Etiology of “Shimagare” dieback and regeneration in subalpineAbies forests of Japan

The “Shimagare” phenomenon relates to a striped pattern of stand-level dieback in the subalpineAbies (fir) forest of central Japan. Dieback fronts move gradually into pure stands ofAbies. As the dieback zones move forward they become regenerated with even-aged cohorts ofAbies which come up in form of waves. The phenomenon occurs mostly on the south slopes near mountain tops, exposing dieback fronts to south (i.e. downwards). Several facts suggest that desiccation stress and mechanical damage accelerated by the prevailing wind trigger the directional dieback ofAbies stands. The dieback is, however, promoted by over-crowded and even-sized stand structure. When less shade-tolerantBetula (birch) trees invade in an early stage of regeneration, mixed stands develop. These hardwood-mixed stands, with less-crowded and various-sizedAbies populations, hardly decay simultaneously. Here, alsoAbies trees become larger and live longer. Thus, the tendency of mass dieback ofAbies is not a characteristic of the species as such, but rather one of dense pure stands of the species.