Protective netting has been wrapped around “valuable” trees in Lancaster city centre after bark was stripped from their trunks.
Lancaster City Council carried out the emergency measure after the trees in Market Square and Cheapside were vandalised over the last two weeks.
Green Party Coun Dave Brookes, who represents Castle Ward, said it is an offence to damage trees in a Conservation area.
The city council said it was now studying CCTV footage to try and find those responsible.
Coun Brookes said: “Some of the trees have had a lot of bark removed, which could severely weaken or even kill them.
“The recent heatwave demonstrated how valuable city centre trees are for providing shade.
“It is difficult to understand why anyone would attack trees, but this level of damage cannot be accidental.”
A spokesman for Lancaster City Council said: “You may notice that protective netting has been added to trees on Cheapside and Market Square in Lancaster.
“This is due to some damage that has taken place over the last couple of weeks, which has seen bark being stripped from the trees.
“We’re studying our CCTV system to try and find those responsible and also looking into some long term tree guards that will prevent similar problems in the future. We’ll be monitoring the trees to closely in case the damage affects their health.”