## 3 Results
### Time-Lag Response of the GPP to Meteorological Variables during the Vegetation Growing Season
Despite a significant decrease in rainfall in this area, forests have a robust water-retention capacity. Furthermore, to ensure food production, there is a high degree of human intervention in farmland, which may weaken the negative impact of rainfall changes on GPP in this region. According to our results, the GPP exhibits clear lag effects in response to meteorological factors such as temperature, rainfall, and radiation (Figure 7b). In China, excluding regions with no meaningful research significance (primarily deserts), areas with no time lag for temperature are the largest, covering 17.74% of the total area. These areas are mainly concentrated in the northwest, where vegetation is predominantly composed of short-growing grasslands. Regions with a 3-month lag effect on GPP in response to rainfall constitute the largest proportion at 26.84%, primarily concentrated in the Loess Plateau region, followed by regions with a 2-month lag effect at 17.59%. For GPP in response to radiation, areas with no time lag are the most prominent, accounting for 23.21% of the total, mainly concentrated in local parts of Regions VII and V, followed by regions with a 2-month lag effect at 19.10%. It is noteworthy that we observed that the proportion of the regions with an accumulating effect of vegetation carbon absorption in response to  
Figure 6: Normalized results (dimensionless) of the GPP and meteorological variables (SR, solar radiation; PRCP, precipitation; GPP, Gross Primary Productivity; TEMP, temperature). meteorological elements is lower than those with lag effects. The proportion of regions with an accumulating effect is less than 40% of the total study area. Lag Statistics of GPP in Different Elevation Ranges, Regions, and Land-Use Types in Response to Meteorological Elements  
The response ratio of vegetation GPP to meteorological elements varies with different altitudes (Figure 8). Specifically, as altitude increases, the proportion of the GPP's cumulative response without lag to temperature increases. Within the altitude range of 3500-5500 m, the proportion of the high-lag 1-0 stage is higher, accounting for 23.38%. The GPP's response ratio to rainfall in stages 3-0 is higher, concentrated in the range of 500-2500 m and above 5500 m. The proportion of GPP's cumulative response without lag to radiation is higher in regions with altitudes less than 500 m, and this proportion tends to decrease with increasing altitude. Additionally, the proportion of the GPP's cumulative response to radiation increases with increasing altitude. From a climatic-zone perspective, there are significant differences in the response to meteorological elements in various regions (Figure 9). Specifically, when examining the lag-accumulation effects of the GPP in response to temperature, the stages with the highest proportion, from Region I to Region VII, are as follows: 3-0 (17.87%), 2-0 (27.37%), 2-0 (22.46%), 0-1 (25.39%), 3-0 (24.45%), 2-0 (15.74%), and 1-0 (35.01%). For GPP responses to rainfall, the stages with the highest proportion of lag are all 3-0, with proportions ranging from 24.42% in Region VII to 32.10% in Region IV. Regarding GPP responses to radiation, the stages with the highest proportion of lag-accumulation effects in these seven regions are as follows: 1-0 (23.24%), 2-0 (19.83%), 1-0 (21.92%), 3-0 (33.56%), 1-0 (28.42%), 2-0 (27.71%), and 0-1 (30.86%). Figure 7: Distribution of the maximum correlation coefficients between GPP and meteorological elements (**a**) and lag distribution (**b**). Apart from regional differences, different land-use types exhibit varying responses to meteorological elements. Therefore, we calculated the proportions of lag stages of the GPP response to meteorological elements in several land-use types with higher representation (Figure 10). For instance, cropland and forest GPP have the highest proportions for the temperature lag stages of 2-0 (18.05%) and 1-0 (17.10%), respectively.