Norway Spruce (Picea abies) is the most common tree in the European mountains and therefore of very great importance in the protection mountain forests provide against natural hazards. The European spruce bark beetle (Ips typographus) may cause extensive dying of spruce. Especially in the alpine and subalpine regions, this may have very serious consequences as the proportion of spruce in the woods here is often near 100%. When the decay of the remaining trees and the decomposition of dead wood proceed too rapidly, there is a risk that for some time the protective properties of the forests will fall below a critical level.
The scope of this study was to analyse the behaviour of dead trees under mechanical loads such as are caused by avalanches and snow movements, rockfall and slope failure, and to determine the effect of wood-rot fungi on the resistance of tree stems.
The research was carried out in a case study in the Gandberg forest near Schwanden, in the canton of Glarus (Switzerland). From 1992 to 1997, in the wake of the storm Vivian (1990), a vast area (about 100 hectares) of a spruce tree mountain forest died off as a consequence of the bark beetle calamity.
The area affected by the bark beetle was not cleared, the dead spruce were left standing. The mechanical stability of these stumps was measured about 10 years after their death. Attempts were made to pull down entire stumps in the field thus simulating a static load, comparable to the pressure of creeping snow in winter. On a test installation, stem parts were crushed by falling weights and the fracture energy calculated. In this way it was possible to simulate the dynamic loads caused by rock impact. The rot agents were isolated and identified by examining the cut surfaces of the stumps and stem sections. As the root system of the stump contributes to the slide-preventing stability of the soil, the tensile strength of the excavated roots of both dead trees and freshly-cut spruce were calculated and compared with each other.
During the attempts to pull down the stumps, three-quarters of the tested stumps failed by stem fracture, and at a lower bending moment than that of the stumps which were uprooted. In determining the fracture energy, stem sections from deadwood that had already been invaded by fungus Fomitopsis pinicola were found to have the lowest fracture energies (25.59 ±7.74 kJ/m²). In stem sections from deadwood in which the fungus was not found, mean fracture energies of 131.89 ±34.52 kJ/m2 were observed. For stem sections from fresh spruce, maximum values of 238.89 ±40.43 kJ/m² were calculated.
F. pinicola was the predominant rot fungus to be found in the stumps ten years after the Gandberg spruces were killed off by bark beetles. It was found in 68.7% (n=90) of all basidiomycete isolates from above-ground deadwood.
If the dead trees break early (within about 5–10 years) and the deadwood comes to rest at right angles to the slope, there is an increase in their protective properties against avalanches and rockfall. However, on steep slopes (with an angle of >70%) the protective properties of dead spruce is rather dubious. The process of decomposition may cause the deadwood itself to come loose and slide down together with the snow mass.
In comparison with the roots of freshly-cut living spruce, the roots of spruce that had died about 10 years previously had 29% less tensile strength and 75% less strain energy. Consequently, where the ground is susceptible to erosion and landslides, the danger of surface slides increases after the death of trees in proportion to the increase in the decomposition of the root wood.
Basically, areas with dead spruce do offer some protection against natural hazards in spite of the decreasing strength of the wood. However, it cannot be excluded that in certain places and under unfavourable conditions, the protective effect of the decomposing matter may not be enough to offer sufficient protection until the next regeneration.