**5. Conclusions**

To our best knowledge, this synthesis provides the first comprehensive analysis of NRE and PRE in planted forests worldwide. Generally, NRE significantly increased along climate zones, while no significant differences were observed for PRE among climate zones. These results sugges<sup>t</sup> differential impacts of climates on NRE and PRE. In terms of plant functional groups, the PRE of conifer trees was higher than that of broadleaf trees; evergreen trees had higher PRE than deciduous trees; non-N-fixing trees had higher NRE but lower PRE than N-fixing trees. These findings indicate that plant functional types exert strong impacts on nutrient resorption. Additionally, multivariate regression analyses showed that variations in NRE were mainly regulated by mean annual precipitation (MAP) and latitude, indicating that the predicted global warming and changed precipitation regimes may profoundly affect N cycling in planted forests. Variations in PRE were dominantly controlled by green leaf nutrient concentrations (N and P), suggesting green leaf nutrient concentrations may be good indicators for PRE in planted forests.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1999-4907/10/3/201/s1.

**Author Contributions:** Conceptualization: X.X.; Data curation: D.J.; Formal analysis: D.J.; Funding acquisition: D.J., H.R., and X.X.; Project administration: X.X.; Supervision: X.X.; Writing—original draft: D.J.; Writing—review and editing: Q.G., Q.L., Y.L., J.V., Z.S., H.R., and X.X.

**Funding:** This study was financially supported by the National Key Research and Development Program of China (2016YFD0600204), the Recruitment Program for Young Professionals (Thousand Youth Talents Plan), the Jiangsu Specially-Appointed Professors Program, the Six Talent Peaks Program of Jiangsu Province (JY-041& TD-XYDXX-006), the Nanjing Forestry University Science Fund for Distinguished Young Scholars (the "5151" Talent Program), the Jiangsu Collegiate Science and Technology Fund for the Excellent Innovative Research Teams, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Doctorate Fellowship Foundation of Nanjing Forestry University, the Research Innovation Program for College Graduates of Jiangsu Province (KYLX16\_0833), and the Scientific and Technological Innovation Program for College Students of Nanjing Forestry University (DXSKC-201617).

**Acknowledgments:** We thank many lab members for their help with the manuscript preparation.

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