The only way that I know of to plant defualt trees is to add them with an autogen tool.  Default trees in FS9 are not library objects as such but are generated by the scenery engine.  It is very common for developers to advise users of the libraries that are needed for their scenery.  Generally I think that is a better approach rather than bundling all the libraries in the download.  With textures this can make a very large download and raises the opposite problem of users getting duplicated libraries on their system.

The tree library you refer to is created by Arno Gerretsen and is pretty commonly in use I would think.  So I would follow the usual route of advising users which libraries are used and where to get them if they do not have them