## 5 Conclusions  
Based on a field survey and statistical analysis of plant diversity, this study revealed the spatial differentiation of plant diversity in the Yellow River Delta under the land-sea gradients, and identified the key driving factors of plant diversity using regression analysis, redundancy analysis (RDA), and the Mantel test analysis. The results were as follows. This study investigated 44 species of herbaceous plants of 36 genera and 16 families in the study area. Among them, plants of the _Asteraceae_, _Poaceae_, and _Amaranthaceae_ families dominated. We recorded 30 species of plants of 26 genera and 13 families in the Yellow River Delta National Nature Reserve, indicating the richness of plant diversity in the reserve. The plants in the study area were mainly hygophytes and halophytes, and the dominant communities were _Suaeda salsa_ and _Phragmites australis_. With the increase of the distance from the sea, the community transitions from the monospecies dominance of _Sueda salsa_ to the herbaceous community, dominated by plants of the _Paoaceae_ family, such as _Phragmites australis_, _Miscanthus sacchariforus_, and _Calamagrostis pseudoplragmites_. Meanwhile, the plant species and distribution changed significantly in this process. The species similarity coefficient of the adjacent environment was higher than that of the non-adjacent environment. Additionally, the species similarity of the coastal area was higher, and the plant composition was mainly halophytes. Overall, the level of plant diversity in the Yellow River Delta was relatively low. From the perspective of spatial distribution, the longitudinal change trends of the Margalef index (R), the Shannon-Wiener index (H), the Simpson index (D), and the Pielou index (J) were basically the same, and the level of plant diversity showed a fluctuating downward trend from land to sea. The plant diversity in the reserve also showed a decreasing trend from land to sea. Additionally, it indicated a peak value in the region of 3-17 km from the sea (DS2) and the region of \(>42\) km from the sea (DS5), which may be influenced by such comprehensive factors as habitat level, soil conditions, and policy protection. From the riverbank perspective, the overall plant diversity in the main stream of the Yellow River was significantly higher than that in the tributaries. Human interference was the key factor affecting the level of plant diversity in the main stream, and soil electrical conductivity (SEC) was the key factor affecting the level of plant diversity in the tributaries. The results of the RDA and Mantel test analysis found that different plant diversity indexes had different responses to different environmental factors. In general, the plant diversity index had a strong negative correlation with SEC, soil moisture content (SMC), the nearest distance from the Yellow River (DR), human interference degree (HID), and longitude (Lon), and a strong positive correlation with height. Among them, SEC had the greatest impact on species diversity in the Yellow River Delta, and its comprehensive contribution rate to plant diversity was 62.37%. Therefore, SEC was a key limiting factor for plant diversity in the Yellow River Delta. Therefore, the implementation of reasonable soil improvement measures to reduce the risk of soil salinization is crucial for the conservation and restoration of ecosystems in the Yellow River Delta. Conceptualization, Y.S. ([PERSON]), W.M., and F.W. ; methodology, Y.S. ([PERSON]), W.M., and F.W. ; software, Y.S. ([PERSON]), W.M., and F.W. ; validation, Y.S. ([PERSON]), W.M., F.W., and Y.S. ([PERSON]); formal analysis, Y.S. ([PERSON]) and F.W. ; investigation, Y.S. ([PERSON]), W.M., F.W., Y.S. ([PERSON]), and M.S. ; resources, Y.S. ([PERSON]) and F.W. ; data curation, F.W. ; writing--original draft preparation, Y.S. ([PERSON]) and W.M. ; writing--review and editing, Y.S. ([PERSON]), W.M., and F.W. ; visualization, Y.S. ([PERSON]), W.M., and M.S. ; supervision, F.W. ; project administration, Y.S. ([PERSON]); funding acquisition, Y.S.