Factors limiting the distribution of the annual legumeKummerowia striata in a river floodplain

The annual legumeKummerowia striata often predominates in river floodplains forming a large population, but it is rare in gravelly areas adjacent to it. In order to clarify the factors limiting the distribution ofK. striata, its distribution pattern, germination and survival of seeds sown artificially were studied in the floodplain of the Ohta River. There was a positive correlation between the cover value ofK. striata and the percentage of fine particles (<0.2 mm) in the soil. In the winter season, there was a number of seeds (up to 47 seeds 20 cm⁻²) on the ground where aK. striata population had existed in the previous year, but they were very sparse in areas apart from the population. Only a small proportion of seeds sown in the gravelly area germinated. Many of the germinated individuals died before the first leaf unfolded, but the rest survived to maturity despite repeated floodings. Plants germinated in the gravelly area and those inoculated with soil from the gravelly area bore nodules, showing the presence ofRhizobium in the gravelly area. It was concluded that the colonization ofK. striata in the gravelly area was limited by slow dispersal, and by a low percentage of germination and high mortality in the early stage of establishment.