I use same kinda software
ok, so there's 2 ways on doing it, first you could download the model on sketchfab instead and selecting GLB instead of FBX, this also fixes some weird models (some of the items were flipped inside out when I imported it in godot as an FBX) or you can make the trees mesh unique, then clicking on the material and making that unique, after that select the black material and in the albedo make the black color translucent by sliding the bar all the way to the left, then make sure the transparency opinion is set on the last opinion