Treebeard is taking Merry and Pippin back home when Pippin tells him stop. He wants to go south.
Treebeard says this will take them past Isengard and Pippin goes on about how being closer to danger is more safe because they'll be unnoticed… yeah, it doesn't make sense to Treebeard either (or Merry for that matter), but Pippin seems to have something up his sleeve.
When they make it to the southern edge of Fangorn, Treebeard is stunned.
All of the trees he's known have been burned to the ground. There is nothing left but ash—the burnt remains of trees.
With a cry of anger and sorrow, Treebeard summons the ents. They will march on Isengard and on Saruman.