 95th percentile values] were 5.41 [3.78, 6.82] GPa and 35.0 [28.7, 41.8] MPa, respectively. With the obtained critical turning moment and MOR, the critical tsunami water depth was estimated by numerical simulations using modelled trees. The numerical simulations revealed that Japanese black pine trees on coastal sand dunes tended to be more vulnerable to uprooting than stem breakage, with taller and more slender trees showing less resistance to stem breakage. The results on the mechanical properties of Japanese black pine are of use to those in the wood science community as well as coastal managers who need to know the mechanical strength of Japanese black pine to help evaluate their resistance against tsunamis.