Topographic variations of soil properties and stand structure in a Pinus thunbergii plantation

Soil properties and above- and belowground forest structure were studied across various topographies in a 20-year-old Pinus thunbergii Parl. plantation on Mt Tanakami, Japan. The soil properties and stand structure varied greatly with slope position from ridge top to valley floor. Soil thickness, fine soil content and soil moisture content were greater in lower slope positions. The amount of organic carbon in the forest floor was greater in upper slope positions. The organic carbon content in the mineral soil was slightly greater in lower slope positions. These changes in soil properties suggested an upslope decrease in decomposition rate and water and/or nutrient availability. The aboveground structure of P. thunbergii was more developed at lower slope positions. The mean stem diameter, height and volume of P. thunbergii increased downslope with decreasing tree density. However, fine root biomass increased greatly upslope. This inverse relationship between tree height and fine root biomass indicated morphological plasticity of P. thunbergii in exploiting environmental heterogeneity. Variations in soil-plant interactions in the stand along various topographies caused spatial heterogeneity in the accumulation pattern of organic matter in plants and the soil.