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

Abstract Information about the variation of carbon density components of larch plantation (LP) with changing overstory stand structure is lacking, which limits multifunctional forest management. This study sought to address these components in the Liupan Mountains of northwest China. It showed that stand age, tree density, tree biomass, and canopy density are important stand structure parameters influencing the amplitude and composition of carbon density. The results showed that, with rising stand age, the carbon density of both overstory tree and humus layers increased, the carbon density of the understory vegetation layer (shrubs and herbs) decreased, and the carbon density of soil layer (0–100 cm) first increased and then slightly decreased. However, the total ecosystem carbon density increased with rising stand age. With rising tree density and canopy density, the overstory carbon density increased, up to the thresholds of about 1800 trees/ha and 0.7 respectively, but after that increased very slowly; while the understory vegetation carbon density decreased continuously with rising tree density, but decreased with rising canopy density, first very slowly up to the range of 0.6–0.7 and after that very quickly. With rising overstory tree biomass, the humus layer carbon density increased, while soil layer carbon density first increased to a certain threshold, then leveled off or slowly decreased. With rising canopy density, the humus and soil carbon density first increased to the threshold of 0.7 and 0.67, respectively, then decreased. Therefore, the optimum canopy density of 0.7 maintains the highest total ecosystem carbon density with its highest or relatively higher components. Such a tradeoff among the carbon components informs multifunctional forest management since every carbon component is related to specific functions/services of the LP ecosystems.