How did Pippin know that Saruman the White was cutting the forest on that scale?
FOR THE MOVIE
Treebeard does know that there a a great War coming. After he lets the hobbits sleep he leaves saying.
"I have business in the forest. There are many to call. Many that must come. The Shadow lies on Fangorn. The withering of all woods is drawing near."
THE LORD OF THE RINGS - The Two Towers - Screenplay by
Fran Walsh & Philippa Boyens & Stephen Sinclair & Peter Jackson
In fact, Treebeard already knew Saruman was doing something but doesn't seem to care.
On the way to the Entmoot to decide if the Ents will take part in the Great War... they see Isengard in the distance..
AERIAL ON: Amidst the lush foliage of FANGORN, TREEBEARD
continues his own arch with MERRY and PIPPIN in tow. In the
distance, ORTHANC is clearly visible in the RING OF ISENGARD...a
dark shape moves across the land towards ROHAN.
PIPPIN: Look. There's smoke to the south.
TREEBEARD: There is always smoke rising......from lsengard these days.
ANGLE ON: MERRY and PIPPIN reach the upper branches of TREEBEARD, just above the tops of the trees.
TREEBEARD: There was a time......when Saruman would walk in my woods. But now he has a mind of metal......and wheels. He no longer cares for growing things.
HIGH WIDE ON: On the FIELDS of ISENGARD, the FLOOD of the URUKHAI
ANGLE ON: MERRY and PIPPIN ride atop TREEBEARD, worried.
PIPPIN: What is it?
MERRY: It's Saruman's army. The war has started.
Eventually, the Ents decide NOT to go to war but rather to let it pass over them as they have always done.
TREEBEARD: The Ents cannot hold back this storm. We must weather such things as we have always done.
Treebeard tries to take them to safety by going to the Western edge of Fangorn...but that's not where Merry and Pippin want to go
MERRY: Our friends are out there. They need our help. They cannot fight this war on their own.
So, Pippin takes advantage of Treebeard's 'unhasty' nature by 'tricking' him into taking them South (since that's the direction the hobbits want to go).
Yes. Exactly. If we go south we can slip past Saruman unnoticed. The
closer we are to danger, the farther we are from harm. It’s the last thing he’ll expect.
But he might have suspected.
It's possible that Pippin suspected that something might be going on after this ...
MERRY (distantly) The fires of Isengard will spread, And the woods of Tuckborough and Buckland will burn. And...
MERRY turns to PIPPIN.
MERRY (CONT’D) (angrily)
...and all that was once green and good in this world will be gone.
MERRY looks into PIPPIN’S eyes intently.