**5. Conclusions**

Our results indicated that the initial C quality of newly shed foliar litter significantly varied with the plant species over time. Newly shed foliar litter of fir, spruce and willow comprising more labile C might contribute to the magnitude of accumulation of the soil C pool in alpine forests. Rosa litter, obtaining more structurally complex recalcitrant C fractions, might qualitatively contribute to the stability and sequestration of soil organic C. Moreover, other litter quality-related factors, such as N, DOC, and Mn, strongly controlled the initial C quality of newly shed foliar litter. In addition, the initial methoxyl C, aromatic C and phenolic C were the best predictors of C loss during litter decomposition in the alpine forests. However, our study did not consider the C quality in soils, and we could speculate that labile and recalcitrant C fractions could exhibit different characteristics in soils. Therefore, future efforts should be devoted to thoroughly studying the C quality based on the continuous process of "newly shed litter-decomposing litter-soils" to obtain the contributions of litter-derived C to soil C sequestration in alpine forests.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/f13111886/s1, Figure S1: Results of stepwise regression analyses of initial organic components ((a): WSE, water-soluble extractives; (b): OSE, organic-soluble extractives; (c): ASE, acid-soluble extractives; (d): AUR, acid-unhydrolyzable residues) using litter quality and environmental conditions (C: carbon; DOC: dissolved organic carbon; N: nitrogen; P: phosphorus; Mn: manganese; MAT: monthly average temperature; MAP: monthly average precipitation). The dominant variables are emphasized by red color in the regression model, Values represent the standardized coefficients, The \* indicates significant effects of variables on C fractions in a given regression model \* *p* < 0.05, \*\* *p* < 0.01, and \*\*\* *p* < 0.001. For example, OSE was significantly impacted by MAT (*p* < 0.05), Mn (*p* < 0.001), P (*p* < 0.001), and DOC (*p* < 0.001) in the stepwise regression testing the effects of litter chemical quality and environmental conditions on OSE (a).

**Author Contributions:** Conceptualization, methodology, validation, resources, supervision and writing—review and editing, J.M. and Q.W.; Project administration and funding acquisition, Q.W.; Conceptualization, data curation, investigation, methodology and writing—original draft, J.Y.; Investigation, Q.D., Y.Y., Y.Z. and C.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (31800521, 32022056, 31800373, 31922052 and 32071747), the Fok Ying-Tong Education Foundation for Young Teachers (161101), the National Natural Science Foundation of Sichuan Province (2022NSFSC0087), the Key R&D Program of Sichuan (18ZDYF0307), the Research Fund of Mianyang Normal University (QD2020A18), and the Open Fund of Ecological Security and Protection Key Laboratory of Sichuan Province, Mianyang Normal University (ESP1807).

**Institutional Review Board Statement:** Not applicable.

**Acknowledgments:** We wish to thank Zhuang Wang, Fan Yang, Xinyu Wei, Ling Mou, Qun Liu, Fujia Wu, Ji Yuan, and Long Jiang for the sample collecting.

**Conflicts of Interest:** The authors declare no conflict of interest.
