Abstract: [Objective] This research was carried out to quantify the variation of biomass carbon density of Larix principis-rupprechtii plantation in Liupan Mountains of Ningxia, NW China, and to detect the influencing factors, e.g. tree age, stand density, canopy density, diameter at breast height (DBH) and tree height. The results can guide the rational management of forests to enhance their function of carbon sequestration.[Method] Sample stands with various ages, slope positions, stocking densities were selected in the semi-humid small watershed of Xiangshuihe and the semi-arid small watershed of Diediegou to measure the biomass carbon density. Besides the newly measured field data, the accumulated data from past investigations at the Liupanshan Forest Ecological Station were also used in this study. The biomass and its components were calculated based on empirical relations from the parameters of stand structure. Then, the biomass carbon density, or called as vegetation carbon density, was calculated using the carbon content of different biomass components.[Result] The biomass carbon density of L. principis-rupprechtii plantation was averaged at 30.96 t·hm^-2, with a variation range of 0.70~74.10 t·hm^-2. The biomass carbon density increased with the increase of stand structure (tree density, canopy density, DBH, tree height, and tree age), until certain thresholds were reached; thereafter it increased slowly or stabilized. The corresponding threshold was 1 300 individual·hm^-2 for tree density, 0.7 for canopy density, 14 cm for DBH, 12 m for tree height. The biomass carbon density increased first slowly with increasing tree age before the age of 18 years, and increased rapidly when it aged between 19~27 years, and then increased slowly again when it was older than 27 years. The biomass carbon density maintained a continuous increase in recent 10 years, with a significantly higher increment rate of 5.57 t·hm^-2a^-1 at the semi-humid area than the rate of 2.58 t·hm^-2a^-1 at the semi-arid area (P<0.05).[Conclusion] Based on the integrated analysis of the research results and the consideration of stand structure for other forest services, the ideal stand structure of multifunctional plantation of L. principis-rupprechtii in the region of Liupan Mountains should have a canopy density of around 0.7, and a stand density of not more than 1 300 individual·hm^-2. Such ideal stand structure will be helpful to realizing the multiple goals of forests simultaneously, including the higher carbon sequestration, more water yield, maintaining of natural regeneration and stable stand structure.

Key words: Liupan Mountains, carbon density, biomass, stand structure, temporal variation