There are some options in cfs3config - either take a look via the exe or go into configoverrides.xml in your Appdata pathway, there are some scenery display options there. Not sure if you will lose only the trees, though.
A more radical option would be to remove the trees .sm (scenery models) from your Terrains folder, in one of the zip files. Then put the tree .sm files back, but into your Assets folder would work just as well as putting them back in the zip file.
HTH,