SPECIES DIVERSITIES ANALYZED BY DENSITY AND COVER IN AN EARLY VOLCANIC SUCCESSION

To evaluate alpha diversities, various variables such as density, cove r, volume, and weight have been used. However, density is often a dist inct variable from the remaining three. To clarify differences in dive rsity measured by those two kinds of variables, the data collected in fourteen 2 x 5 m permanently-marked plots on Mount Usu, Japan, which e rupted during 1977 and 1978 in growing seasons from 1983 to 1989 was a nalyzed, using Shannon's species diversity (H') that is represented as a result of combination of species richness and evenness (J'). H' and J' were evaluated by density (density H' and J') and cover (cover H' and J'). Cover H' and J' were significantly lower than density H' and J', indicating that cover H' has different characteristics from densit y H'. Those differences are due to differences in evenness, because sp ecies richness is the same. The rank orders of species density are dif ferent from those of cover. The predominance of a few perennial herbs greatly decreases cover evenness, while seedling establishment success influences density evenness. Therefore, I propose that, during the ea rly stages of succession on harsh environments such as volcanoes, dens ity diversity represents seedling establishment success rate while cov er diversity expresses vegetative reproduction success rate.