The present paper deals with the age of the latest ash fall from Rishiri Volcano-from the points of view of dynamic forest ecology and shelterbelt establishement engineering-discussed from the development of natural coniferous forest on the sand dune at Hamatombetsu coast, about 90km east from the Volcano (Fig. 1).
Generally, the surface soil of sand dune is the least fertile in physical and chemical properties. And it has only so-called sand dune herbs, or dune-formers, as a pre-existing vegetation of an absolutely strong competitor against new invaders. It may be that what makes Todo-fir (Abies sachalinensis) invade and grow naturally on such a bad soil is a kind of topographic changes of ash fall. An ash fall removes the pre-existing competitor and improves the naked ground physically for trees' invasion.
Sarobetsu Research Group (1966) presumed that the age of the latest ash fall from the Volcano (Hotoku ash fall deposit) might be earlier than 4, 000 years ago. However, at Hamatombetsu, the deposit covers the present sand dune, which may be about 1, 000 years ago, or less (Figs. 2 and 3).
Coniferous forests on the flat land are commonly even-aged, uniform type, influenced by topographic changes. The Todo-fir forest at Hamatombetsu, though with a little larger difference in diameter grade (Table 1), may be an uniform type and a first generation on the sand dune immediately after the ash fall, nearly equal to those at Wakasakinai and Mehkuma. Those three Todo-fir forests on sand dunes (within the area of Hotoku deposit from Rishiri Volcano) may be 150 to 200 years of age.
It is presumed that the Hotoku volcanic ash deposited 200 (to 300) years ago.