Carbon sink potential of natural forests of Picea schrenkiana var. tianschanica based on different site grades

Objective This paper aims to clarify the carbon sink potential of Picea schrenkiana var. tianschanica natural forests in Xinjiang of western China under different site conditions, and to provide theoretical basis and technical support for the enhancement of ecological functions of natural forests in Xinjiang, scientific management, and the achievement of regional carbon neutrality.

Method Based on the data from 150 sample plots of Picea schrenkiana var. tianschanica natural forests in Xinjiang, we systematically assessed the carbon sink potential of stand under different stand conditions by estimating the parameters of stand basal area and carbon storage growth model, and solving the optimal stand density index using the golden section method.

Result (1) The established stand dominant height model (R² = 0.965) divided the Picea schrenkiana var. tianschanica forest sample plots into five site grades (I−V), and the number of sample plots under each grade was 15, 30, 60, 27, 18, respectively. (2) The growth models of stand basal area and carbon storage had good fitting accuracy, in which the basal area model R² = 0.936 (RMSE = 2.931 m²/(ha·year), rRMSE = 0.134) and the carbon storage model R² = 0.861 (RMSE = 5.488 t/(ha·year), rRMSE = 0.164). (3) When the age was fixed, stands with better stand quality had higher carbon sink potential, and stands with poor stand quality had higher potential for enhancement. At the base age, the carbon storage of site grades I−V was 149.487, 143.191, 138.103, 122.681, 116.196 t/ha, and the optimal stand densities corresponded to 1 022, 1 166, 1 343, 1 568, and 1 862 tree/ha, respectively. (4) With the growth of forest age, the carbon sink potential of each grade showed a decreasing trend, among which the decay rate was the largest in young forest stage, and slowed down after the middle-aged forest; the quality of stand was positively correlated with carbon storage, but the low stand quality had a higher density of regulating space.

Conclusion Stand quality classification can effectively characterize the differences in carbon sink potential of Picea schrenkiana var. tianschanica natural forests, and it is recommended to focus on strengthening the precise control of young forest stage of medium stand quality stands (grade III), which accounts for 40% of the total samples, in order to fully explore its potential for carbon sink enhancement.