# Impacts of Heat and Drought on Gross Primary Productivity in China
## 5 Discussion
### Impacts of Heat and Drought on Different Vegetation Regions  
Both heat and drought can cause ENAG, but the degrees of the responses of different vegetation regions to heat and drought differ. According to the total average probability of ENAG under different drought levels (Figure 5), the temperate grassland region (R2) and the warm temperate deciduous broad-leaved forest region (R6) are the most sensitive regions to drought; in these regions, the average ENAG probability is highest under drought conditions. The main reason for this result is probably that most of the temperate grassland region (R2) is located in the semiarid zone of northern China where precipitation has a great impact on vegetation coverage. Due to the overall temperature increase, meteorological drought events have caused soil aridification. Natural water shortages have become a limiting factor restricting grassland productivity [74]. The warm temperate deciduous broad-leaved forest region (R6) is the major crop-producing region in China, and the recognized primary limiting factor for crop production in this region is drought due to low annual precipitation [75; 76]. As one of important agricultural drought mitigation measures, irrigation is popular among farmers in R6 [77]. Irrigation relieves the impact of extreme climate on crops. In theory, without the mitigation effect of irrigation, the ENAG probability would be higher. The most sensitive region to heat is also the temperate grassland region (R2), which has a simple ecosystem structure and a weak self-regulation ability. Heat intensities cause excessive transpiration and water loss in grasslands, resulting in metabolic imbalance. The subtropical evergreen broad-leaved forest region (R4) and the tropical monsoon forest and rainforest region (R5) in southern China are also sensitive to heat because these regions are located in subtropical and tropical monsoon climate zones with adequate water and heat resources. When heat occurs, the stomata of vegetation are closed to prevent excessive transpiration of water, resulting in a decrease in the photosynthetic rate, which directly causes the light reaction to be inhibited. The carbon dioxide absorbed by leaves is also reduced under heat, and the dark reaction is inhibited. Heat at night accelerates organic matter decomposition, which is not conducive to organic matter preservation. All these factors contribute to the reduction in photosynthesis efficiency under high-temperature conditions and affect vegetation growth. [PERSON] et al. [2] studied the effects and drivers of negative extreme events on gross primary productivity and pointed out that extreme GPP events in southern China were mostly associated with temperature extremes, including extreme heat. Both heat and drought have the smallest impacts on the alpine vegetation region on the Qinghai-Tibet Plateau (R3) and the temperate desert region (R1). Alpine vegetation has high altitudes and low temperatures all year, both of which severely limit the productivity of most alpine grasslands at all growth stages, and, appropriately, heat is conducive to vegetation growth in this area. The latest research of [PERSON] et al. [49] showed that GPP was positively correlated with temperature across most parts of the grasslands of R3 during both the entire growing season and different growth stages. Precipitation that affects vegetation growth mainly occurs in warm weather periods [78; 79]. Additionally,the R3 region shows a significant wetting trend, and the drought impact in this region is relatively restricted compared with that in the other regions. The R1 region has a temperate, extremely arid climate with sparse rainfall. The typical vegetation in this region is temperate desert vegetation, which generally exhibits heat resistance and xerophytic characteristics; therefore, R1 is not sensitive to heat and drought. In all the vegetation regions, the ENAG probability caused by heat is much smaller than that caused by drought.