Niche and interspecific relationships of herbaceous plants in the riparian zones of the middle and lower reaches of the Hanjiang River, China

The construction of water diversion projects and cascade dams drastically alters the hydrological regimes of rivers, inducing changes in the habitats of riparian zones and exerting varying degrees of impacts on the interspecific relationships and community stability of riparian plants. Based on the zonal distribution characteristics of plant communities along elevation and moisture gradients in the middle and lower reaches of the Hanjiang River, this study conducted field vegetation surveys by dividing sampling sites into three parallel transects: Transect 1 (near-water vegetation zones), Transect 2 (intermediate zones), and Transect 3 (farthest vegetation zones from the water). Methods including niche breadth, niche similarity ratio, chi-square (χ²) test, Pearson correlation analysis, and Spearman’s rank correlation test were employed to investigate the ecological niches and interspecific relationships of dominant riparian plants, aiming to explore the effects of water diversion and cascade dams operations on plant interspecific interactions and community succession mechanisms. The results showed that there were 44 herbaceous plant species in the study area, predominantly belonging to the Poaceae and Asteraceae families. The species with the highest importance values in Transects 1, 2, and 3 were Typha angustifolia, Phragmites australis, and P. australis, respectively. Meanwhile, the species with the largest niche breadths were Erigeron canadensis, E. canadensis, and Cynodon dactylon in the three transects. The species pairs with the highest niche similarity ratios were T. angustifolia & P. australis (Transect 1), Equisetum arvense & Imperata cylindrica (Transect 2), and Lolium perenne & I. cylindrica (Transect 3). Across all three transects, more than 70% of species pairs exhibited a niche similarity ratio below 0.2, indicating low overlap in resource utilization and relatively independent interspecific relationships among riparian plants in the middle and lower reaches of the Hanjiang River. The χ² test revealed that over 80% of species pairs showed non-significant correlations in each transect, with Transect 3 presenting the loosest interspecific network structure. Further results from Pearson and Spearman’s rank correlation tests demonstrated that negatively associated species pairs dominated in all three transects, suggesting weak interspecific linkages, relative independence among dominant species, and an unstable successional stage of the plant community. Based on these findings, we recommend selecting mutualistic plant species as pioneer species for the conservation and habitat restoration of riparian plant communities in the middle and lower reaches of the Hanjiang River. This strategy will contribute to enhancing the stability of riparian plant communities and maintaining the overall functionality of the riparian ecosystem.