# Vegetation Growth Response and Trends after Water Deficit Exposure in the Loess Plateau, China
## 4 Discussion  
### Response of Vegetation to Water Surplus and Deficit in Different Climatic Zones  
The Loess Plateau is experiencing changes in VWD and vegetation water surplus, with the VWD area contracting and the vegetation water surplus area expanding. This indicates that vegetation growth is hindered by a weakening water deficit and an increase in vegetation water surplus due to the improved humidity conditions. The vegetation water surplus area is relatively small and mainly located in the subhumid climate area on the southern margin of the Loess Plateau, which is prone to rain and flooding during the flood season occurring in different years [62]. The occurrence of rain-caused flooding, especially heavy flooding, negatively affects vegetation or crops with low water demand, resulting in excess vegetation water in a few areas in the southern Loess Plateau. The changes in the VWD area vary among different climate zones, with the arid and semi-humid climate zones showing a trend of contraction, while the semi-arid climate zone exhibits a non-significant increase and a trend of VWD area expansion. The semi-arid climate zone is rich in vegetation types, with cultivated vegetation and grasses being the two vegetation types most affected by drought. In recent years, the implementation of the project of returning farmland to forests and grasses has resulted in the addition of many new areas of cultivated vegetation and grasses in the Loess Plateau, but the water-consuming effect of planted forests is significantly greater than their water-holding effect, leading to widespread dry soil layers in the Loess area [63]. Therefore, the expansion trend of VWD areas in the semi-arid climate zone may be attributed to plantation forestry and grasses, which is yet to be proved by future investigation.  
Response Characteristics of Vegetation Growth to Water Deficit Events in the Loess Plateau at Multiple Timescales  
Vegetation growth responds heterogeneously to drought at multiple spatial and temporal scales. The response time of vegetation to water deficit events varies among different climate zones, with the arid climate zone showing the longest average response time, followed by the semi-arid climate zone. This variability can be attributed to the varying sensitivity, resistance, and resilience of biomes to cumulative water deficit events in different climate zones. Plants in arid regions may have developed mechanisms to rapidly adapt to a changing water supply, which could explain their rapid response to below-normal water conditions. Conversely, plants in semi-arid and semi-humid biomes have different physiological mechanisms that are less adapted to water scarcity and can only tolerate it to a limited extent. However, their response rate to drought is slower than that of arid biomes. Therefore, the average response time of semi-arid and dry-humid zones is shorter than that of arid zones [8]. Control indicators that affect the response of vegetation growth to the water supply include, but are not limited to, soil moisture, soil properties, surface temperature, snow cover area, land use types, and intensity of human activities [36; 64; 65; 66]. Therefore, an exploration of the dominant factors regarding the spatial heterogeneity of the response characteristics of vegetation growth to water availability, which is essential to improve the accuracy of vegetation change prediction, should be considered in the future.