Nope, don't switch them to X either. That means for the trees that are around the edge of the "forest" you won't be able to walk on the flat ground.
You've drawn with an unused tile, and even though there's no sprite, it's still taken the passage settings. To REMOVE unwanted tiles, you need to use the top-left B tile and draw over the unwanted tiles with it.
How do you find them if they have no sprite? Just edit the tileset image in Paint.NET or something and put a big, red X in the tiles that make up the rest of the "rectangle" around the trees (the ones you've drawn with that have no sprite). Then go back to your map with the new tile, and anything that has a big red X in your map needs to be drawn over with the top left tile.