## 4 Discussion  
Vegetation growth and variation in coverage are controlled by several anthropogenic and natural factors [51]. Shaanxi Province spans about 860 km from south to north, is characterize by complex ground surface and climate differences that, and comprises three unique subregions (QBM, GZP and LOP), as well as three climate zones (northern subtropical zone, warm temperate zone, midtemperate zone from south to north). The higher FVC in the QBM area is associated with higher precipitation and temperature, as well as a mountain topography with a lower level of human activity [52]. Lower FVC in the LOP area can be due to the lower precipitation and temperature [53], whereas the heterogeneous FVC distribution in the GZP area can be attributed to the intensive activities such as crop production and building construction. From 2000 to 2018, 73.86% of the FVC area exhibited increasing coverage, and the LOP area showed the highest increase rate of 0.075 year\({}^{-1}\). The LOP area was characterized by low rainfall, high evaporation, dense gullies, severe soil erosion, and intensive agricultural activities, which resulted in low vegetation coverage in this area. The increasing FVC in the LOP area verified the benefits of several vegetation projects such as GGP and the Three-North Shelterbelt Project in this area. Besides the revegetation projects, the variation of FVC in Shaanxi Province is correlated with the characteristics of climate and topography [54; 55]. Precipitation affects plant growth status by altering soil moisture content and humidity to influence vegetation water absorption and transpiration [56; 57]. Temperature affects vegetation growth by influencing vegetation photosynthetic efficiency and water use efficiency [58]. From 2000 to 2018, the climate in Shaanxi Province changed to a warmer and more humid trend with decreasing  
Figure 9: Detection of interaction between influencing factors of FVC change in Shaanxi Province (*: two-factor enhancement, **: nonlinear enhancement). sunshine duration (Figure 10). The time-lag duration between FVC and climate changes was correlated with the topographical characteristics: most of the research area received a 1-2 months' time lag between FVC and PRE and TEM changes, whereas a 6 months' time lag was detected in the conjunction area of GZP, QBM, and LOP. Previous studies have shown that soil temperature and soil moisture exhibit memory effects [59; 60], and climate change can affect the temperature and water content of soil. This is likely the reason for the lagged response between vegetation and climate factors. As shown in Figures 1 and 7, the conjunction area of the three subregions is characterized by the ALT steep drop with a lower ALT in GZP than in QBM and LOP. The steep ALT changes induce slow lateral infiltration of water and heat from higher to lower elevations, leading to a longer lag period. This finding has significant implications for the implementation of ecological measures. Topographic factors indirectly contribute to FVC variation by altering the light, water, and heat flow, which in turn increase the heterogeneity of the spatial distribution of vegetation on a large scale [31; 61]. Most agricultural production and residences were located in the low ALT area, where vegetation types and coverage changes were significantly affected by human activities, and this was consistent with slight FVC degradation in the GZP area. Inversely, a higher and stable FVC was detected in most areas of the QBM, with a higher ALT and only some human disturbances. In the northern LOP and most of the GZP area, the ground was flat with a lower slope that received slight FVC variation. However, in the central LOP area dominated by hills and terraces with 5-25\({}^{\circ}\) slopes, the GGP project was implemented, leading to a significant FVC improvement. Climatic changes and topographical characteristics were dominant influencing factors on FVC variation during the research period in Shaanxi Province.