Factors affecting composition of gravel bar vegetation in middle reach of a lowland river

In this Study, we investigated plant communities in relation to site-specific morphological properties and sediment structure on gravel bars along the middle Drava River in Slovenia. We sampled 143 plots of 5 x 5 m using species cover-abundance estimates. In addition, we considered the following habitat physical variables: elevation above the water line, texture of the substrate (silt, sand, gravel content) and bar age. Based on TWINSPAN method, we distinguished four vegetation types: (G1) xerophilic and thermophilic ruderal stage; (G2) stage with sparse and herb-rich ruderal vegetation with species of semi-dry grasslands; (G3) initial vegetation stage of frequently disturbed mesotrophic and eutrophic sites and (G4) tall-willow stage. The most important factors determining a bar's vegetation were elevation and gravel content. Our study shows that gravel bars of the middle Drava river system form a suitable habitats for species-rich plant communities. Their diversity is a function of high spatial heterogeneity coupled with the opposing forces of stress and disturbance.