And, no, she didn’t need to call the police/council, or threaten litigation to deal with the matter. And, no, the young people in question did not threaten violence against her as a result of her intervention. From the Islington Tribune:

FACED with a group of youths goading a Staffordshire Bull Terrier to strip bark off trees to toughen its jaws, a lesser person would walk away. But 82-year-old deputy mayor and Lib Dem councillor Anna Berent is not a lesser person – she once bit a burglar – and the safety of the tree, in Petherton Road, in Highbury, was more important to her. …

Cllr Berent said: “I was going home and I saw these young men with a dog. It was attacking the tree. I wandered in that direction because I had to go past them anyway and said: ‘Nice dog but you really shouldn’t teach it to rip bark off the trees’.

“They said trees die anyway and I said give them a chance and they will live longer than you. “They ‘Humphed’ and walked away.”

But Cllr Berent was not shaken by the experience. She said: “I was marginally nervous but I always feel that I’m probably the least likely age and appearance to be thumped.
I’m not a challenge to anyone’s masculinity. I’m much too old to be a sex object and I’m not so decrepit that I look like I will fall over if anybody blew.”

She added: “I do feel trees are important. Green issues and the environment are probably the things I feel more strongly about. Dogs stripping bark off trees is a major problem in Islington. It goes with training dogs as fighting dogs. You’ll see a lot of trees with great strips of bark torn off them. If you take a significant amount of bark off it will distort the growth very badly. There are certain things one doesn’t feel entirely you can let go and this is one of them. We as a council spend an awful lot trying to improve our parks in general. It’s something people appreciate. So to have someone negate this does seem very sad. I don’t think you should attack people for doing it but I do think you should walk up to people and tell them not to do it.”