## 4 Discussion  
### Impact of ECIs on Ecosystem Indicators in SWC  
The ecosystems in SWC are significantly influenced by various ECIs, which have differing effects on specific EIs. The region's complex topography and diverse climatic zones [22, 25] contribute to the heterogeneous impacts of ECIs across subregions [38]. Previous studies indicate that in the semi-arid Loess Plateau, drought indices like RDI are crucial for vegetation dynamics, impacting the NDVI and GPP [39]. The WUE rates of all vegetation types decrease with increasing drought index [39], highlighting the important role of drought indices such as the RDI in determining the NDVI and GPP. SU significantly enhance productivity and is projected to increase under global warming, showing strong positive correlations with both the GPP and NDVI (\(p<0.001\)). Conversely, DTR generally inhibits growth, particularly in low-altitude regions, where large temperature fluctuations negatively affect vegetation [7]. While DTR demonstrates a significant overall negative impact across the region, it can also promote growth in some low-altitude areas. To take the most obvious example, in the two areas with large elevation differences, the effect of DTR on HDM is consistently negative and significant, while its effect on SCB is consistently positive and significant. This dual effect stems from variations in drought resistance among different vegetation types, which are influenced by temperature changes affecting the microbial traits of plants and soil. These changes further impact ecosystem decomposition and nutrient cycles, including those of carbon, nitrogen, and phosphorus, ultimately influencing vegetation growth [40, 41]. The GPP is significantly affected not only by temperature-related indices but also by precipitation indicators such as the RDI. Studies have shown that the NDVI and GPP are highly correlated across time scales, with notable differences in their response to precipitation, particularly during extreme events [42]. Water-use efficiency is a key driver of these EIs, significantly altering annual carbon fluxes and water-use efficiency in semi-arid regions. Consequently, extreme precipitation events not only enhance the NDVI, but also modulate the GPP by influencing soil water availability [42]. At an annual scale, the NDVI is influenced by a broader range of factors, including responses to CECIs such as CDW and CDC. Moreover, EIs exhibit significant variations in response to the cold/warm spell duration index (CSDI/WSDI). Specifically, increases in the CSDI tend to favor the GPP but have no significant impact on the NDVI, whereas increases in the WSDI benefit the NDVI without notably affecting the GPP. These differences underscore the distinct roles of the GPP and NDVI, where the GPP reflects photosynthetic efficiency while the NDVI indicates overall plant vigor. Vegetation in SWC is significantly impacted by ECIs, adapting to climate change through alterations in their phenology and physiological functions [43]. During the growing season and summer, standardized indices and CECIs have a more pronounced influence, but their impact remains generally lower compared to direct ECIs. Indices with higher weight and strong correlations with the GPP include the ETI, DTR, and CWD. For the NDVI, the relationships with various indices vary, with stronger responses to CECIs compared to the GPP, suggesting that CECIs partly increase the NDVI while the GPP tends to have a more subdued response. The response of ECIs across subregions also differs. In the HDMs and YGP, ECIs have a similar influence on ecosystem metrics, whereas in the SCB and GXB, responses vary markedly. For instance, in the SCB, indices such as the ETI, DTR, GSL, FD, SU, and TR show anomalous correlations with the NDVI, indicating that high temperatures can inhibit the NDVI. In the GXB, the impact of ECIs on the NDVI is smaller than their impact on the GPP, with temperature remaining the primary influencing factor.