LotR -- Book 3 - Chapter 09 - Flotsam and Jetsam

[The Saucepan Man]

Originally Posted by Lalwendë

It seems that the Ents can resist, but how and why?

During their assault on Isengard, I should imagine that they were not in much of a mood to listen to Saruman, even had he had the presence of mind to try to appeal to them. It is possible that, once the assault was over and the Ents settled down to guard the Tower, Saruman would have attempted to use his "voice" on them. However, Ents are, as we know, not the most hasty of beings and are certainly not moved into precipitative action by mere words (as Merry and Pippin discovered). It is likely, therefore, that in these circumstances they would have taken their time to consider Saruman's words, thus negating their immediacy and therefore, possibly, their "magic".