# Stronger Cumulative than Lagged Effects of Drought on Vegetation in Central Asia
## 4 Discussion
### Vegetation GPP Response to Drought  
Drought has a lagged effect on most vegetation in Central Asia, which demonstrates that early soil moisture conditions largely influence current vegetation growth [91; 92; 93]. The correlation between vegetation and drought lag is higher in the southern and western regions of Kazakhstan, where the main vegetation type is grassland. Grassland comprises shallow-rooted plants, and the source of water during growth is mainly the soil surface water. When drought occurs, these roots are unable to absorb the water deep in the soil or groundwater, thus, grassland has a rapid response to drought, and there is a higher correlation [94], this finding is consistent with the highest sensitivity of grass EVI to drought in Huang-Huai-Hai River Basin, China [20]. The number of lagged months after drought on most of the vegetation was mainly focused on shorter time scales (1-2 months) in Central Asia, indicating that vegetation activities respond faster to wet and dry environments [95]. In forest ecosystems, the average lag time of drought reached 4.96 months, which may be because forests have a deeper root system, and annual tree rings allow water storage in the short term for tree growth; thus, the drought lag time of forests is longer than that of shrubland, grassland, and cropland. The cumulative effect of drought reflects the combined effects of drought on vegetation processes over successive periods of time, from start to finish. In this study, vegetation GPP was positively correlated with the cumulative effect of drought in over 92% of Central Asia, indicating that hydrology is the major limiting condition for vegetation growth in Central Asia [96]. In contrast, drought was negatively correlated with vegetation GPP in a small proportion of forested areas in northeastern Central Asia, where the corresponding accumulated number of months was 1. This region is located in a high-elevation boreal forested zone with adequate soil moisture content. Temperature is the major limiting element for the growth of cold-land vegetation; thus, the forests in this region are less affected by the cumulative effect of drought [97; 98]. This is consistent with the results of the cumulative effect of drought on global vegetation GPP, both of which have the shortest cumulative time scales in forested areas [27]. We found that 75.86% of the region had a cumulative drought time spanning 9 to 12 months. Central Asia has a temperate continental climate with dryness and little rainfall [99], and the vegetation has developed a certain degree of drought resistance to prolonged and intense drought over time in its long-term evolution [100]. Thus, the cumulative effect occurs on a long time scale, which is consistent with the previous study in which the cumulative drought time in this region was found between 40\({}^{\circ}\) and 45\({}^{\circ}\) N [95]. An adequate water supply promotes vegetation development, while a lack of water limits the carbon uptake and transpiration of vegetation [101]. The correlation of vegetation GPP with the SPEI in this study was stronger in relative drought regions, and the correlation of GPP with the SPEI reached the maximum when the average annual SPEI was around \(-0.4\), indicating that moisture conditions had a greater influence on vegetation development under arid climatic conditions; thus, vegetation was more affected by drought in arid and semi-arid areas. For example, the Vegetation Anomaly Index was found to be highly correlated with precipitation in semi-arid regions of North Africa [102; 103; 104]. Meanwhile, the fastest-lagged response of vegetation to drought was observed in regions where the average annual SPEI was approaching \(-0.4\), which was attributed to the fact that the soil surfaces in these areas were more active and variable in terms of moisture, and the vegetation in the area was extremely sensitive to drought and could cope with drought conditions by adjusting its physiology in a timely manner to achieve water balance [105].