you don’t have to physically separate the meshes or split them into separate objects, i meant you should treat each of those parts as separate uv islands.
blender’s lightmap pack will break every face into individual islands, which means each face will need to waste extra space on padding.
instead of lightmap pack, try using smart uv project with an angle between 60 and 80, with island margin of 1. if that still doesn’t give great results, mark edges with seams and manually unwrap, so that you can make better use of the texture space.
if you are still getting problems after all of that, increase the padding between islands, or increase the lightmap resolution of the mesh in ue4.
but if you did want to break the model into separate meshes, it might help you create more variety. with separate meshes for a trunk, a branch, and a sphere, you could make a blueprint with a construction script that uses random numbers to add variation to the orientation and scale of spawned instanceStaticMeshes. basically, you would be making a simple version of speed tree in ue4.
or, if you wanted to skip the coding part, you could just place those tree parts in the level, and when you build a tree you want to reuse, you can convert those meshes into a blueprint, which you can reuse anywhere.