## 4 Discussion
### The Significant Increase in Vegetation and Its Driving Forces  
Annual LAI and GPP in the MHRB increased significantly from 2001 to 2018. Other studies similarly showed rising trends of vegetation in the MHRB after the year 2000 [25; 26; 27]. In the MHRB, around 5% of the areas with significantly increased vegetation between 2001 and 2018 underwent the transformation of other land use types to forest. One of the primary strategies for implementing ecological engineering in the MHRB is afforestation. In comparison to other land use types, forests had much greater LAI and GPP, and their expansion encouraged a rise in vegetation cover in the MHRB. Afforestation is regarded as one of the main causes of the greening trend of vegetation in China [56; 57]. Additionally, we discovered that about 87% of the MHRB's region with considerably enhanced vegetation had not undergone a change in land use. For areas with unchanged land use, the significant increase in vegetation may be promoted by both climate change and anthropogenic measures. Vegetation's spatial heterogeneity and temporal variation both exhibited stronger relationships with moisture-related factors. The humidity index was the most significant climate indicator for the spatial heterogeneity of vegetation (Figure 7). Humidity index and precipitation were more strongly correlated with vegetation in the MHRB from 2000 to 2018 when compared with other climatic factors (Table 5). During this period, the precipitation and humidity index increased in the MHRB (Table 6), which aided in the growth of the vegetation. Other large-scale studies have also revealed that the sensitivity of vegetation changes to moisture is growing [9; 10]. The driving forces of vegetation temporal variation and spatial heterogeneity were not entirely equal. VPD had a limited ability to explain spatial heterogeneity, while the strongest negative correlation between VPD and vegetation change in the MHRB suggested that a reduction in VPD would promote the increase in vegetation. The detrimental effect of VPD on vegetation growth has been confirmed on a global scale [12]. Despite a large \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**MLHRB** & **MBSHRB** & **MYDRRB** & **MDQHRB** & **MZYHRB** & **MZWHRB** \\ \hline \(-\)0.65 \({}^{*}\) & \(-\)0.65 \({}^{*}\) & \(-\)0.8 \({}^{*}\) & \(-\)0.61 \({}^{*}\) & \(-\)0.59 \({}^{*}\) & \(-\)0.65 \({}^{*}\) \\ \hline \hline \end{tabular} Note: \({}^{***}\) denotes that the correlation coefficient is significant (\(p<0.05\)).  
\end{table} 
Table 7: The correlation coefficient between humidity index and ETo in the MHRB from 2001 to 2018.  
rise in VPD globally after the late 1990s was revealed [13], a decreasing trend of VPD was found in the MHRB from 2001 to 2018. The increased precipitation and humidity index and the reduced VPD facilitated vegetation growth in the MHRB from 2000 to 2018. According to studies conducted in the MHRB and its neighboring regions, the wetter climate that began to prevail after 2000 encouraged the increase in vegetation [25; 51]. In the MHRB, temperature and vegetation change were negatively and insignificantly correlated (Table 5). A previous study also found that vegetation change in most areas of the HRB was negatively correlated with temperature [25]. The increase in temperature can increase ETo [58], which tends to make the climate drier. Increasing temperature is prone to increase VPD, which would negatively affect vegetation growth [59]. The correlation between temporal changes in vegetation and solar radiation was weak (Table 5). A study in northern China also showed that the impact of solar radiation on vegetation was weaker during the period 1990-2015, while the impact of precipitation increased [60]. The spatiotemporal changes and corresponding dominant influencing factors of LAI and GPP were similar. However, we also found that GPP was more sensitive than LAI to climate change both in terms of spatial heterogeneity and temporal variation.