[agracier]

You can also name any texture that you wish with '...._Fields.tga' and no more trees will spawn on that texture (unless you create a ... .tree.tga file)

Note that you call a texture whatever you like. If you wish to call a desert sand texture, 'desert_sand_fields.tga', even though there is no trace of a cultivated field in the image, it doesn't matter. No trees will spawn on that texture anymore. It's the filename that is instrumental in keeping spawned trees out of the area.