Intraspecific variation in root system structure in a <i>Pinus thunbergii</i> stand grown in a gravelly spit coast.

Root systems of <i>Pinus thunbergii</i> planted in coastal forests exhibit high phenotypic plasticity under edaphic conditions. The objectives of this study were as follows: 1) to determine intraspecific variation in root system traits, maximum root depth and length of horizontal roots of <i>P. thunbergii</i> in a gravelly spit within a stand by excavation of entire root systems, and root system structure modelling from photographs; and 2) to determine whether the root system traits can be inferred from aboveground traits. The <i>P. thunbergii</i> trees grew twisting roots through the hard soil layer of sand and gravel and exhibited variation in root systems; most individuals had inherent tap root systems, but one had a two-tiered thick horizontal root system with a thinner tap root. Although the maximum length of the horizontal root had no relationship with the aboveground traits, the maximum root depth of <i>P. thunbergii</i> was significantly related to tree height. This suggests that tree height can be a predictive indicator of root depth, which is valuable for the rehabilitation of disaster-resistant coastal forests. Both surface and solid models of root system were produced using structure-from-motion with a multi-view stereophotogrammetry method, which enabled continuous estimation of the sum of the root cross-sectional area at any depth in the depth direction. The new method of reconstructing root system models was effective for post-analysis of traits after excavation. We concluded that intraspecific variations in the root system traits of <i>P. thunbergii</i> were observed even at the stand level on a gravelly spit coast.