My PCs have burned down a forest before. They even got an NPC druid to help them make sure the whole thing burned.
A nearby Far Raelm bleed through point had corrupted a Treant along with some surrounding plant life. The critter moved into a large section of woods near the Captol city. After a few encounters with the psuedonatural treant, the 5th to 6th level PCs realized the ent was actually animating other trees to do the fighting, which then took on its corruption even after de-animating. Their only real weapon against it was the corrupt ent had strong pyrophobia, which they learned through high casualty trial and error...
The PCs, fearing more could likly perish fighting the ent and endless supply of minions it had access to, took the dwarven [alchemist]fire-thrower they had obtained previosly and used it to swiftly ignight the forest perimiter. They encountered the druid when they had over half of the outer forest ring ignited. Having returned to the area because of rumors of a fouless in the forest, the druid was not in a good mood. Through good roleplay they convinced the druid of the nessesity to burn the forest to ash.
Through fleetness of foot, the PCs managed to close the ent in the circle of burning woods, in which it was eventually consumed.