Some Aspects on the Shimagare Phenomenon in the Subalpine Forests in the Kanto and Chubu Districts, Japan

Abstract

“Shimagare” or a phenomenon of blighted trees distributing in stripes is one of the characteristic features seen in the subalpine forests in Japan. The striped pattern of a group of withered trees in the subalpine forests is not only of great interest for those researchers of plant geography and plant ecology, but an important problem to be solved for the protection of nature and forest conservation. The Shimagare is uniquely observed in the subalpine coniferous forests, mainly consisting of Abies veitchii and Abies mariesii, partially mixed with Tsuga diversifolia and Picea hondoensis. The blighted forests present various shapes in the course of development, spots at the initial stage turning into arches and finally into stripes. From the fact that the Shimagare zones appear almost at the same altitude and on the slope facing in the same dierction, and further from the fact that the dead tree stripes move upward on the slope (or leeward) with the lapse of time, it is thought that the phenomenon has close relationship to the prevailing wind direction. To sum up the results of studies obtained so far on this subject, the Shimagare zone is distributed in the subalpine forests, ranging from the Kinki District to the northern part of the Kanto District in Japan; Oku-Nikko, Yatsugatake, Chichibu Koshin Kokkyo, Southern Alps, Central Alps, and Ohmine Mountains. Excepting for the Ohmine Mountains, where the lower limit of the distribution of Abies veitchii is low, this phenomenon appears at an altitude of higher than 2, 000m and mainly on the gentle slopes facing SW-SE. The investigation was carried out in the areas in the Chichibu Koshin Kokkyo Mountains, the Oku-Nikko Mountains and Akaishi Mountains by making use of aerial photographs and the field survey. Belt transects with a width of one meter for all trees with the diameter at breast height of more than 3cm were taken at a right angle at the center of the arch stripe of the Shimagare. At the same time, the height of trees, underbrush, density, and soil profiles were noted (Fig. 1). As a result of the present study, the following facts are clarified: 1) In all such mountains as stated above, the Shimagare phenomena are distributed on the gentle slopes facing SE-SW or failry flat ridge tops. It seems, therefore, that the distribution is related to the radiation, evaporation and prevailing wind direction. 2) The Shimagare strips are found only in the needle-leaved forest in sub-alpine regions. This fact, along with the movement, scale and shape of the Shimagare strips, is enough to make us believe that Abies veitchii and Abies mariesii are liable to be influenced by abrupt change of environments. 3) According to the meteorological observation carried out on the slope of Mt. Nantai, Oku-Nikko, the zone of the subalpin.e forests on the top of the mountain is characterized by the larger number of days with dense mist, frost, stronger wind, and greater cloudiness but by less precipitation than on the foot and mid-slope of the mountain. It can be said, therefore, that the relatively dry condition caused by the stronger wind on this zone might be one of the causes of the Shimagare phenomenon. 4) The soil in the Shimagare zone differs in thickness from place to place, but generally it is thin ranging from 4 to 30cm. The layer is mostly made up of humus underlain by the base rock. In the region, where the Shimagare is lacking, trees grow higher and the soil is thick. On the contrary, many trees fall dead in the Shimagare zone, probably because of the thin soil layer. 5) Comparing the prevailing wind direction estimated by the wind-shaped trees with the arrangement and shape of the Sh.imagare stripes, it was made clear that the direction of fallen trees and the direction of the movement of the Shimagare stripes have close relation to the prevailing southerly winds in spring, summer and autumn.