A modification of NEET ENGINEER's DoronTree shader to add better lighting, randomness settings, and LOD options.
Original here: DoronTree on Booth
Modification originally made for my VRChat world Test Pilots. Tree textures are AI generated. Some AI assisted code.

This shader spawns a cross-plane trees on every vertex of the mesh it's applied to.
You will have to make a your own mesh that conforms to your terrain to make it work well.
I recommend using this tool to export your terrain to create a tree mesh from:
iffn's Terrain Exporter Sacc Mod
Tree meshes should be in clumps rather than having one for the whole terrain to allow for culling.