# Impacts of Heat and Drought on Gross Primary Productivity in China
## 1 Introduction  
Gross primary production (GPP) refers to the total carbon amount fixed by green vegetation per unit area per unit time through photosynthesis and is the material basis for the survival and development of human society; changes in GPP affect the entire terrestrial carbon cycle [1, 2]. CO\({}_{2}\) fertilization and extensions of the growing season are expected to enhance vegetation growth because of ongoing global warming [2], and there is evidence of increased vegetation coverage worldwide, even in some semiarid regions [3, 4]. However, the frequency, persistence, and magnitude of extreme climate events such as droughts, storms, floods, heat, heat waves, extremely low temperatures, and heavy rains are projected to further increase in the mid-to-late 21 st century [1, 2, 5, 6, 7] and may pose potential threats to vegetation growth and terrestrial carbon uptake [8, 9, 10]. Understanding the responses of terrestrial GPP to extreme climate events such as heat and drought in the context of the potential aggravation of climatic extremes is of great significance forpredicting the responses of global terrestrial ecosystems to future climatic changes with confidence [11; 12; 13; 14; 15]. Drought stress is the main environmental factor limiting the productivity of terrestrial ecosystems [16], and heat stress causes physiological damage to vegetation and reduces vegetation productivity (including crop yield) [17]. Scholars have used GPP, net primary productivity, net ecosystem productivity, and vegetation indices to characterize ecosystem productivity in response to these two main climate anomalies, heat and drought, and have performed a series of related studies on the impacts of drought and heat on terrestrial ecosystem productivity on global and regional scales [18; 19; 20; 21]. Several global studies have shown that from 2000 to 2010, droughts caused declines in vegetation productivity in most parts of the world [18; 22; 23]. On a regional scale, frequent drought events in Africa in the past few decades have led to different degrees of famine, causing high life and economic losses and seriously impeding the economic development of some African countries [24; 25; 26]. In addition, due to global warming and drought, Australia has recently experienced bushfires of unprecedented severity and scale [27]. Moreover, droughts with different intensities and droughts occurring in different seasons have diverse impacts on different ecosystems (grasslands, forests, farmland, etc.) and vegetation types (broad-leaved forests, coniferous forests, etc.) [19; 28; 29; 30; 31]. Regarding heat, a study by [PERSON] et al. [32] showed that insufficient precipitation and an extreme increase in summer heat caused a decrease in vegetation productivity in Eastern and Western Europe in 2003. [PERSON] et al. [33] found that GPP decreased linearly under short-term intense heat. Studies focused on grain yield loss under high-temperature conditions have also been performed [34; 35]. The sensitivities of vegetation and adverse ecological effects to temperature rising are particularly concerning [36; 37], especially as the coincidence of temperature rising with drought intensifies the physiological stress and mortality of global vegetation [38; 39]. The impact of the interaction between water availability and temperature on plant physiology is strongly complex. Extremely warm conditions at the beginning of a growing season may compensate for ecosystem carbon losses later during a water deficit; meanwhile, early vegetation activity caused by extremely warm conditions likely also contributes to exacerbating the impacts of drought through reduced initial soil moisture [15]. Moreover, heat and drought often occur at the same time and interact with each other. Heat exacerbates water stress, which in turn increases heat damage. [PERSON] et al. [36] found that the sensitivity of climate warming regions to drought increased significantly in Southern California, indicating that temperature played an important role in increasing vulnerability.