| Summary: | The carbon sink capacity of forest ecosystem and its function of mitigating climate change have been confirmed. As two common coniferous species, <i>Cunninghamia lanceolata</i> (Lamb.) Hook. (<i>C. lanceolata)</i> and <i>Pinus elliottii</i> Engelmann (<i>P. elliottii</i>) are widely planted in southern China, and their carbon sink capacity has always been concerning. According to their diameter class, we randomly harvested 42 <i>C. lanceolata</i> trees and 38 <i>P. elliottii</i> trees from our entire study area, measured their carbon concentration, and constructed biomass models with DBH and tree height as variables. The biomass of the tree layer was estimated by measuring the DBH of all trees in the plots, and the biomass and carbon concentration of shrubs, herbs, dead wood and litter in the plot were measured by harvesting them. The results showed that the total biomass in <i>C. lanceolata</i> and <i>P. elliottii</i> plantations were 117.1 and 151.8 t·ha<sup>−1</sup>; the biomass in the tree layer was 94.7 and 122.9 t·ha<sup>−1</sup>; and in the other parts was 22.4 and 28.9 t·ha<sup>−1</sup>, respectively. In addition, the total carbon densities in the <i>C. lanceolata</i> and <i>P. elliottii</i> plantation ecosystems were 166.3 and 198.6 t·ha<sup>−1</sup>; the carbon densities in the soil were 108.1 and 124.6 t·ha<sup>−1</sup>; and in the other parts, they were 58.2 and 74.0 t·ha<sup>−1</sup>, respectively. These results indicate that there are significant differences in total biomass or total carbon storage between the two coniferous forest ecosystems, and net productivity and carbon sink capacity are higher in the <i>P. elliottii</i> plantation ecosystem. This study lays the foundation for the biomass estimation and carbon trading of these two coniferous forests in northern Hunan.
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