Burnings of the Old Forest

Sauron's forces caused much burning of the Old Forest during the time of the War of the Elves and Sauron. Sauron's forces did this on their way to Eregion and this prompted Treebeard and nature to fight back.

This is shown in the Two Towers when Treebeard talks to Merry and Pippin:

The Two Towers; Treebeard 'Quickly?' said Treebeard. 'Hoom! Yes, indeed. Quicker than I expected. Indeed I have not seen them roused like this for many an age. We Ents do not like being roused; and we never are roused unless it is clear to us that our trees and our lives are in great danger. That has not happened in this Forest since the wars of Sauron and the Men of the Sea.