Season dynamics of carbon use efficiency and its influencing factors in the old-growth Abies fabri forest in Gongga Mountain,western Sichuan,China

Aims Carbon use efficiency (CUE), an important function parameter, reflects the carbon sequestration capacity of forest ecosystems. It is useful in analyzing the temporal dynamics of carbon budgets at the organ, individual and community scales. It can help to determine and predict the carbon sink/source of terrestrial ecosystems, which is a matter of widespread concern.Methods Using the biometric method, we measured and calculated the respiration and net productivity dynamics of different fir organs from an old-growth Abies fabri forest on Gongga Mountain in the eastern Qinghai-Xizang Plateau, China. We studied the CUE dynamics of the tree layer and its organs and analyzed their influencing factors. We also estimated the CUE of whole trees of different diameters at breast-height (DBH) classes.Important findings (1) Monthly respiration rates in both the tree layer and its organs are positively related to temperature, and fine roots have the highest respiration rate of all. There is no significant difference in the annual respiration of whole trees with different DBH classes, and the small DBH trees (30-40 cm) have the minimum annual stem respiration. (2) The monthly net primary productivity (NPP) of the fine root and whole stem in the tree layer increases with temperature, with the fine root accounting for the largest proportion. The small trees have the greatest annual NPP, and their needle NPP is also significantly higher than that of the medium DBH (50- 60 cm) and large DBH (75-90 cm) trees. (3) The CUE of the tree layer and its organs are mostly among 0.30 to 0.60. The monthly changes in the CUE of both fine roots and stems are similar, and their CUE increases with temperature. The CUE of trees and their organs all decrease significantly with tree growth. (4) The CUE of both the stems and fine roots is positively related with the air and soil temperature, while precipitaion has a positive effect on needle CUE. Fine root CUE has negative and positive effects on stem CUE and needle CUE, respectively. The tree layer CUE depends mainly on stem and fine root CUE. These results indicate that old-growth forests have strong and sustainable carbon sink functions, and play an important role in regional carbon storage and the carbon cycle of the forest ecosystem.