Although never widely implemented as a tactic, at least the idea of tree-spiking was once regularly discussed. Nowadays it is dismissed without a thought - much less a second one.
For those of you not familiar with tree-spiking... the tactic involves placing small hard spikes sporadically in some of the trees scheduled to be clearcut in an old-growth forest. The foot-long spikes (made of non-corrosive metal or high-tempered ceramics) do much less damage than the chainsaws and lumber mills which they are designed to destroy on contact.
The ethical argument against this tactic is that chainsaws could snap when they hit the spikes and potentially harm harm loggers. However, clearcutting is ultimately suicidal to begin and nobody is forced to be a logger. Furthermore, if the spikes are placed ten feet high in the trees they probably won't be hit till they reach the expensive saw blades in the lumbermill. Also, if the tree-spiking activity is well publicized, detailing exactly which THP was protected, that is more than fair warning to anyone who still wants to clearcut. Therefore, in my opinion, spiking should be considered a NON-violent action. And again, anyway, clearcutting is a suicidal and ecocidal act to begin with and should be stopped by any means necessary. If on the off chance that the timber industry still chooses to cut in a spiked area, despite the warnings, then THEY should be held responsible for any injuries (which might eventually make them rethink their position). If the tactic ever became widespread enough a mere warning would someday suffice to save a threatened forest.
Although denounced by mainstream forest activists (who want to make friends with the loggers), I believe they contradict the Earth First motto: NO COMPROMISE in defense of Mother Earth. And, despite what anyone might say, I believe it's the lumber company executives who hate tree spiking more than anybody else.