## 4 Results
### Spatial Pattern of the Vegetation Greenness Trends  
From 2000 to 2019, 84.1% of the study region greened up (\(p<0.05\)), although at various magnitudes (Figure 4). Nearly half (49.87%) of the region greened up more than 2.39%/year, and 15.66% more than 3.78%/year, with respect to the average growing season NDVI for the years from 2000 to 2002. Only 0.43% of the study region experienced browning at a 5% level (Figure 4). These browning regions were mostly located in the urbanizing regions of Lanzhou, Baiyin, and Linxia, where vegetation was replaced by impervious surfaces.  
Some forests on Wushaoling and Jishi mountains show no statistically significant trends of growing season greenness (Figure 4), which suggests that there might be limiting factors for the forests to green up, given the warming climate and rising atmospheric CO\({}_{2}\) concentration. In addition, the average magnitude of green-up for forests (1.26%/year) is much lower than those for cropland (2.88%/year), grasslands (2.84%/year), and even barren land (2.77%/year), where sparse short bushes grow (Figure 4). This suggests that the forests in our study region are, in general, quite mature--premature forests usually exhibit higher rates of greening due to natural growth. Nevertheless, some forests greened up at comparable rates to the average green-up rate of the herbaceous biomes. These forests are distributed close to the tree lines on the mountains.  
The trends of growing season NDVI for grasslands are bimodally distributed (Figure 5), with one mode at around 0.5%/year, and the other at around 3.3%/year. This suggests that there are two sub-types of grasslands in our study region: one type greened up much faster than the other. Other than that, the greenness trends of grasslands are similar to those of cropland. This can be explained by the fact that they are both herbaceous. On the other hand, this suggests that agricultural practices, such as fertilization and irrigation, might contribute little to the greenness trends of cropland, while climate and CO\({}_{2}\) fertilization might play a major role in driving the greenness trends of cropped vegetation in this semi-arid region. The mean green-up magnitude of the barren land is similar to that of the herbaceous land cover types (i.e., grasslands and cropland), but the variation of the former is smaller than that of the latter, suggesting that barren land is more homogeneous than grassland and cropland.  
Figure 3: Average growing season Normalized Difference Vegetation Index (NDVI) for the study region during the period from 2000 to 2019. The growing season was defined as May to September. The NDVI data were obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6 MOD13Q1 product, in which NDVI and accompanying quality assurance (QA) fields are provided. The NDVI data corrupted by clouds, aerosols, and cloud shadows were filtered out in the calculation of the growing season mean NDVI.  
Figure 4: Spatial pattern of the trends of growing season mean NDVI for the study region in the period from 2000 to 2019. The trends were calculated using Sen’s method, and were tested at the 5% level using the Mann–Kendall test. Areas with no statistically significant trends are colored white. The unit of the trends relates to the average growing season NDVI for the years from 2000 to 2002.  
Figure 5: Frequency distribution of the trends of growing season NDVI for major land cover types in the study region from 2000 to 2019. Almost all of the trends are positive. The bimodal frequency distribution of the trends for grasslands suggests there are two categories of grasslands.