This is the manual to properly set the billboarding trees in Blender which works with the
rz tree shader billboard multiple V2 for multi trees textures. For single tree texture and
rz tree shader billboard V2 the process is same just the UVMap.002 is not necessary
1) Create desired object (planes) in X/Z plane. Importan is to have scale 0,01 and rotation X 90°. I create several collections with different types of trees.
2) First UV map is covering just the single tree on the texture.
3)Set the scale in Units to 100 to see se proper dimensions of the mesh hence these dimensions will be 100 times smaller
4)UV map 2 is most important for proper rotation of the tree in 3dsimed but the process simple. Turn on the edge length in overlays panel and now set the 2D cursor to coordinated which corresponds with mesh dimensions X is X, Y is Y. Only you have to put negative sign or possitive. When i set positiove for both the tree rotated in opposite direction so i have kept negative for both. Duplicate UVMap to get UVMap.001. Then scale everything down into this cursor position to get one single point. You have to turn on cursor as a pivot point.
5)UVMap3 is similar as UVMap only the UVs are covering entire texture. Not necessary in case of single tree texture. Now the group of trees are prepared for cloning but i suggest to test it before cloning if it rotate correctly in 3dsimed but before you export it to fbx set the origin point of each object to 0,0,0,. Then for cloning it should be back to the individual origin of each object.
6) Create object with points cloud with positios of the trees and use the particles to clone the trees. For different group of trees i create different particle systems. Most important is to say particles system to not scale and rotate the objects so hair lenght should be 100, orientation axis set to Object Y and phase and random phase in rotation toolbar set to 0. Also the scale should be untouched so set to 1.
After everything is set i convert the trees so you will get amount if trees what i set in particles as a single object. Then you have to make these objects single user, join them into one mesh and set the origin point to 0,0,0. Then its ready for fbx export.
Only a bit crude is when you want to create different size of trees to get nice variation so it is necessary after the scalling of the mesh to set the UVMap.001 point again. But it should be possible to create some script to make it automatically so set the UVMap point based on the dimensions of the mesh.