# Responses of Forest Carbon Cycle to Drought and Elevated CO2
## 5 Conclusions and Outlook  
Although we have recognized that drought events and CO2 fertilization have critical impacts on the carbon cycle of forests, it is still challenging to figure out the complex, comprehensive effects of climate change on vegetation.  
We reviewed the current understanding of vegetation response to drought and elevated CO2 from leaf to ecosystem scales. In conclusion, plants initiate drought resistance strategies in arid and warm environments, which likely change the carbon flux of forests. Extreme drought massively killing trees is one of the main reasons for forest ecosystem carbon loss. Elevated CO2 favoring photosynthesis and water use efficiency of plants in a short period of time has been confirmed by a large number of experimental studies, but the effect of CO2 fertilization is limited by other constraint factors such as light, water and nutrients (N, P). Additionally, on a long-term scale, plants have the possibility of acclimating to warming and elevated CO2 with a lower net photosynthetic assimilation. The underlying mechanism has not been explained clearly. There is great instability and unpredictability in the physiological mechanisms of vegetation adaptation and mortality under drought [76]. This instability stems from the drought resistance of trees and elevated CO2 effects on WUE. However, the problem of how far CO2 fertilization can offset the effects of drought has not been resolved. Whether and to what extent the improved water use efficiency by CO2 fertilization can withstand drought damage remains unknown.  
Understanding the interaction effects of rising CO2 and drought is crucial for looking forward to future forest ecosystems growing in a drier and warmer atmosphere. Process-based models are important tools for predicting the future interaction of forest ecosystems and atmosphere, however, accurately extrapolating the conclusions derived from individual experiment reports to large-scale ecosystem models requires more comprehensive observational data and process descriptions. There have been studies that provide information that helps improve model simulations, for example, different vegetation type responses to drought [31; 33; 42] and N, P limitation [53; 54; 55; 57; 58]. Applying that information to model building may help reduce the uncertainty of models. We recommend considering the uncertainties from the following perspectives:  
1. Including observational data across different scales. Flux tower measurements, space-time data from large-scale long-term experiments, observation of climate change and vegetation change recorded by remote sensing.
2. Different responses to drought among vegetation types and changing plant species composition of ecosystems over time.
3. Consideration of constraint factors. Restrictions of nutritional elements (N, P), water and light on CO2 fertilization.
4. Effect of photosynthetic acclimation (down-regulation of photosynthetic capacity).
5. Disturbances. The complexity of the multiple disturbance interactions associated with drought (fires, insects and pathogens).  
Overall, a better understanding of mechanisms and scale-up methods are key issues in current research on forest strategies to combat climate change. However, it is worth noting that forests cannot infinitely resist climate change, and the containment of greenhouse gases emissions is the fundamental and necessary way of mitigating global warming and protecting forest ecosystem functions.  
Writing--original draft preparation, J.-L.X. and W.-Y.S.; writing--review and editing, F.Z. and W.-Y.S.; help and discussion, F.Z., Q.-L.H., Y.-X.Y., X.H. and W.-Y.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Natural Science Foundation of China (Nos. 41975114 and 41830648).  
Not applicable.  
Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article.