## 4 Discussion  
A comparison of survey data from 2019 and 2024 in the sample plots revealed that the survival rate and growth of _Ulmus pumila_ seedlings and saplings were significantly higher than those of _Robinia pseudoacacia_ and _Ailanthus altissima_. The superior growth performance of _Ulmus pumila_ suggests that it has a competitive advantage over black locust and tree of heaven in interspecific interactions, allowing it to better adapt to the local environmental conditions. In contrast, several species--including _Salix matsudana_, _Populus simonii_, _Rhus typhina_, and _Lycium bararum_--disappeared from the sample plots by 2024. This indicates that seedling survival rate is a crucial determinant of species persistence. If seedling survival is low, even initially dominant species may be replaced by other, more competitive species within the community. The disappearance of certain species also highlights the importance of environmental filtering and competitive exclusion in shaping community composition over time. Spatial distribution patterns describe the horizontal arrangement of individuals within a population and reflect their spatial relationships. These patterns are influenced by both species-specific biological traits and environmental conditions. Spatial structure not only reflects past ecological processes but also plays a crucial role in predicting future vegetation dynamics and species interactions [35; 36]. A comparative analysis of the spatial distribution of seedlings and saplings between 2019 and 2024 revealed that the distribution of major tree species in 2024 remained largely consistent with that observed in 2019. In areas where seedlings were densely distributed in 2019, the density remained high in 2024, whereas in previously sparse regions, seedling density showed a moderate increase. These findings align with observed species abundance trends and indicate a degree of spatial stability in the community. The results of a generalized linear model (GLM) analysis revealed a significant positive correlation between seedling height and survival probability. Although the strength of this correlation varied across different levels, a clear trend emerged: taller seedlings exhibited higher survival rates. Height is often a proxy for competitive ability, as taller seedlings are more likely to access light and other resources, thereby increasing their chances of survival [37; 38]. Our findings suggest that biotic factors play a more significant role in seedling and sapling survival than abiotic factors. This is consistent with the conclusions \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Seedling Height** & **Abiotic Factor** & & \\  & **Model** & **Model** & & \\ \hline _Robinia pseudoacacia_ & 608.3882 & 609.7257 & 615.0555 & 616.9202 \\ _Ulmus pumila_ & 5012.960 & 5015.920 & 5012.489 & 5015.178 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: AIC values of seedling survival models at the species level. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Seedling Height** & **Abiotic Factor** & & \\  & **Model** & **Model** & & \\ \hline _Robinia pseudoacacia_ & 154.8690 & 158.1091 & 161.9599 & 164.9411 \\ _Ailanthus altissima_ & 315.0783 & 318.5791 & 311.6675 & 313.9067 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: AIC values of sapling survival models at the species level. of [PERSON] et al. [39], who found that tree regeneration in temperate forests of Japan was primarily driven by biological interactions rather than environmental constraints. Specifically, we observed the following: the survival of Ulmus pumila seedlings was negatively affected by both the basal area and the individual number of conspecific mature trees. This suggests that intra-specific competition, likely for light and nutrients, plays a key role in regulating Ulmus pumila seedling survival.