After we reported that the municipality of Cham had felled sick trees on municipal land, a reader asked the editorial team why the trees by the harbour, at the Siehbach in the Zug area of the city, had fallen victim to the saw.
"We had to cut two willows that were affected by the willow borer," says Claudius Berchtold, head of the Public Facilities department of the city of Zug. The willow borer is a moth that lays its eggs in the tree bark. "The larvae hatching from it then dig through the wood and damage the tree." Younger trees can generally fight back against pests and diseases better than older ones. "For us, felling trees is really only the last resort," emphasizes Berchtold. This has to be done if there is a risk of branches falling or the tree being uprooted in a storm.
The approximately 3,500 trees in the City of Zug are checked regularly. “Every five years by an external company, and annually by employees of the Maintenance Department of the City of Zug.” In order to obtain a better picture of their health, the specialists climb the trees and carefully check the trunk, branches and foliage. If a tree is infected with a disease, a fungus or a pest, they first try to only remove individual branches, or to strengthen the tree in another way so that it can deal with the problem itself, says Berchtold.
"For city trees, which are subjected to more stress than their country colleagues, it’s above all important that they are planted on a good substrate, and in a suitable location where the roots can spread undisturbed." Irrigation also had to be checked more often, because rainfall in the city often insufficiently waters the ground for trees.
"Pests are as diverse as the plants themselves," says Berchtold. "In the past few years, for example, the occurrence of the horse chestnut leaf miner has increased massively." These moths eats the mine between the veins of the leaves, and overwinter in the fallen leaves. “We therefore remove the fallen leaves and take them away, so that this pest cannot spread so easily. “Old trees in a prominent location, such as on the Landsgemeindeplatz, are additionally protected. "We tie the crown branches together with ropes and belts so that stormy winds cannot tear the tree apart." According to Berchtold, these trees would no longer be there without this measure.
Photo 1: The two willow stumps on the Siehbach have become a habitat for numerous insects
Photo 2: A close-up of the stump shows the fascinating work of the insects.
The tree population in Zug is relatively young, with an average age of an estimated 50 to 60 years. For this reason, only about ten fellings have to be carried out each year. The stock is much older in other cities, such as Basel, Bern or Zurich, and is therefore more vulnerable. Basically, a city tree will live for around 50 to 100 years old, although the average age has increased in recent years.
"The stress factors that city trees are exposed to mainly consist of the heat, drought, and damage caused by collisions or vandalism," reports Berchtold. The latter has decreased significantly in recent decades, however. "It is no longer common to carve love hearts in tree trunks." And that’s good for the trees.
Because of climate change, and the resulting more intense and persistent heat and drought in summer, tree species are now being sought that could respond better to these changes. This is a being done in collaboration with city gardens and horticultural offices at home and abroad. Lime trees, plane trees, maple trees and horse chestnuts predominate at the moment.
When a tree is felled, its stump is left to provide a house for insects of all kinds, as long as there is no danger to humans or the environment. This is also the case with the two willows on the Siehbach, on the stumps of which the work of the willow borer and other insect species can be observed very well. “Where possible, we also carry out a replacement planting at the same location or in the immediate vicinity of the felled tree.” The newly planted trees are already ten to twenty years old, as the city obtains them from a Swiss or North German tree nursery.