# Spatiotemporal Evolution and Impact Mechanisms of Gross Primary Productivity in Tropics
## 4 Results
### Impact of Land Cover Type Transformation on Gross Primary Productivity  
The map of land cover types in 2001-2020 (Figure 7) indicates that the following land cover types are more intensively distributed: evergreen broad-leaf forest is concentrated in the Amazon rainforest in the central part of South America, in the central part of Africa, and in Southeast Asia; broadleaved deciduous forest is concentrated in southern and central Africa; cropland is concentrated in eastern South America and southern Asia; grassland is concentrated in central Australia; and bare areas are concentrated in the desert areas of northern Africa. The main land cover types in the tropics are cropland and various forests. The most transferred type is evergreen broad-leaf forest, covering a total of 5.44 \(\times\) 10\({}^{5}\) km\({}^{2}\). It is primarily transferred to mosaic cropland (2.66 \(\times\) 10\({}^{5}\) km\({}^{2}\)), tree mixed (5.80 \(\times\) 10\({}^{4}\) km\({}^{2}\)), and shrubland (5.33 \(\times\) 10\({}^{4}\) km\({}^{2}\)). This is followed by shrubland and mosaic cropland, covering 3.62 \(\times\) 10\({}^{5}\) km\({}^{2}\) and 2.83 \(\times\) 10\({}^{4}\) km\({}^{2}\), respectively. The largest transfer is mosaic cropland, totaling 3.79 \(\times\) 10\({}^{5}\) km\({}^{2}\), composed of evergreen broad-leaf forest (2.66 \(\times\) 10\({}^{5}\) km\({}^{2}\)). The area of water bodies remains relatively stable with little change. The urban area, although relatively small, is increasing, with its transferred area much larger than the transferred area. At the macro level, the land where land cover type transfer occurs accounts for 4.39% of the total area, which is relatively small. The vast majority of the tropics remain unchanged in terms of land cover type.  
From the land cover type maps of the tropical region in 2001 and 2020, we can calculate the amount of vegetation _GPP_ transfer caused by changes in land cover types and derive the land use transfer matrix for the period of 2001-2020 (Table 3). Over the 20-year span, the cumulative area of land cover types transferred out exceeds one million nine hundred thousand hectares. The impact of land cover type conversion on _GPP_ primarily manifests in two scenarios: firstly, the conversion of diverse land types into evergreen needle-leaved forest, cropland, or bare areas; secondly, the transformation of mosaic cropland, grassland, and bare areas into cropland leads to an increase in _GPP_. Cropland exhibited the highest total _GPP_ transfer, with an increase of 5.54 \(\times\) 10\({}^{6}\) G\(\mathrm{\SIUnitSymbolCelsius}\), while grassland showed the greatest decrease in total _GPP_ transfer, amounting to 8.85 \(\times\) 10\({}^{4}\) G\(\mathrm{\SIUnitSymbolCelsius}\). Additionally, the highest total _GPP_ transfer occurred from bare areas, with an increase of 1.06 \(\times\) 10\({}^{7}\) G\(\mathrm{\SIUnitSymbolCelsius}\). Grassland, snow, and ice also exhibited significant total _GPP_ transfers, amounting to 6.84 \(\times\) 10\({}^{6}\) G\(\mathrm{\SIUnitSymbolCelsius}\) and 4.33 \(\times\) 10\({}^{6}\) G\(\mathrm{\SIUnitSymbolCelsius}\), respectively. The most substantial decrease in total transfer was observed in evergreen broad-leaf forest, with a decrease of 5.03 \(\times\) 10\({}^{6}\) G\(\mathrm{\SIUnitSymbolCelsius}\). Moreover, evergreen needle-leaved forest experienced a considerable decrease in total transfer, totaling 3.63 \(\times\) 10\({}^{6}\) G\(\mathrm{\SIUnitSymbolCelsius}\). The data reveals that mutual transfers among forests, grasslands, and croplands are more frequent within the tropics.  
Figure 7: Spatial distribution map of land cover type transfer in the tropical zone from 2001 to 2020.