Worldsdesigner
The logical first step to placing trees in worlddesigner is having them.
We have two options:
They can either be imported as a treepack from here. This is the fast way but gives you less customization. Just download the treepack, unzip it and import it using a structure pool node in the editor.
Or you generate the trees using the built-in tools of the worlddesigner editor. This is more complicated but allows you to create the trees exactly how you want them to look. For a detailed explanation of how this process works, take a look at the detailed explanation post.
Whichever way you choose, you will end up with what the editor knows as a Structure pool. You can think of that as a list of different structures that can be used in different ways. The structure pool is simply the container that holds them. In the editor you can identify nodes of the structure pool type by their yellow color.
This is the interesting part of the process.
The placements of structures and structure pools is done with placements nodes. If you rightclick on the background you can find them under the placement entry. Some nodes are used to place singular structures and others can place structures from a pool.
The simplest way to placing trees in a world is with the Structure scattering node. It places the structures (in this case they are trees) randomly on the surface of the landscape. This means that the tree/structure is always on top, regardless if there is a mountain or a flat grass field.
Such a setup would look like this:
It is also possible to mask out areas of the landscape where no trees are supposed to grow. This can be done for areas like oceans or lakes. You can also create mountains and cliffs that dont have trees because they are too steep for anything to grow on them.
You may want to mix it up a bit and place different kinds of trees. This is possible by combining multiple structure pools that come from different treepacks.
if you want a perfect raster placement of your trees you can use the Raster Placer node. It lets you define a grid where on each intersection of the gridlines a structure is placed. This is intended for structures like buildings but can be used for trees.
If you want to try this yourself, download the worldsdesigner editor and start creating worlds.