# Satellite-Observed Global Terrestrial Vegetation Production in Response to Water Availability
## 4 Discussion
### Terrestrial Ecosystems' Drought Resistance  
Terrestrial ecosystems' drought resistance can be characterized as: a water deficit must continue for a period of time before negative anomalies occur in ecosystem variables [47]. In this study, response time of productivity to water availability was defined according to timescales over which the maximum GPP-SPEI correlations were recorded. To some extent, the response time of vegetation productivity to drought can characterize terrestrial ecosystems' drought resistance. Our research results showed that 56.8% of the global terrestrial ecosystems' response time to water availability is based largely on short- and medium-term time scales. Because different hydrological backgrounds and vegetation composition affect terrestrial ecosystems, their drought resistance demonstrates strong spatial heterogeneity. In Canada, Northwestern Colombia, Eastern Russia, and Northeastern and Southeastern regions China, the response time of vegetation productivity to SPEI is relatively long, usually greater than 12 months. The longer the response time to water availability changes in these regions, the stronger the vegetation productivity's capacity to withstand long-term water shortages. Seasonal changes also affect the resistance of the ecosystem to drought in the same region, particularly in mid-to-high latitude regions (Figure S3). Taking the mid-to-high latitude regions of the Northern hemisphere as an example, it can be seen that the proportion of GPPs in the high latitude with a longer response time to SPEI decreases gradually with the passage of seasons. This result indicates that vegetation has a strong resistance to drought in the early stage of growth, and as subsequent vegetation growth requires considerable water, the occurrence of drought at this stage is more likely to cause vegetation productivity to decline. Climatic conditions and different types of land cover often affect terrestrial ecosystems' drought resistance [48; 49]. Our results showed that as the climatic conditions become gradually humid, the response time of vegetation productivity to changes in water availability increases. This indicates that vegetation productivity in arid and semiarid areas is more vulnerable to drought than in humid and semi-humid areas. [PERSON] et al. [49] also found that arid biomes respond to drought at short time-scales. This may be related to the adaptation strategies of vegetation to water availability in different climate zones. Figure 11: Changes in climate factors during the drought period. (**a–c**) show the anomalies of TEMP (Temperature), RAD (Radiation), and SM (soil moisture), respectively, during drought periods compared to those in non-dry periods. When a water deficit occurs, arid ecosystems adapt quickly to water shortages by reducing water loss and respiratory costs and increasing growth rates. [PERSON] et al. [20] demonstrated that, as there is a long-term water surplus in humid areas, a short-term drought can not cause changes in vegetation productivity easily. Further, our results revealed different land biomes' role in drought resistance. By comparing GPP's response time to SPEI under different land cover types, we found that DNF, OS, EBF, and ENF have a greater ability to adapt to drought, while WSA, MF, SAV, and GRA have a weak ability to resist and mitigate drought. Forests generally have deep root systems, and thus, under severe drought conditions, they can use the water stored in the deep soil to achieve strong resistance to drought. Whether drought disturbs a forest ecosystem is related closely to its intensity and duration. The water use strategy of shrubs is generally to increase the use of surface soil water during non-arid periods and absorb deep soil water during dry periods to maintain their own production needs [50]. However, herbaceous plants' xylem system has low water and carbon storage capacity compared with that of woodland and shrubs, so it is less resistant to drought. [PERSON] et al.