I don't know if I have understand you right, but the variable "TIME_FAC" multiplicated by 16 = FPS. So you can check the fps like this

if(TIME_FAC*16<30) //FPS<30?
{
ent_remove(tree1);
ent_remove(tree2);
camera.clip_far=5000; //Reduce view distance;
}

Regards
TSGames

www.TSGames.de
Bounci Madness - Online Game