Another theme that appears several times in The Lord of the Rings is the conflict between nature and industry. Tolkien had been raised in the countryside and was very attached to nature, so you could understand his disappointment with his fellow humans when industry and machines began taking over. Because of his childhood home, he made a noticeable connection between evil and metal by making the Shire a rural place and filling Mordor and Isengard (the antagonists) with machines, forges, fire, wheels, and other objects associated with manufacturing and war. As Isengard is piled up with machines, Saruman levels the trees surrounding the tower (called Orthanc, by the way) to fuel the fires that power the machines. When Treebeard and the rest of the Ents attack Isengard, they flood the inside of Saruman's territory and destroy the machines, and only when the machines are destroyed, Isengard's armies are scattered at Helm's Deep, and nature returns to Isengard, does Saruman's power begin to dwindle.
Fire also makes somewhat of an appearance in the story, in both protagonistic and antagonistic ways. In The Lord of the Rings, most of the symbolism associated with fire is negative, as Tolkien associates fire with the machines employed by Sauron and Saruman in their conquest of Middle-Earth, and the destruction that results from this. However, in other stories Tolkien also associates fire with creation and birth; for example, Eru Ilúvatar (Middle-Earth's equivalent of God) and other characters in Tolkien's stories call Eru's ability to create life the "Secret Fire" (hence Gandalf's statement that "I am a servant of the Secret Fire, wielder of the Flame of Anor" in Moria).