Population dynamics in overlapping zones of Phragmites australis and sacchriflorus sacchariflorus

Abstract

The water fringes of ponds and lakes in Japan are occupied by Phragmites australis (Cav.) Trin. ex Steud. and Miscanthus sacchariflorus Benth. P. australis occupies a water habitat, whereas M. sacchariflorus thrives on the moist upper shoreline. The two species overlap in the intermediate zone. The present study was undertaken to observe and record features of growth and population dynamics of these two species in a small pond, with particular reference to the overlapping zone. The factors considered were water level, oxygen diffusion rate, redox potential, rate and density of bud and shoot formation, rhizome length, rate of plant growth, population density and mortality rate. Observations and measurements were made twice a month from mid-April to mid-August, with a follow-up in October at the onset of the dormancy phase, and another the following February. Higher mortalities of P. australis in its dominant zone and of M. sacchariflorus in both the overlapping zone and its dominant zone occurred twice early in the growth season when the pond was flooded. After this, new shoots of P. australis formed to replace dead ones, but most of these also died before August; shoots emerging in mid-April in the dominant zone showed the highest survival rate by August. The emergence of P. australis in the drier area of the overlapping zone was delayed by about two weeks relative to that in wet areas. This delay in the emergence of P. australis in the overlapping zone allows M. sacchariflorus to become dominant in spite of the high mortality of the latter caused by flooding.