# Responses of Forest Carbon Cycle to Drought and Elevated CO2
## 3 Elevated CO2 Effect on Plants and Ecosystem
### Ecosystem Responses to Elevated CO\({}_{2}\)  
Plant physiological response to elevated CO\({}_{2}\) at the leaf level has been supported by short-term individual experiments, however, extrapolation of these conclusions from individuals to ecosystems requires multiple data across scales. Over the past decades, research of plant responses to elevated CO\({}_{2}\) scaled up and increased in complexity. Large-scale open-top chambers (OTC) and FACE experiments with an open-air condition reflect plant responses to elevated CO\({}_{2}\) at the field level and extend the timescale of research, however, the incorrect data, experimental deficiencies (such as the unnatural increase of CO\({}_{2}\)) and unsatisfactory future prediction needs still challenge the understanding of global-scale forest responses to climate change. Satellite observations reveal the change in global vegetation on a scale of decades and there is a documented upward trend on vegetation cover in the northern hemisphere over the past two decades ([PERSON], 2018). Recent evidence ([PERSON], 2018; [PERSON], 2018; [PERSON], 2018) indicates that in recent decades, this trend is probably attributed to a combined effect of the elevated CO\({}_{2}\), nutrients, water supply, solar radiation and socio-economic factors (such as land use change and reforestation). For example, according to [PERSON] et al. (2018), global land NPP experienced a climate-induced increase between 1982 and 1999, which is probably due to solar radiation increase rather than CO\({}_{2}\) fertilization in the Amazon rainforest (accounting for 42% of total global NPP increase). [PERSON] et al. (2018) analyze the drivers of the Sahel greening trend through ecosystem modelling and the output suggests that precipitation is the main cause of increased vegetation cover while CO\({}_{2}\) has only a small contribution. However, CO\({}_{2}\) fertilization is considered more obvious in areas where water is the primary limitation for vegetation growth because the increased WUE under elevated CO\({}_{2}\) mitigates water stress ([PERSON] et al., 2018). [PERSON] et al. (2018) indicate that the prediction of CO\({}_{2}\)-stimulated increase in maximum foliage cover is consistent with satellite observations, which suggests that CO\({}_{2}\) does play a key role in warm and arid area greening. [PERSON] et al. (2018) showed consistent results with [PERSON] et al., and they indicated that the WUE of vegetation in Australia increased over the past decades due to CO\({}_{2}\) fertilization. A new method, which scales up from leaf and canopy level, was developed by [PERSON] et al. (2018) to isolate elevated CO\({}_{2}\) effects from other effects and quantify the magnitude of vegetation response globally, and their results supported the effects of CO\({}_{2}\) enrichment on GPP increase and transpiration decrease. Process-based models that involve mechanical process descriptions are feasibly to isolate global forest response to CO\({}_{2}\) fertilization from other drivers on a longer time scale (decades). [PERSON] et al. 's (2018) modelling studies based on ORCHIDEE suggest that 49% of the increase of leaf area index (LAI) in growing season in the northern hemisphere is contributed by atmospheric CO\({}_{2}\) (temperature and precipitation account for 31% and 13%, respectively). Ten carbon cycle models used for the IPCC were evaluated through statistical functions by [PERSON] et al. (2018) and their results show that every increase of 100 ppm CO\({}_{2}\) concentration contributes to 5-20% simulated NPP increase, which slightly exceeds the results from FACE experiments (13%). However, [PERSON] et al. (2018) indicated a decline in CO\({}_{2}\) fertilization effects from 1982 to 2015 based on observation datasets, which related to increased nutrient (N, P) limitations and water limitation.