**5. Conclusions**

The C, N, and P concentrations of plants and soil vary greatly among different species, and these differences may be species dependent. The input of exogenous N and P changes the stoichiometric characteristics of understory plants to different degrees; thus, we believe that different species have different response mechanisms to changes of environmental factors. Different functional plant groups show different changes with the addition of N and P. The addition of N and P had an interactive effect on the N:P of litter, indicating that the growth of understory plants was dually restricted by N and P. Over the long-term, the addition of N and P may change the competitiveness of different functional plant groups in the forest, leading to a change in the community of understory plant species. Therefore, the response of the stoichiometric characteristics of understory plants to different N and P inputs should be studied in future research on understory vegetation.

**Author Contributions:** Conceptualization, L.Z. and J.X.; methodology, L.Z. and J.X.; software, J.X. and M.C.; investigation, J.X., H.F., Q.Z. and Y.G.; resources, L.Z.; data curation, J.P. and J.X.; writing—original draft preparation, J.X.; writing—review and editing, L.Z., X.X., X.F. and J.X.; supervision, X.G. and L.Z.; funding acquisition, X.F. and L.Z.

**Funding:** This research was funded by the National Natural Science Foundation of China (31770749) and the Research Project of Jiangxi Provincial Department of Forestry (No.201806).

**Acknowledgments:** The authors thank the following people for their help in this research. Li-LiMa, Shu-LiWang, Xi Yuan provided field assistance. Jia-Shun Peng assisted with lab analyses. Jiao Wu assisted with syntax modification.

**Conflicts of Interest:** The authors have declared that no competing interests exist.
