Disturbance has a strong influence on the structure and dynamics of subalpine forests. In general, growth and survival of plants is determined by the availability of resources such as light, water and nutrients at their site. But, the structure and dynamics of a forest is not only the result of supply and demand of resources. It is to a large extent effected by disturbance of biotic or abiotic origin, interrupting and hence disturbing the "normal" life cycle of a tree and the dynamics of a forest.
Disturbance in subalpine forests is most frequently caused by either natural hazards such as avalanches or strong winds or by parasitic fungi such as the Fomes annosus (root rot) or by insect calamities such as that caused by Zeiraphera diniana (larchbud moth).
Disturbance by abiotic or biotic agents results in growth reductions or dieback of individual trees or stands of subalpine forests. Consequently, the balance of supply and demand of the required resources light, water and nutrients changes. Resources are newly available for surviving trees or plants newly establishing from seeds. Surviving trees, deprived of their competitors, may increase growth and develop towards dominant individuals. Seeds of plants of pioneer character typically establish first on the bare ground, giving way to succeeding plants and, without disturbance, finally develop into a closed forest.
Disturbance offers temporary habitats for pioneer - and early successional species, whos resource demands are not fulfilled in the closed forest. Sites with intermediate disturbance frequency usually exhibit the highest diversity of species.

Disturbance through avalanches

Disturbance in subalpine areas is frequently caused by avalanches. Forest in steep terrain (> 25° inclination) or immediately below it is exposed to avalanche hazard. Avalanches exert a high pressure on trees in their wake. Trees with elastic main stems are bent down and may overcome the impact without major injuries. Elastic stems are exhibited by smaller trees with stem diameters < 10cm, as well as of krummholz species such as Pinus mugo or Alnus viridis. These trees are often found occupying large areas in avalanche tracks, regularly overrun by avalanches. Trees of stem diameters > 10cm are fully exposed to the pressure of avalanches and may break. In the Swiss Alps, many avalanches yearly cause many m³ of wood to be blown down. In the extremely snow-rich and avalanche-prone winter of 1999, a sum of 450 avalanches were recorded causing more than 1400ha of forest to be blown down, corresponding to 160 000m³ of wood debris. One of the largest avalanches blew down 35ha of forest (8000m³ of wood, Bisistal, Schwyz). Through their destructive forces avalanches cause large openings as vertical tracks in the subalpine forest belt. Depending on the frequency of avalanches and the amount of time since the last event, the vegetation structure exhibits an increased diversity of plant species. Ground cover vegetation and small shrubs profit from the resources newly available through the disturbance of the trees.

Disturbance trough parasitic fungi

Fungal species such as Fomes annosus may cause considerable growth reduction or even die-back by decomposing essential root wood composites of trees. Entering the water conducting elements, they reduce the physiologically active parts of a tree, occasionally leading to tree death. Through their vegetative propagation, parasitic root fungi often cause whole cohorts of trees to die. Acting under ground, this often disastrous activity is rarely attributed to the fungi, but to the secondarily active bark beetles.

Eidg. Institut für Schnee- und Lawinenforschung SLF, Davos Eidg. Institut für Schnee- und Lawinenforschung (Hrsg.), Schadenlawinen - Datenbank. Der Lawinenwinter 1999. Ereignisanalyse, 2000, Davos, Eidg. Institut für Schnee- und Lawinenforschung. 588 p.
Pickett S T A and White P S, The Ecology of Natural Disturbance and Patch Dynamics, 1985, Academic Press, Inc. Orlando. 457 p.
White P S, Pattern, Process, and Natural Disturbance in Vegetation, 1979, The Botanical Review. Vol. 45, No. 3, 230-299.

29 August 2011
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