## 4 Discussion
### NPP Response to Drought Events  
Mongolia experienced six major drought events in 2004, 2005, 2006, 2007, 2009, and 2017 during which more than 60% of the total area suffered from drought, resulting in a decline in NPP to different extents. For the temporal dimension, there was a slight increasing trend in both NDVI anomaly and BEPS-derived NPP. The growing season NPP variance was moderately (\(r\) = 0.61 \(p\) < 0.001) correlated with the variability in wet and dry conditions as expressed by the drought index NDVI anomaly at the country scale, which is consistent with the finding from previous studies [67; 68].  
Spatially, the growing season NDVI anomaly was positively correlated with total NPP for 92.95% of the land, and this association was more pronounced in the western and southeastern regions. This suggests that the growing season productivity dynamics are parallel with the drought conditions in these regions. This also agrees well with the findings of a recent study that used SPEI as an indicator of drought [37]. However, for forested and mountain areas (which constitute 7.05% of the country's land area), the growing season mean NDVI anomaly was negatively correlated with the total NPP. These regions experience a low temperature and relatively high precipitation and do not suffer from drought conditions.  
Droughts have significantly affected the NPP, which was also ascertained in previous studies on a regional to a global scale by [PERSON] et al. [20], [PERSON] [7], and [PERSON] et al. [22]. A recent study based on the MODIS NPP product and a drought index (SPEI anomalies) found that the NPP decreased by 50% in Mongolia at a country level from 2000 to 2014 [22]. However, we found a decreasing trend in NPP during the drought events, and NPP anomalies were the highest when drought severity reached its peak (decreased by 75%). This discrepancy is possibly because of differences between the regional and global scales. As a representative of sparse vegetation, reduced NPP in the eastern regions by severe drought was approximately 10-75% during those events. For mild and moderate drought, a 10-50% decrease in NPP was observed, and they even reduced 50-75% in some parts in the growing season of 2017 (Figure 6).  
More specifically, the impacts of drought on vegetation productivity were intertwined with the severity of drought events and the types of vegetation. The severe droughts had a larger impact on vegetation growth than moderate and mild droughts. This finding agrees with that of [PERSON] et al. [69], who found that NPP decreased significantly with the increase in drought severity. Finally, we found that the correlation between the annual total NPP and NDVI anomaly varied across vegetation types, from 0.39 in the forest to 0.79 in sparse vegetation (Figure 8). This means that the NPP of sparse vegetation is more susceptible to drought conditions than other types of vegetation. This was also consistent with a similar previous study, which showed that NPP of sparse vegetation was highly sensitive to temperature, whereas forest NPP showed a weak response to precipitation [10]. Additionally, it was indicated that the NPP value was highest in the wettest years, while the lowest NPP value was witnessed in the drought years (Figure 6). Thus, drought events that typically lasted for 5 months have largely impacted NPP in Mongolia.