# Spatiotemporal Evolution and Impact Mechanisms of Gross Primary Productivity in Tropics
## 1 Introduction  
Vegetation is the primary component of terrestrial ecosystems and plays a pivotal role in regulating global climate, carbon balance, and other environmental factors [1]. Increasing the carbon sink of terrestrial vegetation has become a necessary strategic choice for humans to adapt to climate change [1, 2]. Gross primary productivity (GPP) refers to the total amount of organic carbon fixed by green plants through photosynthesis per unit area of ecosystem and per unit time and serves as an important indicator to characterize the intensity of plant photosynthesis [3, 4]. This process determines the total amount of the original substances [5]. Vegetation GPP is the largest component of the carbon sink in terrestrial ecosystems and plays an important role in regulating the global carbon cycle [6, 7]. Therefore, studying the evolution of GPP is essential for assessing ecosystem health and the status of forest types in response to climate change. The tropics play a pivotal role in safeguarding the global environment from the adverse effects of global warming and maintaining global biodiversity as a potential carbon sink. They are home to nearly half of the planet's species and about 70%-80% of the world's tree species [8]. The dominant land cover type in the tropics is forest, with tropical forests accounting for approximately 50% of the world's forests and 45% of the carbon content of terrestrial forests [8, 9]. Tropical forests are responsible for the production and storage of 40% of the world's biomass carbon, and they also act as a significant carbon sink, absorbing approximately six times more carbon through photosynthesis than humans release into the atmosphere from fossil fuels. This makes the existence of this carbon sink crucial tothe global carbon cycle and human ecological security [10]. Consequently, tropical forests contribute significantly to the global carbon balance, are the most productive terrestrial ecosystems, and are globally important carbon sinks [11]. Any increase or decrease in their biomass will have a significant impact on the Earth's carbon cycle [12]. The study of the spatial and temporal patterns of GPP in the tropics and its mechanisms has far-reaching implications for the study of the terrestrial carbon cycle and global climate change. The current research on GPP is focused on the following aspects: (1) Analysis of trends in spatial and temporal evolution: Previous studies have demonstrated a significant increase in the occurrence of GPP over the past decades [12; 13; 14]. The largest increase is in the southern part of the Amazon rainforest in the tropics, with an increase of \(5\,\mathrm{g}\cdot\mathrm{cm}^{-2}\mathrm{year}^{-1}\)[15]. Among these regions, the low-latitude tropical region near the equator has the highest GPP, with GPP per unit area exceeding \(2000\,\mathrm{g}\cdot\mathrm{cm}^{-2}\mathrm{year}^{-1}\)[16; 17]. Overall, the regions with increasing trends of GPP in the tropics are mainly north of the equator, while the regions with decreasing trends are mainly south of the equator. Among these regions, the change in GPP in the Australian region is not significant, while the Southeast Asian region shows predominantly a decreasing trend [18; 19]. (2) To investigate the mechanisms underlying GPP, factors such as the CO\({}_{2}\) fertilization effect [20; 21], climate change [14; 22], nitrogen deposition [23], and land cover change [11; 24; 25] have been shown to positively influence the increase in GPP. CO\({}_{2}\) fertilization effect: Increased carbon dioxide concentration leads to increased vegetation productivity, commonly known as the CO\({}_{2}\) fertilization effect (CFE). In recent years, the vegetation response to CO\({}_{2}\) has declined over much of the world's land-mass, implying that the positive impact of increased atmospheric CO\({}_{2}\) on terrestrial carbon sequestration has been substantially reduced [26].