When I was playing SPORE: Galactic Adventures again, I ran into this game-breaking bug. I was horrified that nobody had a surefire fix to this other than "reinstall the games in a certain order." There were so many threads on forums full of frustrated people who couldn't figure out to fix their game.
So, I did something really simple... I tried to open SPORE to see if the original base game had the same issue. It didn't.
So I opened SPORE: Galactic Adventures again... AND THERE WERE TREES.
Turns out you HAVE to let the base game initialize in order to have trees in SPORE: Galactic Adventures... or something.
Watch the video if you need more information, but this put trees back in my game, and let me continue having fun!