Weren't they the ones driving spikes into trees so when the lumberman came along with his chainsaw, the chain would hit a spike and break, with a good chance of causing severe injury?
That's non-violent?
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Sure it is, especially if you tell them about the spikes.
Even if you don't, after the first spike, they can consider themselves warned, can't they?
The chainsaw is violent, not the spike. This is a form of forest protecting jiujitsu.
No one told them they could not use the traditional two-man saw.
Spiking trees like redwoods is a great idea if it prevents them from being felled.