Characteristics of carbon sequestration changes and environmental regulation in a typical artificial coniferous and broad-leaved mixed forest in Beijing

This study aimed to explore the variation characteristics of net ecosystem productivity (NEP) and its environmental regulation in a typical artificial mixed coniferous and broad-leaved forest ecosystem in Beijing, clarify its carbon sequestration function, and to support the goals of carbon peaking and carbon neutrality.

An artificial mixed coniferous and broad-leaved forest ecosystem located in Beijing was selected as the study site. Continuous in-situ observations of NEP were conducted throughout 2023 using the eddy covariance (EC) technique. Simultaneously, environmental factors, including photosynthetically active radiation (PAR), vapor pressure deficit (VPD), air temperature (T_a), and soil water content (SWC), were monitored. The random forest model was used to calculate the importance scores of these environmental factors and to quantify their impacts on NEP and carbon sequestration.

The energy closure of the measurements was 0.72, The average daily and seasonal variations of NEP exhibited unimodal trends, with NEP peaking at 11.95 μmol·m^-2·s^-1 in July during the growing season (March–November). The ecosystem acted as a carbon sink during the growing season, with a net carbon sequestration of 384.83 g·m^-2, whereas it functioned as a carbon source in the non-growing season, releasing 1.18 g·m^-2. The total annual net carbon sequestration was 383.65 g·m^-2. Seasonal variations in NEP showed an initial increasing trend followed by a decrease, significantly influenced by PAR, SWC, VPD and T_a (P<0.01). Among these factors, PAR was the dominant environmental factor controlling NEP and carbon sequestration, with an importance score of 56.42 determined by the random forest model. The thresholds of environmental factors influencing NEP were 400 μmol·m^-2·s^-1 (PAR), 0.17 m³/m³ (SWC), 1.5 kPa (VPD), and 28 ℃ (T_a). Additionally, when SWC was below 0.1 m³/m³, it initially increased with rising T_a, but after exceeding 10 ℃, SWC decreased. The indirect regulatory threshold of T_a on NEP was thus identified as 10 ℃.

The studied artificial mixed coniferous and broad-leaved forest exhibited moderate carbon sequestration capacity. PAR was the primary factor controlling seasonal variations in carbon sequestration. The findings provide scientific support for managing carbon sink enhancement in artificial mixed coniferous and broad-leaved forest ecosystems in Beijing. Additionally, the observational dataset constitutes an important supplement for validating models used in forest carbon estimation.