An old-growth subtropical Asian evergreen forest as a large carbon sink

Old-growth forests are primarily found in mountain ranges that are less favorable or accessible for land use. Consequently, there are fewer scientific studies on old-growth forests. The eddy covariance method has been widely used as an alternative approach to studying an ecosystem’s carbon balance, but only a few eddy flux sites are located in old-growth forest. This fact will hinder our ability to test hypotheses such as whether or not old-growth forests are carbon neutral. The eddy covariance approach was used to examine the carbon balance of a 300-year-old subtropical evergreen broadleaved forest that is located in the center of the largest subtropical land area in the world. The post-QA/QC (quality assurance and control) eddy covariance based NEP was ∼ 9 tC ha−1 yr−1, which suggested that this forest acts as a large carbon sink. The inventory data within the footprint of the eddy flux show that ∼6 tC ha−1 yr−1 was contributed by biomass and necromass. The large-and-old trees sequestered carbon. Approximately 60% of the biomass increment is contributed by the growth of large trees (DBH > 60 cm). The high-altitude-induced low temperature and the high diffusion-irradiation ratio caused by cloudiness were suggested as two reasons for the large carbon sink in the forest we studied. To analyze the complex structure and terrain of this old-growth forest, this study suggested that biometric measurements carried out simultaneously with eddy flux measurements were necessary.

Research highlights
► The post-QA/QC eddy covariance based carbon flux data indicates that a 300-year-old subtropical evergreen broadleaved forest acts as a large carbon sink of ∼ 9 tC ha−1 yr−1 during our investigation interval.
► Inventories within the eddy flux footprint support the eddy flux results and indicate that ∼6 Mg C ha−1 yr−1 can be attributed to increases in plant biomass and necromass.
► The larger trees continue to strongly sequester carbon in the studied forest, with about 60% of biomass increment in trees > 60 cm.
► We suggest that low temperature and cloud-related high diffuse to direct radiation ratios may account for the large carbon sink in this.
► In complex terrain and forest structure, biometric measurements simultaneous with eddy flux appear to be necessary.