# Responses of Forest Carbon Cycle to Drought and Elevated CO2
## 2 Drought and Forest Carbon Cycle
### Forest Carbon Cycle Feedbacks to Drought
Second, studies from North America and Europe suggest drought duration and timing are crucial factors that affect the dynamics of forest carbon balance between gross primary productivity and ecosystem respiration [29; 30; 31]. For example, the increase of forest carbon uptake in a warm spring is eliminated by subsequent summer drought [29; 30; 31; 32], and early warming-induced consumption of soil moisture may intensify the water stress and heating in summer [30]. In addition, the sensitivity of carbon flux to warming and drought varies among vegetation types [31; 33]. Alaskan deciduous forests showed a much higher net carbon uptake during the growing season than evergreen forest because of leaf area increasein warm spring, which enlarged the difference of response to summer drought between deciduous forests and evergreen forests [33]. This result indicates a bigger contribution of CO\({}_{2}\) uptake by deciduous forests in the northern hemisphere. Figure 1: Forest carbon and water fluxes change as the intensity of drought increases. (**a**) Before drought events. (**b**) Moderate drought without tree mortality. (**c**) Severe drought with large biomass loss. Orange arrows show the carbon flux; blue arrows show the water flux. The magnitude of fluxes is indicated by the size of arrows. Numbers of (+) and (\(-\)) indicate that the degree of fluxes increase and decrease, respectively. Plant respiration includes leaf and stem respiration; soil respiration includes autotrophic soil respiration (roots) and heterotrophic respiration (litterfall, microbial community, soil organic matter, etc.). Third, the response of ecosystem respiration is expected to contribute to the uncertainty of the net CO\({}_{2}\) exchange, because if Re and GPP decrease with a similar magnitude, the net primary productivity is expected to be unaffected by drought. The magnitude and direction of Re variation depend on duration and severity of drought and regional characteristics (such as soil moisture and vegetation types). [PERSON] et al. [34] global study suggests that the direction of ecosystem respiration change depends on whether heat stress coincides with drought because heat and drought have the opposite impacts on Re. Re will keep stable if a warming-induced increase in Re is offset by water limitation. For example, ecosystem respiration in Alaskan forests increased during the 2004 summer drought because warming-caused increases in respiration exceeded the effects of limitations such as soil moisture and oxygen content [33]. Moreover, the European hot drought in 2003 led to a reduction in ecosystem respiration (autotrophic and heterotrophic respiration decreased) [16; 35; 36]. [PERSON] et al. [35] suggest that less carbon assimilation led to plant respiration decrease, and the negative effect of higher water stress on heterotrophic respiration exceed the positive effects of warming on it. Similar results were found in [PERSON] et al. [37], they suggest that most forests in their study experienced a decline of net ecosystem productivity (decrease both in GPP and Re) in the 2018 summer drought across Northern Europe, and the forest carbon balance change was largely explained by decreased heterotrophic respiration and lack of precipitation. \begin{table} \begin{tabular}{c c c c c c} \hline \hline
**Location** & **Period** & **Forest Carbon Variations** & **Causes** & **Reference** \\ \hline North American & 20 th Century & \(\bullet\) & Restricted carbon uptake & \(\bullet\) & Heat stress & [PERSON] et al., 2000 [38] \\  & & & & \(\bullet\) & Water deficit & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ \end{tabular} 
\begin{tabular}{c} \(\