## 3 Results
### Spatial Distribution Pattern of Plant Diversity in the Yellow River Delta  
As shown in Figure 5, the value of the plant diversity index is relatively low. The Margalef index (R) value ranges from 0 to 0.939, with an average value of 0.4, indicating \begin{table}
\begin{tabular}{c c c} \hline \hline
**Distance from the Sea (km)** & **Partition** & **Important Plants** \\ \hline \(<\)3 & DS1 & Suaeda salsa \\
3–17 & DS2 & _Phragmites australis, Suaeda salsa, Sonchus brachyotus, Artemisia_ \\  & & _scoparia, Glycine soja_ \\
17–26 & DS3 & _Phragmites australis, Suaeda salsa, Miscanthus sacchariflorus_ \\
26–42 & DS4 & _Phragmites australis, Miscanthus sacchariflorus, Artemisia scoparia,_ \\  & & _Cypureurs scrotimes_ \\ \(>\)42 & DS5 & _Phragmites australis, Erigeron canadensis, Potentilla supina,_ \\  & & _Calamagrostis pseudophragmites, Cyperus rotundus_ \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The plant composition under different distance from the sea. Figure 4: The condition of the species similarity. relatively low species richness in the study area. The value of the Shannon-Wiener diversity index (H) ranges from 0 to 1.144, with an average value of 0.486, reflecting relatively low diversity. The value of the Simpson index (D) ranges from 0 to 0.644, with an average value of 0.265, suggesting that many plots have limited species dominance and low diversity level. The value of the Pielou index (J) ranges from 0.038 to 0.784, with an average value of 0.485, showing the species distribution evenness among plots was relatively low. From the spatial characteristics (Figure 6), the longitudinal change trends of the R, H, D, and J indexes were basically the same. In general, they showed a fluctuating downward trend from land to sea, and the plant diversity was poor in the areas closer to the sea. In the national nature reserve, the plant diversity also showed a decreasing trend from land to sea. From the level of the plot, the sampling sites with the highest plant diversity indexes were M12, S2, M8, M1, and M2, while the sampling sites with the lowest plant diversity indexes were X3 and Y3. In addition, we can clearly see from Figure 6 that most of the sampling plots with the greater plant diversity are located in the riverbank of the main stream of the Yellow River. Through one-way ANOVA, there were significant differences in the R, H, and D indexes between the main stream and tributaries of the Yellow River Delta (\(p<0.05\)), but no significant differences in the J index (\(p>0.05\)) (Table 8). The overall species richness, dominance, and diversity were generally higher in the main stream of the Yellow River than in its tributaries. In order to further probe into the land-sea differentiation of plant diversity, the plant diversity indexes were analyzed and evaluated at different distance from the sea (Figure 7). There were significant differences in the four indexes of R, H, D, and J among the different land-sea zones (\(p<0.05\)). In Figure 7, the level of plant diversity was lower in the zones closer to the sea, and higher in the zones further from the sea. The plant diversity indexes showed a peak value in the region of 3-17 km from the sea (DS2) and the region of \(>42\) km from the sea (DS5). The DS2 zone was affected by multiple factors, such as river siltation, tidal overflow, and salinization, forming a unique ecological environment and breeding a variety of plants adapted to different soil conditions, which made the plant diversity relatively rich. In addition, a part of the sampling sites in this zone was located in the national nature reserve, which was affected by policy protection and less human interference. Therefore, the plant diversity has been protected and restored to a certain extent. As the distance from the sea increased, the soil properties also changed. Due to its suitable climate, terrain, hydrology, and soil conditions, the DS5 zone was suitable for the growth of some plants, and provided suitable living conditions for a variety of plants.