## 3 Results
### Spatial Pattern and Elevational Gradient of Cropland Ecosystem NPP  
Figure 3 shows spatial pattern of the averaged cropland ecosystem NPP during 1981-2014 and its elevational gradient. Mean annual cropland ecosystem NPP of the entire study area was 752 gC m\({}^{-2}\) year\({}^{-1}\), with a total value of 71 Tg C. The cropland ecosystem NPP increased from east and north to south and west (Figure 3a). The south part of the study area is located in the subtropical humid climate zone, and cropland ecosystem NPP is higher than 700 gC m\({}^{-2}\) year\({}^{-1}\) in the area. Cropland ecosystem NPP is the lowest in east part of the Yunnan-Guizhou Plateau, the west Sichuan Plateau, and the Songpan Plateau. Along increasing elevations, cropland ecosystem NPP converted from increasing between 1000-1500 m ASL to decreasing between 1600-4500 m ASL (Figure 3b). Below 1500 m ASL, cropland ecosystem NPP increased significantly by a rate of 50 gC m\({}^{-2}\) year\({}^{-1}\) 100 m\({}^{-1}\) (R\({}^{2}\) = 0.99, \(p\) < 0.001) along increasing elevations. Between 1600-2600 m ASL, cropland ecosystem NPP was the highest, with an average value of 837 \(\pm\) 25 gC m\({}^{-2}\) year\({}^{-1}\) (range: 801-866 gC m\({}^{-2}\) year\({}^{-1}\)). Above 2700 m ASL, cropland ecosystem NPP decreased significantly by a rate of \(-\)38 gC m\({}^{-2}\) year\({}^{-1}\) 100 m\({}^{-1}\) (R\({}^{2}\) = 0.99, \(p\) < 0.001) along increasing elevations. Figure 3: Spatial pattern (**a**) and elevational gradient (**b**) of mean annual cropland ecosystem NPP during 1981–2014. Figure 2: Validation result between model simulated NPP and field survey data. The dot line was the 1:1 line. The solid line was the linear-fitting line, and the dash line was the line of linear-fitting without constant. Details of the survey methods can be found in Section 2.2, while the concurrent model-simulated NPP values are extracted at the same site. Figure 4 shows the interannual change trends in cropland ecosystem NPP and climate factors from 1981 to 2014 across the entire study area. The study area experienced significant climate warming and drying and increasing solar radiation. Annual mean temperature and total solar radiation underwent significant increasing trends by rates of 0.03 \({}^{\circ}\)C year\({}^{-1}\) (R\({}^{2}\) = 0.51, \(p\) < 0.001) and 15.19 MJ m\({}^{-2}\) year\({}^{-1}\) (R\({}^{2}\) = 0.47, \(p\) < 0.001), respectively, but annual total precipitation decreased significantly by a rate of \(-\)3.23 mm year\({}^{-1}\) (R\({}^{2}\) = 0.12, \(p\) < 0.05). Under the climate warming and drying and increasing solar radiation, cropland ecosystem NPP increased significantly by a rate of 3.85 gC m\({}^{-2}\) year\({}^{-1}\) year\({}^{-1}\) (R\({}^{2}\) = 0.43, \(p\) < 0.001), resulting in an increase in amount of NPP of 131 gC m\({}^{-2}\) year\({}^{-1}\) during the 34-year. Spatial Patterns and Elevation Gradients of Interannual Change Trends in Cropland Ecosystem NPP and Climate Factors  
Figure 5 shows the spatial patterns and elevational gradients of interannual change trends in cropland ecosystem NPP and climate factors from 1981 to 2014. Cropland ecosystem NPP increased significantly with the increasing of solar radiation and climate warming in most parts of the investigated area. Temperature trends were positive significantly at all elevations. Along increasing elevations, the warming trends increased significantly by a rate of 0.0008 \({}^{\circ}\)C year\({}^{-1}\) 100 m\({}^{-1}\) (R\({}^{2}\) = 0.87, \(p\) < 0.05) from 1000 m ASL to 2600 m ASL, then were around 0.04 \(\pm\) 0.003 \({}^{\circ}\)C year\({}^{-1}\) above 2700 m ASL. Precipitation trends were negative significantly between 1400-1800 m ASL. Solar radiation trends were positive significantly at all elevations.