Role of seed settleability and settling velocity in water for plant colonization of river gravel bars
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A natural gravel-bed river is an important habitat for xerophytic plant species that prefer dry and periodically disturbed environments. However, human-impacted gravel bars covered with fine sandy sediments are often rapidly occupied by hygrophytic and mesophytic species, including weedy species, after a flush event. What is the mechanism by which these plant seeds are carried on gravel bars through a flush event? Can the seed traits of floating capability (buoyancy or settleability) and settling velocity in water explain seed accumulation in sandy sediments?
The Tama River, central Japan.
Seed buoyancy/settleability and settling velocity in water were examined for 70 herbaceous species growing on newly deposited sediments of a river gravel bar. Settling velocities of sand particles of different sizes were estimated using theoretical formulas to compare with those of plant seeds. In addition, actual seed content of 36 samples of fresh sandy sediments were examined using a germination test to confirm coincident sedimentation of seeds and sand particles.
Of the 70 species examined, only nine had apparently buoyant seeds and the remaining 61 species had non-buoyant seeds. We found that 53 (86.9%) of the 61 species having non-buoyant seeds had settling velocities comparable to those of very fine sand and fine sand particles, ranging from 0.062 to 0.25 mm in diameter. Furthermore, fresh sandy sediments deposited by a flush event contained many non-buoyant seeds of common species of above-ground vegetation. Both the germinated seedling number and species richness in the sediment samples showed positive correlation with the percentage weight of very fine sand and fine sand.
Seeds of most of the species that appeared after a flush event were considered carried by submerged dispersal through flowing water than through floating dispersal, because of their settleability. Many plant seeds could deposit together with fine sandy sediments as water velocity declined, since they had correspondent settling velocities. The main reason for rapid colonization by hygrophytic and mesophytic species was the settleability and settling velocity of the seeds.