Characteristics of secondary dispersal of seeds and its effects on the soil seed bank of a drawdown zone in the Xiangxi River

Seed secondary dispersal through water flow is a major pathway of riparian plant seed dispersal in a drawdown zone. In order to uncover the pattern of seed secondary dispersal and its effects on the development of a seed bank, we tested the species composition, biodiversity, and density of secondarily dispersed seeds and a soil seed bank in the drawdown zone along the Xiangxi River in China through field sample collection and germination experiments. The results showed that there were 42 and 50 species in the secondarily dispersed seeds and soil seed bank with average seed density 1876.27 (± 518.87)/m 2 and 7322.18 (± 678.44)/m 2 , respectively. Seed quantity of the secondary dispersal accounted for 20.40% of the soil seed bank. The secondarily dispersed seeds and soil seed bank were mainly composed of annual and perennial herbaceous species. Dominant species included Cynodon dactylon and Paspalum distichum in the secondary dispersal, as did Solanum nigrum and Rumex dentatus in the soil seed bank . Under the influence of the reverse seasonal water level fluctuation, seeds of the secondary dispersal and of the soil seed bank displayed similar spatial distributions. Species diversity and seed density changed with the increasing altitude gradient, and the highest species biodiversity and seed density occurred in the middle upper part of the drawdown zone. There were 37 common species between the secondarily dispersed seeds and soil seed bank with a relatively high similarity coefficient. It was concluded that secondary dispersal is the main seed source of the soil seed bank and has an important influence on the development and spatial distribution of the soil seed bank.