Look what I found on IMDB! I never knew this! 

“The soundtrack reflects the natural vs. man-made nature of the scene. For example, the opening song “Thneedville” describes the thoroughly artificial town with completely electronic music. Meanwhile, the finale “Let It Grow” and the planting of an all-natural tree is made with almost all acoustic instruments. “Let It Grow” even includes a capella choral music, i.e. with no man-made instruments at all. ”