Silent Running (movie, 1972)

In a future Earth barren of all flora and fauna, the planet's ecosystems exist only in large pods attached to spacecraft. When word comes in that the pods are to be jettisoned into space and destroyed so that the spacecraft can be reused for commercial purposes, most of the crew of the Valley Forge rejoice at the prospect of going home. Not so for botanist Freeman Lowell who loves the forest and its creatures, so decides to take matters into his own hands to protect what he loves.