Subalpine Forest and Treeline Ecotone under the Influence of Disturbances: A Review

Abstract

This article considers disturbances caused by abiotic and biotic factors and human impact in the ecological region extending from subalpine forest to the upper tree limit. Both abiotic and biotic factors may cause reversible or irreversible disturbances. Disturbances by mass movement and avalanches give the subalpine forest and the treeline ecotone a distinct spatial pattern characterized by forest on safe topography and sites that preclude forest. Removal of the upper subalpine forests by humans has enlarged the snow-catchment area of avalanches and elongated the avalanche pathways. Consequently, avalanche destructive potential has increased. Hazards will probably increase due to climate change. External factors, like cyclonic storms, may cause fundamental disturbances. Fires have played a major role in the removal of high-elevation forests. Forest destruction by fire is often followed by soil erosion. Wild fires are likely to increase as a result of warming climate and would possibly prevent climatically-driven treeline advance. Cyclic or episodic mass outbreaks of defoliating insects and bark beetles, and pathogens also cause severe disturbances. Oversized populations of wild ungulates impede tree regeneration and can cause local soil erosion. Inadequate game management is the primary cause of intolerable ungulate numbers. Due to man-caused habitat fragmentation, the animals’ impact on the remained habitats has increased. Subalpine forest may recover from disturbance or become replaced by a substitute formation (e.g. krummholz). A subsequent absence of natural disturbances may also be considered a disturbance initiating a new development. Both natural and anthropogenic disturbances may counteract positive influences of climatic warming on subalpine forests and treeline. Effective measures to reduce or prevent abiotic and biotic disturbances of high-elevation forest may contribute to greater safety for people living in the endangered areas of the mountain valleys and also improve other ecosystem services of the subalpine forest.