For network rendering it is not allways neccesary to copy the libraries to the render node. When you select a item from library, the geometry and materials are merged to the scene so the library itself is not fully neccesary.
One exception for this are the texture maps. For the HQ Plants free, the maps are included into the library folder and a small maxscript included with the library, check for the maps when your import the first tree. For network rendering you could copy these maps to your shared maps folders, or run the library once time on every node. For HQ Catalog libraries the maps also checked and installed on the same way. For your own libraries there is not maxscripts that does this, so your must take care of these.
Other exception is the Xref import mode (available from library settings). When it is active the trees are XRef'd to the scene so you'll need to install them on all nodes or use shared folders as for the maps.