I'd actually recommend going with a geometry node setup instead of a particle system at this point. Seems a bit easier to control once you've learned the systemUse the particles system. Create vertex group on your terrain where you want to plant trees. There are many, many other parameters to set, I'm sure there's a tutorial about this somewhere. It's important that you set the tree objects origin at the base of the trunk. This is what will determine the height location by the particle system on the terrain map