At the beginning of the footpath called "Metzgerwegli" near the stop Uitikon Waldegg of the Uetliberg train, a glance into crown canopy of the trees leaves one feeling distinctly puzzled and somewhat irritated. A single spruce is swaying backwards and forwards while the surroundings trees are still. One finds the explanation for this phenomenon about 100 metres further on. 4 metres above the ground, the suspicious 40 metre high tree is attached to the steel winding cable of a servo-hydraulic cylinder, which pulls regularly at the trunk and causes the tree to sway. While oscillation of the trunk is just 5 to 10 centimetres, it is several metres at the crown of the tree. Movements at ground level are far smaller and the earth around the trunk seems to be breathing.

From roots to crown

The person responsible for these strange happenings is Pierre Vanomsen, graduate student of Jean-Philippe Schütz, ETH Professor of Silviculture (1). In collaboration with EMPA (2) Vanomsen is assessing the spruce in a number of ways. An acceleration receiver is fixed at a height of 25 metres with three axes measurements, there are two gradient gauges a metre above the ground, and a steel frame around the trunk measures the movements of roots and earth in eight directions. In addition, an underground cable is connected to a microphone to record breaks in the root system. The dynamic tackle experiment with a traction charge of up to 16 tonnes will be done using an apparatus that the EMPA uses for static and dynamic constructions.

Makes spruce sway: the servo-hydraulic cylinder from EMPA with all its installation apparatus. large

Home game

Vanomsen will not only be making one spruce tree sway. Trees of comparable size will be submitted to the procedure in two different types of forest; four trees in areas where weak and four trees where strong thinning intervention has taken place. The same thinning trial was carried out in this research forest in 1951. The trees Vanomsen will be using are trees from stock that was planted in 1913. "My aim is to examine the influence of thinning interventions on a tree's root system and capability to withstand wind," Vanomsen explains.

The spruce shaker: Pierre Vanomsen observing the dislocation of one of his spruces at earth level. large

This "home game" offers two opportunities for research. Not only is it possible for Vanomsen to compare the effects of various thinning measures in the forest, these trees will be felled in autumn in any case to investigate the root system. This would hardly be possible if the forest belonged to someone else.

Natural frequency testing

There is another particularity in this examination. This is the first such dynamic traction experiment with trees of these dimensions. Up until now experiments were usually carried out on smaller trees with static traction to determine the point at which trees fell. Such tension is not the same as that which is caused by a hurricane, however, as Vanomsen explains. During a storm the strength of the wind is not constant but comes in gusts, which make the trees sway. Over the past years the forester has determined the natural frequency of "his" trees using an accelerated receiver. It is around 0.2 hertz. This is precisely the frequency that Vanomsen will apply to the trees in accordance with the main wind direction of the area. He hopes that this will simulate, as closely as possible, the dynamic conditions of a storm. The forest engineer thus leads back to the underlying reason for this experiment: hurricane Lothar.

Standing up to future "Lothars"

Hurricane Lothar swept through Switzerland in December 1999 leaving a trail of devastation in its wake: 13 million cubic metres of wood and wind damage to 46,000 hectares. Since then a regeneration of the forest has taken place to a degree that never would have happened without the storm. Moreover, it raised the question of how to manage regeneration in order to minimise damage from any future Lothars. One management method is the use of silvicultural tending measures. This is where Vanomsen's work comes in. Should it transpire that thinning leads to heightened stabillity, then this measure would be especially advisable for spruce, as this species is always worst hit in storms occurring in Switzerland. Before recommendations can be made, however, those spruce must continue to swing, whether artificially or naturally.