Reconsideration on the Distribution of the Shimagare Phenomenon in Japan

Abstract

The Shimagare phenomenon which is called ‘wave-regeneration’ in the United States, is characterized some crescent-shaped bands consisted of a mature forest, dead trees and young seedlings. Many ecologists, dendrologists, geographers and others have already examined the phenomenon from their own viewpoints. The purpose of this paper is to reconsider the distribution and the locational condition of this phenomenon in Japan.As a result of aerial photograph analysis, some areas have newly been added to the distribution area of the Shimagare phenomenon. It becomes clear that the reexamined distribution area fundamentally coincides with the geographical distribution area of Abies mariesii and A. veitchii. It is noticeable that the scale of the Shimagare strip, crescent-shaped band, is smaller in Tohoku district (north-eastern Japan) than in central Japan. It can be said that the reasons why the Shimagare strip is smaller in Tohoku district are as follows: 1) the mountain areas are consitituted of small geomorphic surfaces, 2) the sub-alpine forests are dominated by A. mariesii, stronger species, 3) the Shimagare does not appear on southfacing slopes more influenced by sunshine effects.The Shimagare strips mostly appear at upper sub-alpine forest one near the top of the mountains, and also on south-facing gentle slopes except in Tohoku district. These facts point out that the Shimagare strips are distributed where pure fir forests are liable to be formed and to be damaged by the desiccation.On the other hand, the arrangement of the Shimagare strips characterized by dead trees show more concentrated direction. Comparing this direction with the direction of wind-shaped trees, both are not necessarily the same with each region. For example, both directions almost agree on Mt. Hakkoda and Mt. Higashi-azuma in Tohoku district, but not Mt. Nantai in Kanto district. These facts closely correspond to the pattern of seasonal variation of wind direction obtained by instrumental observation. For example, meteorogical records of the Mt. Nantai show the considerable frequency of southerly wind corresponding to the direction of arrangement or movement of the Shimagare strips during warm season, but not during cold season. With these facts, prevailing wind during warm season is considered to act as an important factor for the movement of Shimagare strips. And, if the direction of strong wind coincides with prevailing wind direction during warm season, the Shimagare strips will be moved faster.