## 3 Results and Discussion
### Spatial Pattern of Terrestrial NPP in China  
National average NPP in China was 322.05 g C m\({}^{-2}\) yr\({}^{-1}\) over the period of 1961-2016. As shown in Figure 4, NPP in China showed a gradient decreasing from the southeast to the northwest. The annual NPP was mostly below 50 g C m\({}^{-2}\) yr\({}^{-1}\) in extensive areas of Northwest China and western areas of Inner Mongolia covered with desert or the Gobi. In these areas, the climate is characterized by low temperatures and scarce precipitation. In the Greater and Lesser Xing'an and Changbai mountain  
Figure 3: The temporal variations of the total NPP (solid black line) in China and multivariate ENSO index (MEI) (gray chart) from 1961 to 2016. EI Niño and La Nina events are marked with red and blue circles, respectively. areas of Northeast China, temperate needle forests or temperate deciduous broad-leaved forests are extensively distributed, and the annual NPP was in the range of 500 to 600 g C m\({}^{-2}\) yr\({}^{-1}\). Southwest, South central and East China lie in the subtropical monsoon climate zone, where abundant precipitation and warm temperatures are suitable for vegetation growth. The annual NPP in these areas was above 500 g C m\({}^{-2}\) yr\({}^{-1}\). Among these areas, Hainan province had the highest NPP, i.e., above 800 g C m\({}^{-2}\) yr\({}^{-1}\), in which tropical rain forests and monsoon forests were widely distributed. In the Yunnan-Guizhou Plateau, the Hengduan mountains areas, and the southeastern coastal areas, the vegetation types were mixed with tropical rain forests, subtropical evergreen broad-leaved forests and temperate transitional forests, and the NPP ranged from 600 to 700 g C m\({}^{-2}\) yr\({}^{-1}\). In addition, the northeastern Plain and North China Plain are major grain crop and economic crop production areas of China. Temperature and precipitation are colimiting factors of vegetation growth, and the annual NPP here ranged from 200 to 400 g C m\({}^{-2}\) yr\({}^{-1}\). The Sichuan Basin is also a well-developed agricultural area with suitable climate conditions for plant growth, and the mean NPP was in the range of 400-500 g Cm\({}^{-2}\) yr\({}^{-1}\). The Qinghai-Tibetan Plateau and Inner Mongolian Plateau are major pasture areas of China, where precipitation is the major limiting factor affecting vegetation growth. The mean NPP was in the range of 200-400 gCm\({}^{-2}\) yr\({}^{-1}\). The total NPP varied greatly in different regions of China. Totals and averages of NPP were calculated for six geographical regions for the periods of 1961-1980, 1981-2000, 2001-2016, and 1961-2016 (Figure 5). Northwest China had the largest area (32.37% of the national total) but the lowest total NPP among the six geographical regions. The total NPP and average NPP in Northwest China were 0.31 Pg C yr\({}^{-1}\) (9.68% of the national total) and 101.43 g C m\({}^{-2}\) yr\({}^{-1}\), respectively, and both were much lower than those in other regions. Southwest China had the greatest portion of the national total NPP (0.90 Pg C yr\({}^{-1}\), 29.05% of the national total) due to the second largest area (24.41% of the national total) and high average NPP (392.84 g C m\({}^{-2}\) yr\({}^{-1}\)). South-central China had the highest average NPP (566.98 g C m\({}^{-2}\) yr\({}^{-1}\)). Total NPP here was 0.58 Pg C yr\({}^{-1}\). These regions were followed by East China, Northeast China and North China in the contribution to national total NPP. The total  
Figure 4: Spatial distribution of simulated NPP (g C m\({}^{-2}\) yr\({}^{-1}\)) averaged over the period from 1961 to 2016 in China. NPP in these three regions was 0.47, 0.43, and 0.41 Pg C yr\({}^{-1}\), respectively, and their average NPP was 561.45, 537.38 and 263.69 g C m\({}^{-2}\) yr\({}^{-1}\). The transient trends of annual NPP in six geographical regions of China averaged over the periods of 1961-1980, 1981-2000, and 2001-2016 are shown in Figure 5. The total NPP showed gradually increasing trends in Northwest China, Southwest China, South-central China, and East China.