## 4 Discussion
The northeast part surrounding Sichuan Basin is the junction area of the basin and the alpine region, with steep slope and -frequent landslides. Most of the southwest part are dry-hot river valleys with a foehn effect on cropland ecosystem. Soil in these areas was characterized by steep slopes, thin soil thickness, frequent landslides, and serious soil erosion, resulting in a susceptible surface hydrological trait to climate warming and drying. Precipitation of these areas has displayed an decreasing trend and a strong seasonal anomaly during the past decades [12; 52]. The decreasing precipitation would reduce soil moisture and lead to deficit of water available in cropland ecosystem [53]. At high elevations above 2600 m ASL, annual mean temperature dropped to below 10 \({}^{\circ}\)C, and the photosynthetic rate abated under worsened heat condition. Climate warming enhanced crop photosynthesis and had a significant positive effect on cropland ecosystem NPP at high elevations with colder temperature [54]. Absolute values of temperature SRC were larger at high elevations than at low elevations, featuring a stronger driving effect on cropland ecosystem NPP at high elevations. The elevation-dependent driving effect of climate warming on ecosystem has also been found in the adjacent plateau, i.e., the Tibetan Plateau [55; 56; 57]. The alpine ecosystem has to develop stronger ecosystem properties, such as foliar nitrogen content [58], to cope with harsh environment at high elevations. Soil microbial community also aggregates cold shock, nitrogen cycling, and sulfur cycling genes to adapt cold environment at high elevations [59]. The stronger ecosystem and soil properties caused cropland to be more sensitive to climate warming at high elevations than at low elevations. It is worth noting that croplands at elevations below 1700 m ASL were vulnerable to climate drought. Since the 1950s, drought frequency and its influenced area have increased in the study area [19; 60; 61]. In particular, the northeast and southwest parts exhibited high frequency of seasonal precipitation anomaly and climate drought. During 2000-2017, more than 10 drought periods were tracked in the two parts by Self-calibrating Palmer Drought Severity Index [62]. Seasonal precipitation anomaly and drought, especially occurring before growing season, has greater impact on cropland ecosystem NPP than other periods [63]. Moreover, crops assimilate CO2 through photosynthesis and evapotranspiration to regulate growth [50]. In a tropical monsoon climate, surface energy is primarily allocated to latent heat flux [64], and cropland ecosystem evaporation constitutes 71-74% of the total evapotranspiration during the growing season, which is principally controlled by temperature and solar radiation [65; 66]. Under the significant climate warming and increasing solar radiation, cropland needs more water to assimilate CO2 at low elevations of the investigated area. The conflict between climate drying, frequent drought at low elevations, and the stronger water requirement of croplands resulted in that croplands at low elevations of the study area were more vulnerable to climate drought than other regions. The elevation-dependent effect of climate warming on increasing NPP and elevational gradient of cropland distribution led to different results when we analyze climate driving effects on cropland NPP at different spatial scales. Cropland area percentages were much larger at low elevations than at high elevations (Figure 11). The distribution of cropland and the negative driving effect of climate warming on cropland ecosystem NPP at low elevations brought about nonsignificant negative impact of climate warming on increasing NPP across the entire study area. Meanwhile, the false driving effect of climate warming on interannual change trend in cropland ecosystem NPP across the entire study area also affected driving effects of precipitation and solar radiation on cropland ecosystem NPP in a multiple linear regression analysis.