## 4 Discussion  
### Plant Composition and Diversity Under the Land-Sea Gradient in the Yellow River Delta  
Previous studies on changes in plant composition and diversity along land-sea gradients are often limited to specific geographical areas, such as estuarine wetlands or nature reserves [27]. Due to the relatively confined spatial scope, these studies might not comprehensively reflect the overall trends of plant diversity changes along land-sea gradients. By contrast, this study focused on a larger regional scale, establishing different survey routes based on the flow directions and distributions of various rivers. By setting up distinct research areas according to varying land-sea gradients, this study systematically investigated the changes in plant diversity and habitat characteristics across these gradients, thereby contributing to the understanding of the complex mechanisms underlying the changes in plant diversity along the land-sea gradient. The plant species composition is the result of long-term interactions between plants and their environment, influenced by a combination of biotic and abiotic factors [46]. Variations in community habitat conditions lead to differences in plant composition. In the Yellow River Delta, the predominant plants belonged to the families _Pooraeae_, _Asteraceae_, _Amaranthaceae_, and _Fabaceae_, which were well adapted to the high moisture and salinity conditions of the soil. In the DS1 zone, halophytic plants, such as _Suaeda salsa_ and _Salicornia europea_, predominantly grew along the coast and tidal flats in dense patches. With the continuous expansion of the habitat of vigorous _Suaeda salsa_, the soil salinity in the DS2 zone experienced some regulation and improvement. Under the influence of human protection efforts, species such as _Phragmites australis_ began to grow in smaller areas within this habitat, resulting in relatively rich plant diversity. As the distance from the sea gradually increased, the terrain rose, salinity decreased, and water levels declined, the community transitioned from the monospecies dominance of _Suaeda salsa_ to the herbaceous community dominated by plants of the _Pooraeae_ family, such as _Phragmites australis_, _Miscanthus sacchariforus_, and _Calamagrostis pseudoplragmites_. Throughout this process, the community  
Figure 11: The Mantel test analysis of plant diversity and influencing factors. SPH: soil pH; SEC: soil electrical conductivity; SMC: soil moisture content; DR: the nearest distance from the Yellow River; Lat: latitude; Lon: longitude; HID: human interference degree. richness gradually increased, with both the variety and number of species rising, while interspecies competition intensified, leading to an overall enhancement of species diversity and evenness. Regarding the similarity of plant compositions, the Jaccard index was higher among plots closer to the ocean, indicating a greater number of shared plant species. Due to their proximity to the sea, these areas shared similar geographical environments, climatic conditions, ecological processes, and mechanisms of plant dispersal. Such similarities contributed to a high degree of overlap in plant species among the coastal regions of the Yellow River Delta, where salt-tolerant and alkaline-tolerant plants flourished abundantly. This study revealed the characteristics of plant diversity in the Yellow River Delta by calculating different diversity indexes. However, further analysis showed that species richness and distribution uniformity were inconsistent in some sampling plots. Conceptually, the Margalef index (R), the Shannon-Wiener index (H), and the Simpson index (D) mainly focus on the number, dominance, and overall diversity of species, while the Pielou index (J) further measures the evenness of species distribution on the basis of the above. For example, when the R, H, and D indexes are similar in two sampling plots, the J index may be different, indicating that, although species diversity is similar, there may be differences in the evenness of population distribution.