How do I make animated .fbx file detect collision with another .fbx file?

So I've modeled an axe and an Evergreen in Blender, and I've been trying to make it detect collision (I want it so that I have to chop the tree five times, then let it delete) but no matter how hard I've tried, no collision is detected. I can make cubes detect collision though, and balls (both added from unity). How can I fix this?