Above- and belowground biomass and net primary production in a cool-temperate deciduous forest in relation to topographical changes in soil nitrogen

Patterns in above- and belowground allocation of biomass and net primary production (NPP) along a topographic and soil N availability gradient were measured in a cool-temperate deciduous forest in central Japan. In this study site, soil N availability changed along the topographic sequence and decreased up the slope. Total NPP ranged from 8.8 to 14.1 t ha^-1 and showed no trends along the topographic sequence, while the percentage of belowground NPP to total NPP ranged from 15.2 to 55.1% and increased up the slope. The aboveground NPP ranged from 5.6 to 8.6 t ha^-1 and decreased up the slope. Belowground NPP ranged from 1.5 to 7.7 t ha^-1 and increased up the slope. Fine root production contributed to this trend. Allocation to structural components in the lower slope exceeded that on upper slope positions, whereas higher allocation to fine roots on the upper slope may result in higher annual belowground litterfall. Our results suggest that differences in the carbon allocation pattern of plants between structural and litter components may be a driving force to create variations in forest structure and nutrient cycling along the topographic sequence.