In August 1992 a hailstorm destroyed 73 ha of protection forest on the north face of the Rigi. Many spruces were not wind thrown, but their stems were broken off at heights ranging from 3 to 15 metres. As the trees had been growing on steep slopes, the local forest service decided to leave the stumps standing as a protection against rock falls and other natural hazards. The question as to whether this procedure would further the propagation of bark beetles, in particular Ips typographus, came under debate.
An experiment was conducted to determine whether girdling of these stumps would accelerate their drying out and thus more rapidly render them less attractive to bark beetles.
Results: Two measurements of wood moisture showed that the drying out of the stumps was not influenced either by girdling or by the height at which the crown broke off. The only factor was the diameter of the stem: thin stumps dried out more rapidly than thick ones.
In the case investigated, Ips typographus invaded only broken crowns lying on the ground, which could be removed in time to prevent the beetle from spreading. One year after the hailstorm all the stumps left standing had dried out to such an extent that they were no longer attractive for dangerous forest pests, so that here it was worthwhile to leave the stumps standing. Their protective function proved itself to a large extent, and there was enough manpower to remove the infested wood and that which might attract the pests.
Without doubt, it is thinkable that if the bark beetles underwent a population explosion, even the stumps may have been invaded. Therefore it was essential to maintain observation for two further years.