Watchwood

A forest of ancient trees set around Isengard after its capture by the Ents. The trees of the Watchwood stood throughout the valley of Nan Curunir, put in place to look for any sign of Saruman secretly escaping from his Tower. Treebeard boasted that even a squirrel could not enter the Watchwood without his knowledge, and that Saruman would remain trapped for many years.

However, within a few months of this, Treebeard relented and allowed Saruman to leave Orthanc. It is unclear whether the Watchwood remained in Nan Curunir: after Saruman's release, there would be little purpose in the trees remaining, and they presumably returned to Fangorn.