# Characterizing Growing Season Length of Subtropical Coniferous Forests with a Phenological Model
## 5 Conclusions  
Using a total of 27 site-years of GPP and environmental observation data, the phenological characteristics of three subtropical forest sites with different soil water availability were discussed. We found that the impact of prescribed fire on ecosystem-scale phenological processes is limited. In this study, fire only weakly affected the timing of the start of the growing season and had no direct impact on subsequent ecosystem phenological processes. Increased air temperature and spring radiation were found to significantly extend the growing season. Increased annual precipitation positively impacted sites with better soil water holding capacity; however, excessive late autumn and winter precipitation may have a negative impact on early spring and autumn phenology in sites with poor soil water holding capacity. Restoration of precipitation after short-term summer drought is extremely important for the recovery of ecosystem productivity, which will directly affect the end of summer, and sufficient precipitation in August will extend the end of summer. This is of particular importance for subtropical forests, as the timing of precipitation is predicted to be altered with climate change. Further study of the phenology of these forests could greatly improve our understanding of future GPP patterns, and ultimately their carbon dynamics. Conceptualization, Y.G. and C.L.S. ; methodology, Y.G. ; software, Y.G. ; formal analysis, Y.G. ; investigation, G.S. ; data curation, S.W. ; writing--original draft preparation, Y.G., G.S. and C.L.S. ; writing--review and editing, Y.G., Y.Z., C.L.S., S.W. and G.S. ; supervision, G.S. and C.L.S. ; project administration, G.S. ; funding acquisition, G.S. All authors have read and agreed to the published version of the manuscript. Y.G. was funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). C.L.S. and G.S. acknowledge support from the U.S. National Science Foundation (DEB EF-1241881). C.L.S. also acknowledges support from the U.S. National Science Foundation (DEB EF-1702029). Not applicable. **Data Availability Statement:** Publicly available datasets were analyzed in this study. This data can be found here: ([[https://ameriflux.lbl.gov/](https://ameriflux.lbl.gov/)]([https://ameriflux.lbl.gov/](https://ameriflux.lbl.gov/))) as sites US-LL1, US-LL2, and US-LL3. **Acknowledgments:** The authors thank the members from the Starr laboratory (The University of Alabama) for their constructive comments on the revision of the article. The authors also thank the National Key Research and Development Program of China (No. 2016 YFC0502704) for financial support. **Conflicts of Interest:** The authors declare no conflict of interest. Figure A2: Cont. There was no summer short-term drought or extreme air temperature events in the mesic site in 2009 [37] and thus this site-year was representative of phenological characteristics associated with normal weather conditions (Appendix B, Figure A2). The spring recovery line (RL, \(A_{RL}(t)\)) had a slope of 0.08 g C m\({}^{-2}\) d\({}^{-1}\) d\({}^{-1}\), and the autumn senescence line (SL, \(A_{SL}(t)\); right black dotted line) had a slope of \(-0.06\) g C m\({}^{-2}\) d\({}^{-1}\) d\({}^{-1}\). Modeled maximum daily GPP occurred at 177 d (8.47 g C m\({}^{-2}\) d\({}^{-1}\)), which corresponds with the date when GPP growth rate is zero. The \(t_{PRD}\) occurred in mid-spring (117 d, 0.08 g C m\({}^{-2}\) d\({}^{-1}\) d\({}^{-1}\)), and \(t_{PSD}\) occurred in mid-autumn (291 d, \(-0.06\) g C m\({}^{-2}\) d\({}^{-1}\) d\({}^{-1}\)). During 2009 in the mesic site, summer was estimated to occur from 147 d to 241 d, and autumn was from 241 d to 365 d with a length of 124 d. The ecosystem-scale growing season was from 44 d to 365 d, indicating that the LOS was 321 d. The modeled daily peak photosynthetic capacity was 8.47 g C m\({}^{-2}\) d\({}^{-1}\) with a LOP/summer of 94 d. The length order of four seasons for the mesic site in 2009 was: autumn \(>\) spring \(>\) summer \(>\) winter.