LONDON — In February 2015, Simon Bramwell climbed into a tree near Bristol, in southwest England, to prevent workmen from cutting down a whole row of trees for a bus lane.

“I’m terrified of heights, so it was quite a prospect,” Mr. Bramwell, 47, said in a telephone interview.

Along with other activists, he made platforms to sit on, then spent days and nights in the trees. He relieved himself in a bucket. It was so cold, he added, that he often woke up with frost in his beard.

The protesters were evicted after a few weeks, and the trees were cut down. But that failure set Mr. Bramwell on a new path. “It was this hammer blow of, ‘O.K., that’s it, I’m done with this kind of organizing,’” he said. “‘There’s got to be a different way.’”