Estimated biomass carbon in thinned Cunninghamia lanceolate plantations at different stand-ages

Chinese fir (Cunninghamia lanceolate [Lamb.] Hook.) is a fast-growing species which is not only important as a timber-supplier, but also as an available sink for carbon (C) storage in biomass. Stand age and density are two critical factors that can determine tree C sequestration as interrelated drivers through natural self-thinning. C. lanceolate were planted using 1-year-old bare-root seedlings at the initial density of 1800 stems ha−1 in a 15-ha montane area of Hunan Province, China in 1987. The plantation was thinned twice 10 and 20 years after planting to leave trees of 437.5 ± 26.6, 675.0 ± 155.2 and 895.8 ± 60.1 stems ha−1 as low, medium, and high densities, respectively. Tree height and diameter at breast height (DBH) were measured every 2 years beginning from 23 years (2009) to 31 years (2018) after establishment, timber volume (TV) and biomass C were estimated accordingly. We did not find any interactive effect of age and density on any variables except for height. Both TV and biomass C increased with stand age or decreased in higher densities. The allometric height-DBH relationship can be fitted by an exponential rising-to-maximum model with higher maximum value over time. The decline of biomass C along density fit with the inverse first-order polynomial model which indicated that at least 1300–1500 stems ha−1 may be needed to maximize TV and biomass C for a longer term over 20 years. Therefore, to control the density to a reasonable level, over 1300 stems ha−1 in a rotation over 20 years old will be practical for tree biomass C in Chinese fir plantations.