## 4 Discussion
### Ecosystem Resistance  
Ecosystem resistance in Central Asia differed between 2002 and 2003. In this study, strong ecosystem resistance was observed in 2002, and resistance increased with increasing environmental wetness. The main reason for these findings is that the vegetation in arid areas can show vigorous growth once sufficient water to support vegetation growth is obtained because it has a high photosynthetic capacity [65; 66]. However, when extremely wet events occurred for two consecutive years (with a higher wetness level in 2003 than in 2002), we found that ecosystem resistance began to decrease. Ecosystem resistance decreased with increasing wetness in 2003. This pattern was different from that observed in 2002, in which continuous extremely wet events similarly reduced ecosystem resistance. A possible reason for this phenomenon is that the water used for photosynthesis was in surplus, and the maximum amount of water required for photosynthesis had been reached [67; 68]. Excess water flows into the soil or is intercepted by the canopy. A high-temperature environment increases evapotranspiration, resulting in a decrease in WUE, which may also be a reason for the decline in resistance. In addition, during rainy weather, light intensity is decreased, which limits photosynthesis. The inability to use more water leads to a decline in resistance. This observation shows that as water resources become abundant, the productivity of an ecosystem will increase at the same time as the evapotranspiration of the ecosystem. Previous studies have shown that with increasing precipitation, biomass and NPP usually increase and that the WUE of the ecosystem increases at the same time; these three variables exhibit similar distributions of temporal and spatial changes [69]. These findings are consistent with only the results for 2002 in this study; they are inconsistent with the results for 2003. Another possible reason for the difference is that the continuous wet years promoted the conversion of open shrubland to grasslands. In a previous study in Central Asia, the WUE of shrubland was found to be higher than that of grassland [10]. Therefore, the overall WUE during the wet period was lower than that during normal years. This phenomenon is another reason for the decline in ecosystem resistance in Central Asia. In 2002, as the degree of wetness increased, ecosystem resistance showed an increasing trend. Under mild, moderate, and extremely wet conditions, ecosystem resistance was greater than 1; after two consecutive years of wetness, ecosystem resistance decreased as the degree of wetness increased. These findings further show that adequate wetness increases the resistance of an ecosystem, but continuous extremely wet events reduce it. In 2008, Central Asia experienced a drought event in which regional resistance in 93.13% of the study area was less than 0, and it was less than \(-1\) in southeastern TKM and greater than 1 only in high mountain areas. These findings indicate that resistance to drought is weak in Central Asia. Studies have shown that the WUE of an ecosystem is negatively correlated with drought during a drought period in Central Asia [10]. Drought significantly reduces ecosystem WUE and has an obvious legacy effect [65; 70; 71; 72; 73]. In this study, ecosystem resistance increased with increasing drought intensity. This finding is consistent with the previous finding that WUE increased with increasing drought intensity [27] and explains why drought resistance was higher in arid areas. However, resistance did not reach a maximum but declined under extreme drought conditions, with the weakest resistance observed in southeastern TKM. Furthermore, the increase in shrubland areas and the decrease in grassland areas after drought events are additional reasons for the higher WUE and drought resistance in arid areas than in other areas.