[Sardy]

A question - after the battle of Helm's Deep, did the Ents (or more likely the Huorns) remain "rooted" outside the Gate, thus creating a new forest and semi-permanently changing the geography of Middle-earth for the Fourth Age? Or did they return (were they herded) back to Fangorn?

A related question, did the number of Ents, Huorns, trees marching to
Isengard (and Helm's Deep) comprise a significant amout of trees to noticeably reduce the size of Fangorn?

The only quote I've been able to find addressing this,

"Wailing they passed under the shadows of the trees; and from that shadow non ever came again."

While certainly not specific and much open to interpretation, the above quote (to me) seems to indicate that the new forest remained in place for a substantial period of time...