# Spatiotemporal Evolution and Impact Mechanisms of Gross Primary Productivity in Tropics
## 4 Results  
Characteristics of the Spatial and Temporal Distribution of Gross Primary Productivity of Vegetation in the Tropics  
1.1 Characteristics of the Temporal Distribution of Gross Primary Productivity of Vegetation in the Tropics  
The magnitude of total vegetation primary productivity (\(GPP\)) is an important indicator for evaluating the quality of vegetation in terrestrial ecosystems. The graph illustrating the trend of the annual average value of the gross primary productivity of vegetation in the study area from 2001 to 2020 (Figure 3) shows that the overall \(GPP\) exhibited a fluctuating and increasing trend over the past 20 years. The correlation coefficient R\({}^{2}\) is 0.90, indicating a strong linear relationship. Specifically, the average \(GPP\) of vegetation increased from 2557.08 g\(\cdot\)cm\({}^{-2}\)a\({}^{-1}\) in 2001 to 2757.11 g\(\cdot\)cm\({}^{-2}\)a\({}^{-1}\) in 2020, representing an increase of 7.82% (Figure 3). The average \(GPP\) over the 20 years was 2665.19 g\(\cdot\)cm\({}^{-2}\)a\({}^{-1}\), with an average annual rate of increase of 10.21 g\(\cdot\)cm\({}^{-2}\)a\({}^{-1}\), which was statistically significant (\(p<0.05\)). Vegetation \(GPP\) varied from 2511.28 to 2757.11 g\(\cdot\)cm\({}^{-2}\)a\({}^{-1}\), with the lowest value occurring in 2002, representing a 5.77% decrease from the 20-year average \(GPP\), and the highest value occurring in 2020, representing a 3.45% increase from the 20-year average \(GPP\). The fluctuation of the data over the 20 years was small and observable. The \(GPP\) data exhibited minor declines in the years 2002, 2005, 2014, and 2016, indicating a slight decrease in the gross primary productivity of vegetation compared to the previous year. Conversely, small peaks were observed in 2001, 2006, 2015, and 2020, indicating a slight increase in the gross primary productivity of vegetation compared to the previous year. These fluctuations may be influenced by various factors, such as climate change, land cover change, and human activities. Despite some annual fluctuations, the overall trend shows steady growth. 1.2 Characteristics of the Spatial Distribution of Gross Primary Productivity of Vegetation in the Tropics  
The spatial distribution of vegetation GPP in the tropics (Figure 4) demonstrates that the tropics as a whole exhibit a high level of gross primary productivity, with high _GPP_ concentrated in a limited area of southern North America, the majority of northern and central South America, central Africa, and the southern islands of Asia, while low _GPP_ is observed in the western mountain ranges of South America, northern and southern Africa, the Indian Peninsula, and the majority of Oceania. In general, _GPP_ is higher in the inland plains of the tropics and lower in coastal areas and in areas with special topography, such as deserts and mountains. With the exception of areas with special topography, such as deserts, _GPP_ in the tropics tends to be higher in the northern hemisphere and lower in the southern hemisphere. The analysis of the trend of the annual average value of vegetation GPP (Figure 5) reveals that the majority of regions exhibit an increasing trend in GPP. Conversely, the regions displaying the least notable increase in GPP are concentrated in mountainous and plateau regions of South America and Africa, coastal areas of the southern islands of Asia, and the transition between shrubland and grassland in Oceania. Significant decline is observed, with the most notable losses occurring on the island of New Guinea. Figure 4: Spatial distribution of the average annual vegetation GPP from 2001 to 2020 in the tropical zone. Figure 3: Interannual variation of vegetation GPP in the tropical zone from 2001 to 2020 (COD indicates the coefficient of determination for the data). Conversely, the areas with a non-significant decrease are more dispersed and mainly located in the grassland region of Oceania. Finally, the areas with non-significant change are concentrated in the bare areas of Africa, specifically the Sahara Desert.