25-35 is great for some adventure game or presentation , simulation. Not for a game with too much additionnal stuff : complete UI, characters on the scene, special effects, Some basic AI etc ...
For shadows, you should use some UT3 clever technique : texture projection, used from lot of years , same result : Your trees won't move from their place And biggger performance (why not tunning in otpions like static or real time for trees ?)
You should use billboars that orient to camear direction for far distant trees, instead fo some static 2 crossed planes : they are too much visible, a billboard would give lot more best result for even less processing : only 1 plane !