Variation of carbon density components with overstory structure of larch plantations in northwest China and its implication for optimal forest management

•The carbon density of larch plantation ecosystems varies with tree age.•Tree density and canopy density affect the overstory and understory carbon density.•Canopy density and overstory tree biomass affect the humus and soil carbon density.•The tradeoff among carbon density components in overstory, understory, humus and soil layers can be realized through the optimization of stand structure.•Optimum canopy density is required for multifunctional management of larch plantation.