Forest doesn't require a license for render nodes when used without any interaction of the user. This includes unattended render modes as Backburner, Deadline, Distributed Rendering, etc.
Instead, if Max is open manually, it works as a workstation, and requires a license. Regarding licenses, Forest works in the same way than 3DS Max.
What method are you using for your render nodes ?
When rendering we get red words LICENSE all over the trees.
That may be from other plugin. Forest or RailClone don't show that message in the scene.