# Responses of Forest Carbon Cycle to Drought and Elevated CO2
## 2 Drought and Forest Carbon Cycle
### Forest Carbon Cycle Feedbacks to Drought  
Seventy-eight percent of the changes of global gross primary production (GPP) in the past three decades are related to extreme events, while drought events account for 60-90% of these climate extremes [14]. The water limitation and high air temperature directly affect the carbon cycle of forests through plant responses to stress. Resistance strategies of plants coping with drought include changes in plants physiological traits [15]. At the leaf level, plant responses to drought are reflected in the reduced stomatal conductance and changes in photosynthetic and respiratory rates, which directly affects the carbon exchange between vegetation and the atmosphere (Figure 1). [PERSON] et al. [16] demonstrated that soil moisture is the dominant limiting factor for forest carbon exchange in the 2003 Europe drought event because the GPP and ecosystem respiration (Re) both dropped sharply when the relative extractable soil water was lower than about 0.4. Furthermore, the ecosystem feedbacks may aggravate the severity of drought because: (i) The evaporative cooling effect is reduced due to stomatal closure, so the high leaf temperature may cause heat stress; (ii) high-light stress aggravated by drought leads to the occurrence of photoinhibition or even photodamage; (iii) high temperature, high radiation and rising vapor pressure deficit (VPD) exacerbate drought by higher evapotranspiration [17]. Forest carbon uptake and sequestration can rapidly decrease after widespread tree mortality [18], and the impacts of the decline in forest net primary productivity (NPP) could last for decades. Moreover, the decomposition process of the remnant biomass and underground dead roots releases stored carbon back to the atmosphere (Figure 1c), which can transform forests from carbon sinks into sources. Tree mortality is usually induced by the combination of multiple driving factors such as climate stress, diseases related to pests and pathogens, the stand life histories and fires [19]. Drought may serve as an inducer of the death of trees that are already under stress. The complexity of these drought-induced death mechanisms of trees makes the monitoring of tree mortality challenging. [PERSON] et al. [20] proposed three hypotheses about the mechanisms of drought-induced plant die-off: (i) Hydraulic failure; (ii) carbon starvation, where water limitation-induced stomatal closure blocks carbon uptake; and (iii) biotic attack, where changes in populations of pests and pathogens under continuous warming lead to excessive tree damage and mortality. Hydraulic failure is considered to be the main factor leading to plant mortality related to drought [21; 22]. The hydraulic failure hypothesis states that low soil water potential and soil hydraulic conductivity lead to an increase in the vapor pressure difference between leaves and atmosphere and the rising demand for evaporation. The xylem conduits and rhizosphere are filled with air so the plant hydraulic system cannot transport water from the rhizosphere to leaves resulting in plant tissues desiccation and death [20]. Drought-induced tree mortality directly influences the land-atmosphere interaction because of the loss of vegetation function of exchanging water, energy and carbon. Literature reporting forest carbon cycle responses to drought events in the last two decades is summarized in Table 1. The main causes of forest carbon budget variations related to drought are summarized below. First, increased tree mortality induced by drought explains large forest carbon storage loss in North America [17; 23; 24; 25] and the Amazon [26; 27; 28]. As an example, in the 2010 Amazon drought event, an estimated 2.2 Pg C storage was impacted [27] with a wider impact area than the 2005 event (1.6 Pg C [26]). This impact was mainly derived from the temporary suspension of biomass growth and tree mortality, which reversed the forest from a net carbon sink to source [27].