# Characterizing Growing Season Length of Subtropical Coniferous Forests with a Phenological Model
## 4 Discussion
### Response of Ecosystem-Scale Summer Phenology  
Since the three study sites are in a warm subtropical region, rarely are there light or temperature limitations which constrain GPP. Therefore, water availability is an important factor which supports and affects GPP (undersatory and canopy; Appendix F, Figure A5) and thus day-to-day phenological processes at the ecosystem scale [40] (Appendix B, Figure A2).  
Water availability has been shown to have a lagged effect on productivity of subtropical forests [50]. Our study weakly supports this finding with evidence in the intermediate site in 2010 when precipitation led to a small increase in GPP after short-term drought, but this recovery was not significant compared to pre-drought physiology. The reason may be that after the drought event in July (34.3 mm mo\({}^{-1}\)), the recovery of precipitation in August (85.3 mm mo\({}^{-1}\)) was not enough to offset the water deficit which occurred the month prior (Appendix B, Figure A2). Subsequent above average precipitation is required to offset carbon loss during water stress (Appendix A, Figure A1) and stabilize photosynthetic capacity in summer, as additional precipitation demand is related to evapotranspiration during and after water stress [60]. In addition, increased precipitation following the drought offset carbon loss by reaching the \"effective precipitation\" threshold, which is defined as the ratio of precipitation to evaporation [61]. This increased water availability had a positive effect on GPP (Appendix F, Figure A5). However, when short-term drought lasted from June to August, the ecosystem EOP date was advanced which led to a shorter summer at all sites (Appendix B, Figure A2). This may have been associated with the fact that many conifers--when experiencing drought--will flush one cohort of needles earlier in the summer to limit water loss and reduce drought stress which will reduce GPP rates [62].  
We also observed that following summer water stresses, August precipitation shortened the senescence phase (Appendix F, Figure A5). While September precipitation had less of an effect on the end date of summer, this corresponded more closely to a decline in photoperiod and a reduction in air temperature [15] (Appendix A, Figure A1).  
Although sufficient precipitation during the growing season can extend the end of summer (Appendix F, Figure A5), precipitation is not the only controlling factor for summer ecosystem productivity. Higher vapor pressure deficit (VPD) is known to cause stomatal limitation in longleaf pine ecosystems, which leads to declines in GPP [44; 45; 63]. The sites experienced this type of event in the summer of 2015; although drought was absent, there was still a significant decline in GPP associated with higher VPD (Appendix B, Figure A2). Yet we as a scientific community still have not determined the specific tipping point when stomatal limitations would limit GPP in this system. Thus, to better understand the effect of summer phenological processes of subtropical forests associated with VPD, additional experiments are needed that link physiological activity to VPD thresholds.