## 4 Discussion
### Spatiotemporal Dynamics of ESs  
This study analyzed the spatiotemporal dynamics of the ESs over a continuous 23-year period. The analysis revealed that SC and HQ exhibited downward trends over time in the SHMB, while WY, FP, and CS showed overall upward trends. WY and SC displayed significant interannual variations, with pronounced fluctuations. The HQ values exhibited a slight decline, likely due to urbanization and land use change. The conversion of forest, grassland, and arable land to impervious surfaces threatens habitats, resulting in a decline in HQ [35]. Spatially, HQ and CS exhibited similar patterns, with high-value areas in the eastern and northern regions, where forests provide habitats and enhance CS. Generally, regions with high forest cover and extensive grassland distribution also exhibit higher levels of CS [36]. The extensive forest ecosystems in the region, combined with a favorable climatic environment, provide a conducive habitat for biodiversity. Additionally, these conditions facilitate CS [35]. The high-value areas of SC and FP were mostly in the areas with dense vegetation cover and continuous cultivated land, and the retention effect of dense forest vegetation on precipitation effectively alleviates soil erosion. SC is also closely related to topographic factors. The areas with low SC values included karst rocky desertification regions and certain mountainous areas, which exhibit low soil conservation capacity due to factors such as concentrated precipitation, low vegetation cover, and human activities [37]. FP generally increased but dipped in 2005 and 2010 due to natural disasters affecting agriculture, such as floods and heavy rains in Guangxi and other southern regions of China. Since 2010, the Natural Forest Protection Project and the Grain-for-Green Program have boosted CS. The spatial distribution of WY closely resembled that of precipitation. The high-value areas of WY were primarily attributed to higher precipitation, which provides more water to flow into surface runoff or percolate underground as groundwater, thereby increasing water yield. Additionally, WY is also influenced by vegetation. In general, vegetation helps to mitigate surface runoff and enhances groundwater recharge. Through the root channels of vegetation, water can more easily percolate into the soil, replenishing the groundwater supply [38].  
The spatial changes in the ESs across different stages are also noteworthy. From 2000 to 2010, the primary changes in WY occurred in northern Guangdong and southeasternGuizhou. This can be attributed to variations in precipitation over the years, including changes in the frequency and intensity of extreme weather events, which directly impacted runoff formation [39]. With the implementation of the Forest Quality Improvement Project and Drinking Water Source Protection Project from 2010 to 2022, WY in Guangdong has been greatly improved. SC exhibited less variation during 2000-2010 but more pronounced changes from 2010 to 2022. This shift may be attributed to ecological protection policies implemented in China over the past few decades, such as reforestation and converting farmland to grassland, which have facilitated vegetation recovery and subsequently enhanced SC [40]. HQ changed little in the two stages, but the overall ecological environment quality declined, which is related to the impact of urban construction on regional ecology [41]. FP showed an upward trend over the past 23 years, indicating that the implementation of policies such as reforestation and farmland protection in the SHMB has been effective during this period [42].