SAM: What is that? GIMLI: Nothing, it's just a wisp of cloud. BOROMIR: It's moving fast, against the wind. LEGOLAS: Crebain! From Dudland! ARAGORN: Hide! […] SARUMAN: Cuiva nwalca Carnirasse; nai yarvaxea rasselya! (Wake up cruel Redhorn! May your horn be bloodstained!) LEGOLAS: There is a fell voice on the air GANDALF: It's Saruman! ARAGORN: He's trying to bring down the mountain! Gandalf! We must turn back!
Saruman's use of the elements of the world, both the mountain storm and the cloud of crows, pit the fellowship in the classic duel between man and the natural elements in the world… or maybe the supernatural elements in this case.
BILBO [on hobbits]: But where our hearts truly lie is in peace and quiet and good tilled earth, for all Hobbits share a love of all things that grow.
Man (or hobbit) and nature aren't always fighting against one another. In the Shire, the two coexist perfectly and peacefully. This part of Bilbo's description features shots of Sam gardening, so we already know to associate him with caring and kindness.
ORC The trees are strong, my Lord. The roots go deep. SARUMAN Rip them all down.
Saruman is ripping down trees like a lumber company in the Amazon. This is probably the most visceral man (or orc) versus nature scene of the movie as the fire and iron of Orthanc contrasts the giant trees as they're torn down and thrown into the pit to forge the orc's armor and weapons; trading the life and peace of the forest for death and war.
Gandalf carries an old, twisted, wooden staff and travels the world. Saruman's staff is angular, made of steel; he sits in his tower, Orthanc.
Gandalf seems to be much more in tune with nature and his surroundings that Saruman who sits alone on his iron throne. The juxtaposition of these characters furthers our trust of nature and our hate of man (or wizard's) interference with it.