It's called tileset mixing. You should have two unused GFX slots in your level which you can use to accomplish what Mikul suggested.
1. In the Super GFX Bypass menu (red poison mushroom in Lunar Magic), enable it and select the GFX tileset containing the Forest tiles (1F) for either BG2 or BG3.
2. Go into the Map16 editor (question block), find page area of Map16 tiles you were assigned for this contest, and open up the 8x8 tile editor (quartered question block in the editor).
3. Find the tree tiles in the 8x8 editor (should be a few pages down), select them, place them into your Map16 area using the 8x8 tile editing function in the Map16 window (the quartered turn-block).
4. You will need to place duplicate 8x8 tiles and flip them around to get them to look identical to original SMW trees.
5. Lastly, make sure your trees "Act as" behavior is set to 25 so that Mario and everything else can pass through them. If you want, you can also turn on priority so Mario and sprites can hide behind them!
6. Once you're done creating the tiles save your Map16 data to the ROM
(it's the floppy disk icon in the Map16 editor). Go into the Add Objects -> Direct Map16 Access, locate your tiles, and place them in your level. Test your level to make sure they look and function right in-game.
Just like this, you can add extra tilesets into your level without having to change an existing one! You probably could've found this info in a tutorial, but I'd rather give you the help before anyone makes a snarky comment about your inexperience.
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