Heat and bark beetles force felling of 500 spruce trees near Uster

More than 500 spruce trees have been felled in a forest near Uster after heat, drought and bark beetle infestations weakened the trees, SRF reported. Forestry workers must now decide which species can survive in the forest for decades to come.

A spruce more than 100 years old was among the trees brought down. The felled trees must be removed quickly before the beetles emerge and infest more spruces.

Forester Ruben Menzi said the Buchdrucker, a species of bark beetle, was the main problem in the Uster forest. The team looks for early signs of infestation, including sawdust at the base of the trunk. The beetle creates a main tunnel beneath the bark, while its larvae feed on side tunnels and develop into new beetles.

The trees are particularly vulnerable because of the local soil and increasing drought. Water is poorly retained in the gravelly ground, Menzi said. Longer periods of heat and drought caused by climate change are conditions that spruce does not tolerate well.

Water shortages also weaken the trees’ natural defences. Healthy spruces can use resin to stop invading beetles, Menzi said, while weakened trees cannot withstand as many insects. The bark beetle is part of the ecosystem and mainly attacks weakened trees.

Menzi said the beetle effectively shows foresters where past decisions were wrong. Too many spruces were planted in locations where they cannot thrive in the long term, he said.

A single beetle can produce about 40 new larvae, and hundreds of beetles may be found on one trunk, creating exponential growth. Infested spruces therefore have to be felled and removed from the forest quickly.

By mid-August, the amount of rainfall recorded during a summer half-year had never been lower since measurements began. The Swiss Federal Council has responded by making more than 70 million Swiss francs available to help forests adapt to climate change.