The tree geometry is stored by default in the scene, you can see them as normal 3dsmax objects hidden in the forest_templates layer. Normally these objects should cleaned when they are unused, however you can check if there is some huge geometry there loading your scene.
To prevent to load so much your scene files, you can convert these templates to XRef's and configure the library browser to create new objects on this way. You can change the default behaviour on the Library Browser-> File -> Preferences, and set import mode as XRef as seen on the attached image.
In any case, if your scenes are really big and you can't find the reason, try to create a new clean scene and merge all items from the old one, sometimes this gives surprising results.