Abstract
In order to look into the adaptive role of surviving rate and mortality in grasses, mortalities under different environments were observed in perennial ryegrass (Lolium perenne) and orchardgrass (Dactylis glomerata). Three varieties of each species were tested. Four hundred viable seeds per plot (1 m^2) were sown in 1970 September in pure stand (6 varieties) and in species mixture (1: 1, 9 varietal combinations). The fifteen populations were grown on road side (2 replications) and in an experimental field (four different managements; clipping, herbicide treatment, and two controls with and without white clover). Number of surviving plants were counted four times until 1973 April. Measurements of several characters were also taken. Since the experimental plants on road side died in the next year on account of vigorous gorwth of summer weeds, data obtained from field plots were mainly analyzed. Seedling mortality was obviously higher than adult mortality. Orchardgrass had higher seedling mortality but the remaining plants survived longer than those perennial ryegrass. A negative exponential model, Y_t=Y_o・e^<-bt>, was used for representing the survival curve of each variety. Variance analysis of mortality data showed that the two species significantly differed in mortality as well as in its response to enviromental stresses. Varietal differences within the same species were partly significant. Generally, plant density and mortality were positively correlated, while seedling mortality were negatively correlated. This suggests that death was at least partly density-de-pendent. "Self-thinning " might have worked to regulate the number of plants per unit area. Perennial ryegrass had a higher ability of sexual reproduction and produced more offspring by seed than orchardgrass. The degree of phenotypic plasticity in response to clipping was higher in perennial ryegrass than in orchardgrass. Mortality was generally lowered by clipping or by herbicide treatment, the tendency being more pronounced in perennial ryegrass than in orchardgrass. Perhaps perennial ryegrass copes with environmental stresses by its high survival rate at the cost of plant size. The effects of species competition was represented by the differences in chara values and mortality between pure and mixed plots. Orchardgrass was advant in competition to perennial ryegrass and the relative frequency of orchardgrass plots rapidly increased. It may be concluded that perennial ryegrass has a relatively short life resistance to enviromental stress. In contrast, orchardgrass seems to span, higher vegetative reproduction rate and stronger competitive two species seem to differ in adaptive strategy. (J. Japan. Grassl.
