Lightmapping outdoor scene not working, no shadows

I have a simple outdoor beach scene with 3 trees and a directional light for the sun. I checked generate UVs for each tree model and set the trees and terrain to static in the lightmapping options. I then bake the scene but the trees disappear. I tried setting only the terrain to static and the trees don't disappear but the trees don't cast any shadows and the lightmap seems to have no effect. What am I missing?