Ecosystem carbon storage of natural secondary birch forests in Xiaoxing’an Mountains of China

The ecosystem carbon storage (vegetation and soil), net primary productivity, and annual net carbon sequestration of natural secondary birch forests on seven site types (the top, middle, and bottom of the sunny slope and shady slope, and the valley floor) were studied by use of relative growth equations and carbon/nitrogen analytical approach in Xiaoxing'an Mountains of China, in order to probe into the influence of site type on the carbon sinks in birch forest and provide scientific basis for the management of forest carbon sink in northeast China. The results showed that: 1) The vegetation carbon storages ((49.39±3.09)-(89.20±10.17)t/ha) at the valley floor, the bottom of the sunny slope, and the top and middle of the shady slope were significantly higher than those at the middle and top of the sunny slope and the bottom of the shady slope (38.2%-80.6%, P0.05); 2) The soil organic carbon storages ((136.28±21.39)-(273.67±22.67))t/ha at the middle and top of the shady slope and the valley floor were the highest (which were significantly higher than that at the middle and top of the sunny slope and the bottom of the shady slope (27.9%-85.8%, P0.05), followed by the storages at the bottom of the sunny slope (which was only significantly higher than that at the bottom of the shady slope (53.3%, P0.05), yet they were the lowest at the middle and top of the sunny slope and the bottom of the shady slope; 3) The ecosystem carbon storages ((207.88±16.07)-(357.85±20.80) t/ha) in four of the seven site types (the valley floor, the bottom of sunny slope, the top and middle of the shady slope) were significantly higher than those at the middle and top of the sunny slope and the bottom of the shady slope (33.2%-72.1%, P0.05); 4) The net primary productivities of vegetation ((5.80±0.26)-(8.87±1.17) t/(ha·a)) at the bottom of sunny slope and the top and middle of shady slope were the highest (which were significantly higher than it at the bottom of the shady slope (31.2%-52.9%,P0.05)), followed by the net primary productivities in the middle at the top and middle of sunny slope and the valley floor (which were higher than that at the bottom of shady slope (15.5%-26.4%,P＞0.05), and it was the lowest at the bottom of shady slope; 5) The annual net carbon sequestration ((2.76±0.10)-(4.15±0.32)t/(ha·a)) also were highest at the bottom of the sunny slope and the top and middle of the shady slope (which were significantly higher than it at the bottom of the shady slope (27.9%-50.4%,P0.05), they were in the middle at the top and middle of sunny slope and the valley floor (which were higher than that at the bottom of shady slope (12.3%-23.9%,P＞0.05), and it was the lowest at the bottom of shady slope. Overall, this kind of natural secondary birch forest in Xiaoxing'an Mountains took on a relatively low carbon sink function because their carbon sequestration decreased (15.3%-43.7%)) compared to the average carbon sequestration of terrestrial vegetation in China.