Meteorological Tower Observed CO₂ Flux and Footprint in the Forest of Xiaoxing’an Mountains, Northeast China

The Xiaoxing’an Mountains, located in the temperate monsoon climate zone in Northeast China, have the largest and most complete virgin Korean pine forest in Asia, which has great potential for carbon sequestration. Based on the observational data of the eddy-covariance system at Wuying National Climate Observatory in January 2015–November 2017, the CO₂ flux in the forest ecosystem around the observatory was quantitatively studied and the distribution characteristics of the flux source area were analyzed by the Kljun model and the Agroscope Reckenholz–Tänikon footprint tool, providing references for assessing the carbon source/sink potential of the unique forest area. The results showed that the annual total carbon flux around the observatory in 2015, 2016, and 2017 was −756.84, −834.73, and −629.37 gC m⁻², respectively, higher than that of other forest ecosystems. The forest of the study area in the Xiaoxing’an Mountains was a strong carbon sink, with the strongest carbon fixation capacity in June and weakest in October, and the carbon flux of each month was less than zero. The flux source area under stable atmospheric conditions was larger than that under unstable conditions, and the source area was larger in the nongrowing season than in the growing season. The size of the source area was largest in winter, followed by spring, autumn, and summer. The maximum length of the source area was 1614.12 m (5639.33 m) under unstable (stable) conditions when the flux contribution rate was 90%. The peak flux contribution was located near the sensor (i.e., within 200 m) in all seasons. The contribution of the source area from the coniferous and broadleaved mixed forest on the west side of the observatory was greater than (3.4 times) that from the Korean pine forest on the east side.