Use any non-Tree placeholder object instead and after generating use Edit -> Search & Replace with Current World Only to replace the placeholders with the actual Tree objects.
Well I thought it was a good workaround, but unfortunately, it'll only let you replace the generated static with another static. Tree objects don't appear in the 'replace with' list.
I tried making a new static and assigning a tree mesh to the new static, but it only crashed the CK, so no joy there either

[Edit]
Hahaha,...you beauty,...it does work.
I didn't notice at first, but the search/replace box has a small tick box with 'Same base object type only' beside it. I un-ticked it and it let me replace the statics with trees. Happy Chappy
[Edit - 2]
Looks like Maegfaer was typing this vv, while I was typing this ^^
