## 4 Discussion  
Cropland ecosystem NPP converted from increasing to decreasing along increasing elevations in the alpine region of the southwest China. The high values above 800 gC m\({}^{-2}\) year\({}^{-1}\) were at elevations between 1600-2600 m ASL. Crop photosynthetic capacity has a positive correlation with temperature, but this positive correlation also applies to photorespiration and dark respiration [39]. Thus, the maximum NPP tends to occur under medium temperatures [40]. In the study area, mean annual temperature decreased along increasing elevations [12]. The medium temperatures existed between 1600-2600 m ASL, which was located in the central part of Yunnan Province. Its mean annual temperature was between 10.11-15.35 \({}^{\circ}\)C, while its annual total precipitation was between 983-1098 mm. Below 1500 m ASL, croplands were located in east and northeast parts of the study area, the river valley in the southern part of the Yunnan-Guizhou Plateau. These regions were situated in the humid subtropical climate zone with air temperature being between 15.64-15.91 \({}^{\circ}\)C and precipitation being between 1103-1150 mm. In the humid subtropical climate zone, sufficient hydrothermal conditions were conducive to photosynthesis, but the higher temperature caused autotrophic respiration to consume more photosynthetic products [41]. Moreover, the east part of the study area is the main karst area in China, with poor soil and hydrothermal conditions. In this region, soil erosion is easily triggered under a subtropical monsoon climate [17; 42]. Above 2600 m ASL, croplands were located in the west Sichuan Plateau, the Songpan Plateau, and the Hengduan Mountains. Annual mean temperature in these alpine croplands was below 10 \({}^{\circ}\)C. Crop photosynthetic rate and carbon assimilation was limited by the low thermal condition [43; 44]. The significant increasing trend in cropland ecosystem NPP in the study was in accord with previous studies in the southwest China or its alpine region. For example, during 1961-2010, wheat potential productivity increased by 283 kg ha\({}^{-1}\) per decade significantly in southwest China [45], while maize potential productivity increased in its alpine region [29]. In the Hengduan Mountains, ecosystem NPP increased in 74% of the total area [46], and cropland ecosystem NPP increased with a rate of 5.38 gC m\({}^{-2}\) year\({}^{-1}\) from 2000 to 2016 [44]. It is generally believed that climate change has improved the cropland ecosystem NPP in alpine region of the southwest China. However, previous studies tracked different change trends in air temperature. The southwest China experienced a climate-cooling trend during 1900-2000 [47] but underwent a climate warming with rates of 0.015 \({}^{\circ}\)C year\({}^{-1}\) and 0.04 \({}^{\circ}\)C year\({}^{-1}\) during 1960-2013 and 1982-2013, respectively [1]. The different change trends in temperature during different periods suggested that period after the 1980s was the warmest 30-year period of the last 1400 years [48]. Along increasing elevations, increasing cropland ecosystem NPP was significantly driven by increasing solar radiation positively at all elevations but was driven by climate warming from negative at low elevations to positive at high elevations. Climate warming had an elevation-dependent effect on increasing NPP, which resulted in that NPP trends of all elevations were mainly driven by temperature trends positively. At low elevations below 1700 m ASL, croplands were located in east, northeast, and southwest parts of the study area with a subtropical climate. It has been shown that significant climate-warming trends can enhance crop autotrophic respiration and cause a negative impact on cropland ecosystem NPP [36; 49]. Moreover, crops need more evapotranspiration to assimilate CO\({}_{2}\) in the subtropical climate zone [50; 51]. Cropland in the east part of the study area is the main karst area in China, with fragile ecosystem traits.