Alpha Trees comes with the Weight Layers Addon built in, which allows you to create fully procedural weight maps using layers such as slope, proximity and height.
You can then use these weight maps to control the distribution and scale of your trees.
When you add or select a new vertex group, in Alpha Trees, the addon will automatically create a new weight layer stack for that group, which you can then use to customise the distribution to your liking.
It's worth noting that distribution is per object, rather than tied to biome settings like the other settings. This is because vertex groups can only exist on a single mesh, and can't be directly copied between them.
Both density and scale have a row containing the following settings/buttons:
Vertex group: Lets you select an existing vertex group to use to control either the density or scale of the trees.
Vertex group name: The name of the currently selected vertex group.
New vertex group: Adds a new vertex group and selects it automatically.
Edit vertex group: Puts the user in weight paint mode, and selects the correct vertex group for painting.
Invert influence: Whether to invert the vertex group influence.
Remove vertex group: Remove the current vertex group. This removes it from the entire object as well as from being selected.
When you add a new vertex group, you can choose from a list of presets to apply to the new vertex group. They are added using Weight Layers, and can be edited in the Weight Layers panel.
Blank: Adds a default blank vertex group where all vertices have a value of zero
Height: Creates a gradient based on the Z coordinates of the points. The axis can be changed in the Weight Layers panel. It is in local coordinates, so the gradient will rotate with the object.
Mesh proximity: Gives each point a value based on its distance from a specified object's mesh
Point proximity: Gives each point a value based on its distance from a single point. This can be set to an objects' location for easy distance culling, for example.
Procedural texture: Generates a procedural texture, the settings for which can be edited in the weight layers panel. You can also change the type of texture e.g. to image, Musgrave, or noise etc.
Slope: Points that are facing straight up will get a value of one, and points facing horizontally will get a value of zero.
This shows how the distribution panel is used: