Abstract: The area of natural Picea mongolica forests in Baiyinaobao National Nature Reserve, the eastern edge of Hunshandake Sandy Land, was quantified based on the Landsat TM/ETM+ satellite remote sensing data from 1975, 1981, 1990, 2000, 2013 and 2020 combined with field investigation and laboratory experiments. A remote sensing estimation model for biomass was established, and then the biomass and carbon of natural Picea mongolica forests across different periods were inverted. Combined with the measured carbon content of organs (stem, branch, and leaf) of trees with different diameter classes, aboveground carbon storage and its spatiotemporal variation in the natural Picea mongolica forests across different periods were clarified. The results showed that: (1) The area of natural Picea mongolica forest in 1975, 1981, 1990, 2000, 2013 and 2020 was 1573.66, 1684.31, 1775.21, 1937.16, 1968.27, and 1999.69 hm², respectively, indicating that the area had continuously increased over the past 45 years. The natural Picea mongolica forest showed a spatial distribution pattern with more in the west and less in the east, with 95.1% being distributed in the western part of the study area. (2) The remote sensing estimation model for biomass of natural Picea mongolica forest is: y=7.556(1/b₅)²-6.9174(1/b₅)-20.917 (b₅ is the near-infrared band, R²=0.73). The biomass of the natural Picea mongolica forest was 7.53×10⁴, 7.82×10⁴, 8.36×10⁴, 9.17×10⁴, 9.70×10⁴ and 10.14×10⁴ t in  1975, 1981, 1990, 2000, 2013 and 2020, respectively. (3) The carbon content of different diameter classes and organs ranged from 501.10 g·kg^-1 to 545.09 g·kg^-1. The average carbon content of the aboveground part of Picea mongolica tree was 524.16 g·kg^-1 based on the weighted average method of the biomass of each organ. The carbon storage of natural Picea mongolica forest in 1975, 1981, 1990, 2000, 2013 and 2020 was 3.92×10⁴, 4.07×10⁴, 4.35×10⁴, 4.77×10⁴, 5.06×10⁴ and 5.27×10⁴ t, respectively. During 1975-2020, aboveground carbon storage of natural Picea mongolica forest increased by 1.35×10⁴ t, with a growth rate of 34.4%, indicating that Picea mongolica forest had a significant effect on carbon sequestration and sink enhancement, and also reflecting the gradual improvement of carbon sink capacity in the region. Spatially, the carbon storage showed significant heterogeneity (2.14-181.94 t·hm^-2), with the medium and low carbon storage grades (20-40 t·hm^-2) being the main ones (with the largest proportion), but the area of mature forests with high carbon storage grades (≥40 t·hm^-2) increased significantly over time, indicating that the regional carbon accumulation was tending towards a stable growth stage. The area expansion of natural Picea mongolica forest and the increases in carbon storage have significantly enhanced the regional carbon sink capacity. The dynamics and spatial distribution characteristics of its carbon storage provide a scientific basis and data support for the protection and management of natural forests as well as for the achievement of China’s carbon neutrality goals.

Key words: Picea mongolica, diameter class, carbon content, carbon storage, allocation pattern