## 4 Discussion
For example, in this study, _Fagus_ is the vegetation type that exhibits the greatest difference in drought severity between the two regions, and it is present in very low proportions in the karst area. This is likely due to the shallow root system of _Fagus_, which limits its ability to absorb water from deeper soil layers [53]. Similarly, shrub encroachment in the karst areas has also intensified the severity of drought. We found that _Pinus_ species account for approximately 68% of the vegetation area in karst areas. This finding is consistent with previous studies, which indicate that much of the vegetation in Guangxi has degraded into secondary _Pinus_ species forests, suggesting a process of shrubification due to the poor drought resistance of shrubs [52]. This leads to greater vulnerability of vegetation in karst areas. Unlike other regions, where shrubification is primarily driven by human activities, the shrubification in the karst area of Guilin is also influenced by the unique karst geological structure, which accelerates this process. Our results indicate that the karst topography in the study area amplifies the impact of extreme drought events, making karst areas more severely affected than non- karst areas. This ultimately results in prolonged drought stress in karst areas. Since NDVI directly reflects vegetation cover, LAI represents structural characteristics, and GPP indicates the rate of carbon sequestration [54; 55; 56], their responses to drought stress vary. Specifically, as drought induces plant wilting or mortality, NDVI values exhibit a greater decline. This observation is consistent with the results of concurrent studies conducted in the Southwest region [31]. Meanwhile, the vegetation ecosystem may not have been immediately affected by the drought. Changes in LAI tend to lag, possibly because LAI represents the leaf area of vegetation, rather than immediately reflecting the vegetation's health status. GPP reflects the rate of photosynthesis, and during the first month of drought, it remains relatively stable. Furthermore, a consistent trend of NDVI and GPP indicates good overall ecological health, while a larger discrepancy between the two suggests significant ecological stress [29] During the pre-mid drought period in 2022, although there are some difference in response time between NDVI and GPP, the overall trend remains consistent. However, in the late stage of drought, we observe a significant divergence in the trends of NDVI and GPP, while LAI remains close to the baseline year. This opposing trend suggests that certain vegetation may be suppressed due to insufficient water, while other plants may survive in the limited water conditions through deeper root systems or other physiological mechanisms. This competition and growth pattern may help maintain overall LAI stability. Due to the unique topography of karst areas, vegetation ecosystems show distinct responses under drought stress. We found that the various vegetation indices responded differently to drought stress between the karst and non- karst areas. At the early to mid-stages of drought (July to September), the NDVI, LAI, and GPP in the non- karst areas remained higher than those in the baseline year, indicating that vegetation growth had not yet experienced severe drought impacts. In July and September, which constitute the middle of the vegetation growing season, the occurrence of abundant solar radiation can promote vegetation growth. Similarly, although the degree of drought was higher in the karst areas than in the non-karsat areas, the vegetation indices did not significantly change under water stress in July and September. This could be attributed to the leaf anatomical structure of the main plant species in the karst areas, which exhibit well-developed palisade tissue, tightly packed spongy tissue, small intercellular spaces, and thick cuticles, enhancing their drought resistance [57]. Additionally, the thick wax layers and developed cuticles could reduce water loss during transpiration.