Actually, the material does not change when you unwrap, it's the UV map that changes and your material uses the default UV map of your object as a texture coordinate :
Select one of the leaves and go to UV Editing tab and you will understand :
This is the map that is used for the material now. But this is what happens when you unwrap the mesh again :
The map is completely distorted and since you only have one UV map, you just overwrote the one that your material uses, hence the unwanted change that you noticed :
To fix that, undo everything and go to the Mesh tab from the properties editor and under UV Maps click on the
+ icon to add a new UV map :
Make sure that this UV map is the one selected before you hit
Unwrap and your original UV map would be safe.
I hope it helped you solve your problem.