In Lands there isn’t any difference in memory consumption using plants individually or in forests. In both cases plants are block instances.
The ideal would be that detailed plants exist only on the Octane memory space. However, since we want plants get rendered on any Rhino renderer, we finally opted to put the detailed plants into the Rhino document just before the render, and removing them once finished. This ensure that any renderer will render them properly. Lands uses a lot of strategies to reduce the memory consumption: block instances, levels of detail depending on the distance of the tree in respect to the camera… But given that all renders are going to create its own geometry on its own memory space, we have the plant mesh info duplicated. The only renderer where this doesn’t happen is Flamingo nXt. We use the Flamingo SDK to add plants to the Flamingo memory space directly instead of adding them to the Rhino document. We would like to do something similar in VRay, using proxies, but we haven’t found yet the way to do it automatically (user can do it manually).
Finally, I must say that not all .ArPlantX are equal. The plant editor shows you the number of objects of the actual plant. You can edit your plant definition to reduce the number of objects keeping the overall appearance. This can save a lot of memory.
Regardig the documentation, the main source of information is the Lands Users Guide included in the software. You can access it from the drop-down menu, the toolbars, and the “_laHelp” command.